The rich array of conventional and exotic electronic properties that can be generated by quantum effects in oxide heterostructures is of potential value for device applications. We have therefore explored the possibility to fabricate field effect transistors and integrated circuits from quantum materials, using in particular the two-dimensional electron liquid generated at the LaAlO₃/SrTiO₃ interface as electron system for the drain-source channel. These results illustrate the practicability of non-mean field systems for electronics.
This work was done in collaboration with Rainer Jany, Christoph Richter, Carsten Woltmann, Georg Pfanzelt, Benjamin Förg, Marcus Rommel, Thomas Reindl, Ulrike Waizmann, Jürgen Weis, Julia A. Mundy, David A. Muller, and Hans Boschker.

In recent years, complex-oxide heterostructures and their interfaces have become the focus of significant research activity, primarily driven by the discovery of emerging states and functionalities that open up opportunities for the development of new oxide-based nanoelectronic devices. The highly conductive state at the interface between insulators LaAlO₃ and SrTiO₃ is a prime example of such emergent functionality, with potential application in high electron density transistors. Here, we demonstrate a new paradigm for voltage-free tuning of LaAlO₃/SrTiO₃ (LAO/STO) functionality, which involves the mechanical gating of interface conductance through stress exerted by the tip of a scanning probe microscope. The mechanical control of channel conductivity results in transistor functionality with zero gate voltage, and the long retention time of the induced resistance states also enable the use of LAO/STO heterostructures as mechanically-operated memristors.

Mechanical stimuli induced by the tip of an atomic force microscopy (AFM) is the basis for the generation of different types of phenomenologies, from flexoelectric fields that can lead to mechanical writing in ferroelectric materials [1] to piezochemical effects due to the dynamics in ionic systems [2].
In this presentation, I will introduce a new electromechanical effect that can be studied by AFM: the piezoresistance coefficients and gauge factors of sensor materials. In this work [3], we have induced an insulator-to-metal transition by applying uniaxial pressure to the material through an Atomic Force Microscope (AFM) tip, measuring a record gauge factors for an oxide material at the nanoscale. We achieve the reversible mechanical control of dielectric gap in a semiconductor oxide that lead to metal-insulator transitions induced by uniaxial stress, demonstrating that local electronic structures can be locally changed by applying uniaxial pressure through an AFM tip. The AFM tip also acts as a sensor and transport measurements through the AFM tip are done through different approaches. In all cases the experimental setup consist of the sample and tip placed in series resulting in a capacitor where the tip is the top electrode and an LSMO thin film substrate between SIO and STO is the bottom electrode. While the features of the I(V) for the lowest applied forces resemble those of a semiconductor, linear Ohmic behavior is achieved for increasing forces with increasing slopes. From the obtained results, we observed an outstanding and reversible decrease of the resistance of the Sr₂IrO₄ thin film as a function of increasing mechanical loading force on the AFM tip. We attribute this behaviour to an insulator-to-metal transition caused by pressure induced changes in the Ir-O-Ir bond angle in the plane which produce a closure of the band gap.
[1] H. Lu, et al., Science 336, 59 (2012).
[2] Y. Kim, et al.,Nanoletters 13 (2013) 4068.
[3] N. Domingo, et al, submitted for publication.

Due to unique attributes of complex oxides, such as the fine balance between competing electronic and magnetic phases, and their sensitivity to defects and doping, electric fields can be used to craft electronic order, alter chemistry, apply strain, and tune spin-orbit couplings (e.g. in the 5d oxides). This presents an opportunity to create novel functionalities, in principle, enabling device concepts that go far beyond what conventional semiconductor transistors can do (i.e. using a scheme that mimics the circuits in the human brain). In particular, the very high charge density induced by an electric double layer (EDL) formed at an electrochemical solid-liquid interface has recently been used to induce or “gate” exotic phase transitions, therefore electronic ground states of strongly correlated oxides in the interfacial region, via ‘field-effect doping&’. However, a number of intriguing aspects of the gate “knob” for researchers are still poorly defined and would be fertile ground for exploration in a broad range of oxides that exhibit such functionality. One outstanding argument to name: Is the EDL gating purely an electrostatic (electronic phenomena), or chemical redox effect (field-driven ionic motion), or interplay of both? To address this intriguing question, we carried out in-situ and real-time X-ray study to probe interface structural evolution during EDL gating of complex oxide heterostructures, such as LaAlO₃/SrTiO₃ interface and perovskite nickelates (e.g. NdNiO₃). The experimental findings from X-ray investigations illustrate two intrinsic but contrasting structural responses to EDL field manipulation in the two representative systems although both exhibit drastic metallic to insulating state transition due to the field effect. The structural behaviors, static and dynamic, in LAO/STO during EDL gating can be reconciled with the synergy of polar reconstruction, electrostriction and surface defects, which are intimately linked with the mechanism responsible for the establishment of 2DEG at this interface. In contrast, the structural evolution in NdNiO₃ during EDL gating is more consistent with controlling a metal-insulator transition by manipulating O vacancies

Phase transitions induced by carrier concentration changes in bulk complex oxides are conventionally achieved by chemical doping. Electric double layer transistors (EDLT) with ionic liquid also enable one to dynamically induce phase transitions in a reversible manner when working with thin film oxides. There are however concerns whether these emergent properties originate from pure electrostatic effects or electrochemical reactions, because the response of such EDLTs sometimes far exceeds what one would expect from simply the screening length of the channel. The question is therefore whether one can control and exploit such electrochemical effect in a deterministic way to study emergent transport and consider applications involving ionic-electronic transport such as neural circuits or memory and fluidic interfaces for programmable switches. In this presentation, we will present results on our studies on ionic liquid gated correlated oxides such as vanadium dioxide (VO₂) and rare earth nickelates (RNiO₃). It will be shown that channel conductance modulation by the gate voltage can be due to electrostatic or electrochemical doping operating at different time scales. The differences in the time constant of the charging of the gate capacitance and that of conductance modulation can be used to identify fundamental mechanisms involved. The non-electrostatic effect can be reversible but hysteretic, leading to potentially interesting applications in memory and neuromorphic devices. We will then present on-going studies to quantify the field effect phenomena to build small signal models for such transistors.

VO₂, a typical material for strongly correlated transition metal oxides, shows first order Metal-Insulator Transition (MIT) at 340K accompanied by a resistivity change over 5 orders of magnitude. From the nanoscopic point of view, VO₂ shows mixed electronic phases consisting of metallic and insulating domains around the MIT temperature, which plays an important role in the underlying physics of such materials [1]. With reducing the geometry of this material down to comparable size to the domains, individual domain behavior will be observed and artificially controlled, and it is of advantage for technological application such as memristive or switching devices. To investigate behavior of single electronic domain, we artificially fabricated VO₂ nanowires on Al₂O₃ (0001) substrates by nanoimprint lithography [2] so that sample size of VO₂ film was reduced to comparable to that of a single domain. In a VO₂ nanowires with a width of 200thinsp;nm, a temperature induced multistep MIT was observed with larger change in resistivity [3]. In more details, we performed numerical simulation based on random resistor network model to investigate the origin of the observed large transition and relationship between domain configuration and resistivity. Based on this simulation, these multistep resistivity jumps can be understood as a transition of a single domain, whose size is estimated to be around 50-70thinsp;nm from numerical calculation.
As further development, electrical control of multistep MIT is interesting for device application. We also observed that the nanowire exhibited multistep MIT induced by bias voltage while the thin film sample shows continuous change of resistivity as observed in temperature-induced case. This seems to be caused by the nano-confinement effect of the nanowire whose conductive behavior becomes 1D-like. There results indicate that the dimensionality of domain configuration plays an important role in the electrical control of the multistep MIT of nanowires. In comparison of a performance of the artificial nanowire with other VO₂ two-terminal devices, this voltage controlled MIT nanowire device shows the high On/Off ratio with low energy consumption, and also has an advantage in high controllability of size, shape and position in large area.
[1] M. M. Qazilbash et al., Science 318 (2007) 1750
[2] H.Takami, H.Tanaka et al,., Appl. Phys. Lett.101 (2012) 263111
[3] H. Takami, H. Tanaka et al, Appl. Phys. Lett. 104 (2014) 023104.

The relatively simple binary oxide, VO₂, has served for decades as a prototypical material challenging the ability of scientists to understand how a high-temperature, metallic conductor emerges from a low-temperature band (Peierls) or strongly-correlated (Mott) insulator. A predictive microscopic description remains elusive and controversial. The first-order metal-insulator transition (MIT) in VO₂ occurs just above room temperature (T_c~340 K), where the conductivity changes by four orders of magnitude, and concurrently, the lattice structure changes from high-temperature tetragonal (rutile) to low-temperature monoclinic (M1). A fundamental gap in our knowledge is the lack of an accurate description of changes in lattice dynamics associated with the MIT in VO₂. Phonon dispersion curves normally obtained using single-crystal inelastic neutron scattering (INS) measurements are not available due to the large incoherent vanadium cross section. We have now determined the changes in lattice dynamics and vibrational entropy associated with the MIT. We used inelastic neutron scattering at the SNS/ARCS spectrometer to obtain the Q-integrated phonon density of states, x-ray scattering at APS/33BM to obtain 3-dimensional maps of energy-integrated thermal diffuse scattering, inelastic x-ray scattering at APS/HERIX to directly measure phonon dispersion along symmetry axes, and ab initio calculations to elucidate the atomic mechanisms driving the MIT. Our results show that the transition entropy change driving the MIT is dominated by vibrational rather than electronic contributions. Moreover, we show that proposals of an “R-point soft mode” phase transition are incorrect, and instead, we identify strongly anharmonic lattice dynamics as the underlying mechanism stabilizing the metallic phase.
Research supported by DOE Office of Basic Energy Sciences, Materials Sciences and Engineering Division (MSED). Research by JWH supported by the Center for Accelerating Materials Modeling (CAMM), funded by the U.S. DOE, BES, MSED. SNS and APS facilities supported by DOE-BES, Scientific User Facilities Division.

Ternary Sn(IV) oxides have attracted recent interest because of high mobilities found in the n-type cubic perovskite transparent conductor BaSnO₃ and the ferroelectricity of LiNbO₃ structure ZnSnO₃. In addition to these materials, a number of stable orthorhombic perovskites are known including CaSnO₃ and SrSnO₃, as are layered perovskite compounds. These compounds, while relatively less studied, do show an unusually wide range of optical band gaps. This suggests considerable structural sensitivity and perhaps tunability of the electronic structures of these compounds. Our first principles calculations for strained bulk compounds and heterostructures show that there is in fact exceptionally strong sensitivity of the electronic properties to structure, but in a very different way from the more studied transition metal oxides, such as titanates and niobates. This can be understood in terms of the s-like conduction bands of these Sn(IV) oxides.
This work was supported by the Department of Energy, Basic Energy Sciences, Materials Sciences and Engineering Division and was done in collaboration with Qiang Xu, Khuong P. Ong, X.F. Fan and Xin Chen

Perovskite oxides have been widely investigated as one of the largest material groups indicating unique electronic and magnetic functions since decades. Cation substitution in perovskite oxides is an effective way to develop the functionalities through carrier doping, band engineering, application of chemical pressure, and so on. Recently, improvement of synthetic techniques allowed us to investigate the properties of anion-mixed perovskites. Among them, perovskite oxynitrides especially gain interests because of their potential usage as pigments, visible light active photocatalysts and dielectrics. Although having been widely argued, the relationship between the unique electronic properties and anion arrangement has not been fully clarified yet. This is partly because of the difficulty in controlling anion arrangement in oxynitride compounds with the conventional nitridation method using high temperature ammonia.
In this report, we investigated the possibility of controlling anion arrangement in d⁰ perovskite oxynitride SrTaO₂N using epitaxial stress. We focused on their dielectric properties, because they are, in general, sensitive to structural changes.
We adopted nitrogen plasma assisted pulsed laser deposition to synthesize thin films of the perovskite oxynitrides. A sintered ceramics of Sr₂Ta₂O₇ was ablated with KrF exicimer laser under nitrogen activated with a plasma source and (001)-oriented thin films were epitaxially grown on perovskite oxide single crystal SrTiO₃ substrates. Though the SrTaO₂N films were partially relaxed from the substrate due to the too large lattice mismatch (minus;3.2%), the tetragonality (c/a =1.02) was much higher than bulk specimens (c/a =1.002), indicating that the thin film material was highly compressed by the stress from the substrate. Piezoresponse force microscopy of the SrTaO₂N epitaxial thin films revealed that small regions with 10-100 nm in scale exhibit ferroelectric behavior at room temperature [1]. This is the first observation of ferroelectricity in perovskite oxynitrides, to our knowledge. First principles calculations suggested that the ferroelectricity originates from trans-type nitrogen ordering, which can be stabilized by compressive epitaxial strain. The ferroelectric SrTaO₂N films also showed visible light absorption with a bandgap of 2.3 eV. The present SrTaO₂N epitaxial film realizing small bandgap and room temperature ferroelectricity at the same time is a promising material to be used in ferroelectrics-based photovoltaic cells. These results indicate that controlling anion arrangement through epitaxial stress an effective strategy for developing new functionalities in perovskite oxynitrides.
[1] D. Oka et al.Sci. Rep.4, 4987 (2014).

Hexagonal orthoferrites RFeO₃ (R = rare earth) are new multiferroic materials that are generally unstable in the bulk form, but can be epitaxially stabilized as thin films on lattice-matched substrates. In this work we will discuss the growth and characterization of hexagonal RFeO₃ thin films, where R ranges from Lu to Tb, and the doped phases such as Lu(Fe,Co)O₃ films, obtained on the (111)ZrO₂(Y₂O₃) and (111)MgO substrates. Because these are new multiferroic materials, we performed a broad study of their structural and physical properties, with an extensive use of transmission electron microscopy (TEM), X-ray diffraction, magnetic and spectroscopic measurements. In addition to our originally reported growth of pure hexagonal phases, in the present study we show that, surprisingly, in the thin films of LuFeO₃ the continuous epitaxial nanolayers of Fe₃O₄ can appear at partial oxygen pressure being 10³ times higher than the equilibrium p(O)₂ for magnetite formation [1]. The attempt to dope LuFeO₃ with Co³⁺ ions resulted not in the formation of LuFe_1-xCo_xO₃ solid solution, but instead in the thin films comprised of LuFeO₃-Lu(Fe,Co)₂O₄ layered structures [2]. In terms of physical properties, we found that RFeO₃ thin films exhibit weak ferromagnetism at temperatures T=100-130 K, a behavior very distinct from what is observed in well-known hexagonal RMnO₃ [3]. In order to study ferroelectric order, we used high-resolution TEM and observed peculiar ferroelectric domain structure of the expectedly improper ferroelectricity [4], which together with a relatively small optical band gap of 1.9 eV [5] can make the material attractive for photovoltaic applications. This work was supported by RFBR (13-03-01249), the NSF under DMR under 1124696 and by the ARO under W911.
[1] Akbashev et al. Reconstruction of the polar interface between hexagonal LuFeO₃ and intergrown Fe₃O₄ nanolayers. Scientific Reports 2, 672 (2012).
[2] Akbashev et al. Reconstructed stacking faults in cobalt-doped hexagonal LuFeO₃ revealed by mapping of cation distribution at the atomic scale. CrystEngComm 14, 5373-5376 (2012).
[3] A.R. Akbashev, A.S. Semisalova, N.S. Perov, and A.R. Kaul. Weak ferromagnetism in hexagonal orthoferrites RFeO₃ (R=Lu, Er-Tb). Appl. Phys. Lett. 99, 122502 (2011).
[4] V.V. Roddatis, A.R. Akbashev, S. Lopatin and A.R. Kaul. Complex structural-ferroelectric domain walls in thin films of hexagonal orthoferrites RFeO₃ (Rthinsp;=thinsp;Lu, Er). Appl. Phys. Lett. 103, 112907 (2013).

The amazing properties of strongly-correlated transition-metal oxides stem from the interplay of between many-body effects, charge, spin and orbital degrees of freedom and lattice distortions. It is typically very difficult, however, to disentangle these effects and identify the actual mechanism beyond a specific phenomenon.
In this talk, using a combination of ab-initio and many-body techniques, I will show how these effects can be disentangled in paradigmatic cases: Orbital-order and orbital-ordered melting phenonema (manganites and other oxides) and spin-state transitions (rare-earth cobaltates).
I will show that, in all considered systems, many-body super-exchange is not the driving mechanism behind orbital-order and orbital-order melting phenomena. It plays instead a crucial role for spin-state transition in cobaltates.
References:
[1] E. Pavarini and E. Koch, Phys. Rev. Lett. 104, 086402 (2010)
[2] E. Gorelov, M. Karolak, T.O. Wehling, F. Lechermann, A.I. Lichtenstein, E. Pavarini,
Phys. Rev. Lett. 104, 226401 (2010)
[2] A. Flesch, G. Zhang, E. Koch, and E. Pavarini, Phys. Rev. B 85, 035124 (2012)
[3] G. Zhang, E. Gorelov, E. Koch and E. Pavarini, Phys. Rev. B 86, 184413 (2012)
[4] A. Flesch, E. Gorelov, E. Koch, E. Pavarini, Phys. Rev. B 87, 195141 (2013)
[5] C. Autieri, E. Koch, E. Pavarini, Phys. Rev. B 89, 155109 (2014)

The vast majority of perovskite oxides are centro-symmetric and only a few mechanisms, such as second order Jahn-Teller distortion at the B-site (e.g. Ti, Nb) or lone-pair active cations at the A-site (Pb, Bi), can break inversion symmetry, which is generally limited to special chemistries. New mechanisms, such as hybrid improper ferroelectricity (HIF) have been recently proposed to construct non-centrosymmetric perovskite superlattices (SLs) of the type (ABO₃)_n/(A'BO₃)_n from centro-symmetric building blocks, which are predicted to possess large polarizations [1]. Specifically, builk SrTiO₃ (STO) and CaTiO₃ (CTO) are non-polar at all temperature [2,3], however layering these materials together can induce HIF due to the large octahedral rotations in CTO which persist to ~1500 K [3], making (STO)_n/(CTO)_n SLs a promising candidate for high-temperature piezoelectric applications.
We have grown a series (n=1-6) of (STO)_n/(CTO)_n SLs on (001)LSAT substrates using the hybrid molecuar beam epitaxy approach [4], which combines thermal and metal-organic sources. This growth technique enables self-regulated stoichiometric growth windows for both STO and CTO, ensuring ultra-low defect concentrations in the SLs. The SL structures were assembled in a layer-by-layer mode by sequentially shuttering the respective STO and CTO sources, where n was precicely controlled using in-situ reflection high-energy electron diffraction monitoring. X-ray diffraction analysis revealed the high quality of the SLs evidenced by the occurence of all SL diffraction peaks.
Using optical second harmonic generation (SHG), we discovered that a room temperature polarization emerges in this SL series. The temperature dependence of the SHG shows exotic behavior that varies drastically with n, and exhibits an unusal re-entrant poalrization behavior for n>4 SLs at ~150 K, where octahedral rotation transitions for STO occur [2], which is coupled to the ferroelectric transition of the strained CTO/(001)LSAT system at ~150 K [5]. The n=2 SL shows the strongest polar response which persists to room temperature, and has a linear temperature dependence, indicating that the observed polar phenomenon for the n=2 SL structure is due to HIF. Temperature dependent impedance spectroscopy will be correlated with the SHG results to further elucidate the emergence of HIF in this SL system.
This work is funded by the National Science Foundation through the Penn State MRSEC program DMR-0820404.
[1] Rondinelli, J.M., Fennie, C.J., Adv. Mater. 24, 1961 (2012).
[2] Fluery, P.A, Scott, J.F., Worlock, J.M., Phys. Rev. Lett. 21, 16 (1968).
[3] Gu, Y.J., Rabe, K., Bousquet, E., Gopalan, V., Chen, L.Q., Phys. Rev. B85, 064117 (2012).
[4] Engel-Herbert, R., Molecular Beam Epitaxy, edited by Mohamed, H., 417-449 (Elsevier, Oxford, 2013).
[5] Biegalski, M.D., Qaio, L., Mehta, A., Takamura, Y., 16^th US-Japan Seminar on Dielectric and Piezoelectric Materials, p. 27-29 (2013)

The prospect of creating a ferroelectric directly on a semiconductor has been a subjet of research interest due to the possible applications in electronic devices such as non-volatie transistors. Here, we focus on the properties of crystalline, epitaxial thin films of BaTiO₃ on the (001) surface of Ge which have atomically abrupt interfaces. We summarize first principles results based on density functional theory describing a number of aspects of this system: (i) the interfacial structure which is compared to X-ray diffraction experiments, (ii) the polarization profile and interfacial pinning of the polarizaiton, (iii) the electronic band alignment across the interface, and (iv) the energetics of oxygen vacancies in this system as a function of both position and concentration.

The ABO₃ perovskites are remarkable in their ability to accommodate high degrees of cationic off-stoichiometry, particularly in thin films where epitaxial constraints can promote single phase formation. However, there have been relatively few systematic studies aimed at understanding how macroscopic electronic and structural properties in perovskite films vary with the A:B ratio. Here, we utilize Rutherford backscattering spectrometry (RBS), x-ray diffraction, transmission electron microscopy, and electronic transport measurements to explore off-stoichiometry-induced effects in LaNiO₃ (LNO). LNO is widely-studied metallic perovskite that is of interest as an electrode layer for oxide-based devices. The effects of oxygen stoichiometry on electronic conductivity have been widely studied in LaNiO_3-δ, however the effects of cation content is an area that has not been widely explored. We present the dependence of structural and electronic properties on cation stoichiometry in films with macroscopic La:Ni ratios varying from 0.75 to 1.09. LNO thin films with varying stoichiometry were deposited via molecular beam epitaxy onto SrTiO₃ substrates. Cation compositions were quantified via RBS in the same films from which x-ray diffraction, electron microscopy, and temperature dependent resistivity measurements were obtained. We find a reduction in the c-axis parameter with increasing La deficiency. Cross sectional TEM was used to investigate changes in the microstructure resulting from the substantial off-stoichiometry. Surprisingly, the resistivity is found to decrease with increasing La deficiency, and serves to illustrate the non-trivial dependence of off-stoichiometry on electronic behavior in complex oxide films.
This work is supported by the Office of Naval Research (ONR N00014-11-1-0109).

Transition metal oxides exhibit a rich and diverse range of chemistries and physics that has long established their application in a number of electrochemical energy conversion and storage technologies. The ability to design oxides expressly tuned for these applications is rooted in fundamental understanding of the relationships between structure, chemical composition, electronic properties, and electrochemical functionality.
In particular, the perovskite family (ABO₃ oxides, where A is typically a rare-earth or alkaline-earth metal, and B is typically a transition metal) has been a central focus for studying oxygen evolution reaction (OER) electrocatalyst design due to its chemical flexibility and diverse electronic properties. However, detailed experimental investigation of how the oxide chemistry modifies the electronic structure and its consequent influence on oxygen electrocatalysis has yet to be explored. In this work, we probed the valence electronic states for various first-row transition metal perovskite OER catalysts with a range of activities, using combined analysis of X-ray emission, absorption, and photoelectron spectroscopy (XES, XAS, XPS). We examined major compositional design approaches to better understand how to tune perovskite electronic structure - specifically A- and B-site substitutions, oxygen vacancy content, and the double perovskite family. Through these studies, we find clear principles for tuning the nature of the bulk metal-oxygen bond, which can explain the differences in mechanism and kinetics on different perovskite surfaces

The design of efficient and cost effective catalysts is fundamental for many electrochemical energy conversion and storage reactions such as Oxygen Evolution Reaction (OER) or Oxygen Reduction Reaction (ORR). However, only the understanding of materials structure - properties relationship can provide sufficient insights to design new active and cost effective catalysts. The family of perovskites is the perfect playground for chemist as it provides flexibility to tune the metal - ligand bond. We recently reported a correlation existing in between the OER activity of perovskites and the computed energy level of the oxygen 2p-band.¹ We here go further and explain that the computed bulk electronic structure not only correlates with the OER activity of perovskites family but also influences elementary reaction steps in the OER mechanism. Coupling electrochemical measurements, electronic structure computation (DFT), and X-ray absorption measurements, we demonstrate and discuss the existence of three OER mechanisms.

Ginzburg-Landau-Devonshire theory is employed to study domain walls in multiferroic BiFeO₃ (BFO) with two sets of order parameters: spontaneous polarization and oxygen octahedral tilts. The rhombohedral BFO can form 71^o, 109^o and 180^o domain walls, with the angles measuring the direction changes of polarization vectors in neighboring domains. We find that the wall energy sequence is 109^o < 180^o < 71^o, which is different from the results with polarization as the only order parameter. 71^o, 109^o and 180^o domain walls are determined to be Bloch type, which is consistent with density function calculation and experiments. Phase field simulation of the domain structures in bulk BFO shows that the 109^o and 180^o domain walls are dominant, which agrees with experiments. Our results reveal the important role of oxygen octahedral tilts in determining domain wall properties of BFO.

Pulsed laser deposition (PLD) is a commonly used growth technique for the synthesis of thin films. However, unlike the other popular oxide thin film growth techniques, such as molecular beam epitaxy (MBE), PLD from a single ceramic target does not offer complete control over single monolayer growth of the binary oxides making it more difficult to create metastable-layered compounds such as the Aurivillius and Ruddlesden-Popper phase materials. In this work we overcome this with growth from binary oxide targets. This approach to PLD combines the advantages of growth kinetics close to thermodynamic equilibrium, high reproducibility and precise composition control.^[1] In order to control these growths the monitoring of intensity oscillations from in situ reflection high-energy electron diffraction (RHEED) is utilized to tune the dose from each target to obtain layer by layer growth of stoichiometric films.^[2,3] However, the mechanisms of the RHEED intensity oscillations is still unclear for ternary oxides grown in this scheme. Most work in this area is focused on the increase and reduction in surface roughness during the growth as a source of the RHEED oscillations,^[2,3] in an analogy to the growth of semiconductor materials.^[4] Thus from this model of RHEED oscillations, for the growth of ternary oxides from binary oxide constituents, a full surface coverage of each of these binary oxide should lead to a single oscillation of the RHEED intensity. Yet, a single unit cell per RHEED oscillation is only realized when half of an oscillation is obtained for each alternating binary oxide layers, i.e. half of a RHEED oscillation is needed for each TiO₂ and SrO layer to grow SrTiO₃, suggesting that this growth mechanism is different. In this work we will show that the RHEED oscillations in the growth of SrTiO₃ from binary oxides is controlled by the crystallization of the SrTiO₃ layers and discuss the implications of this on the observed RHEED oscillations.
This research was conducted at the Center for Nanophase Materials Sciences, which is sponsored at Oak Ridge National Laboratory by the Scientific User Facilities Division, Office of Basic Energy Sciences, and U.S. Department of Energy. AH, TZW, and GE were supported by the US DOE Office of Basic Energy Sciences, Materials Sciences and Engineering Division. AH and KD are grateful for the support by SFB 762.
[1] X. X. Xi, Guozhen Liu, Ke Chen, Q.Y. Lei “Atomic Layer-by-Layer Growth of Homoepitaxial SrTiO₃ Films by Laser MBE,” Workshop on oxide electronics 18, Napa Valley, California, Sept. 27, 2011.
[2] J. Haeni, C. Theis, and D. Schlom, Journal of Electroceramics 4, 385 (2000).#8232;
[3] C. M. Brooks, L. F. Kourkoutis, T. Heeg, J. Schubert, D. A.Muller, and D. G. Schlom, Applied Physics Letters 94, 162905 (2009).
[4] J. H. Neave, B. A. Joyce, P. J. Dobson, N. Norton AppI. Phys. A 31, 1-8 (1983).

Ferroelectric films have been widely investigated for various applications. Due to the close correlation between the ferroelectricity and the crystal structure, crystal structure analyses have been carried out for the as-deposited ferroelectric films. However, the crystal structure analysis for the ferroelectric films including non-180° domain after applied electric field is essential to understand the ferreoelectricity of the films because irreversible domain switching is pointed out to be occurred. In the present study, the crystal structure of rhombohedral Pb(Zr,Ti)O₃ epitaxial films before and after applying an electric field was characterized by focusing X-ray diffraction using synchrotron X-ray source.
(111)/(11-1)-oriented rhombohedral Pb(Zr_0.65Ti_0.35)O₃ films with different domain fraction were epitaxially grown by metalorganic chemical vapor deposition. The volume fraction of (111) polar axis domain (V_pol) in the as-deposited films was controlled by selecting the single crystal substrate with different thermal expansion coefficient. Saturation polarization (P_sat.) values of the films were larger than the estimated one from V_pol, especially for the films having small V_pol. The irreversible non-180° domain switching from (11-1) non-polar axis to (111) polar axis had been occurred by applied the electric field. The same behavior was also observed for (001)/(100)-oriented tetragonal Pb(Zr_0.40Ti_0.60)O₃ epitaxial films. The observed P_sat. values can be explained by V_pol after applied the electric field. The interesting point is that the switched domain fraction from non-polar axis to polar one is found to be almost constant of 40% in non-polar domain regardless of the kinds of substrate and the film composition.
The present results clearly indicate the impact of the evaluation of the crystal structure of not only as-deposited films but also domain-switched films after applied electric field to understand the ferroelectric properties of Pb(Zr,Ti)O₃ films.

Transition metal oxides have emerged as candidate materials for next generation, multifunctional electronic devices due to the presence of electronic phase transitions that are accompanied by abrupt changes in their macroscopic electronic and magnetic properties. In order for these transitions to be exploited in such devices, however, it is crucial that the nature of electronic transport and charge localization in these materials be well understood, especially in thin films. Here, the electronic transport properties of one such material, La_1#8725;3Sr_2#8725;3FeO₃ (LSFO), are investigated. The electronic phase transition in LSFO is both intriguing and unique since it occurs without a change in crystal symmetry, causes a change between two insulating conduction mechanisms and introduces an unusually long-range antiferromagnetic and charge ordering at the same transition temperature. Epitaxial LSFO films are probed via temperature dependent resistivity, magnetoresistance and Hall effect measurements. Occurring around 190thinsp;K, the electronic phase transition results in a change in conduction mechanism from nonadiabatic polaron transport above the transition temperature to resistivity following a power law temperature dependence at low temperatures. This conduction mechanism transition occurs concomitantly with an abrupt increase in Hall coefficient and apparent mobility below the critical temperature (T*). The unique low temperature transport properties are attributed to an unusually long-range periodicity of the antiferromagnetic ordering, six pseudo-cubic d-spacings along the [111] direction. The presence of low temperature antiferromagnetic ordering is manifested in the electronic transport via negative magnetoresistance below T* and a sign reversal of the Hall coefficient at T*. Comparison of differing film thicknesses, stoichiometry, and strain states demonstrates that the observed conduction behavior is intrinsic to LSFO and not introduced through any extrinsic phenomena.
This work is supported by the Office of Naval Research (N00014-11-1-0664).

Control of electric phases in a field-effect transistor (FET) structure has been an emergent concept, and the merit of FET structure is exceptionally valuable in strongly correlated electron materials (CEMs), where electric-modulation of electric phases has attracted considerable interest due to their great functionalities originate from the interplay between spin, lattice, charge, and orbital [1]. Since the complex correlation results in a multiple domain structure, the phenomena on CEMs are more complicated than that on conventional materials. The intrinsic inhomogeneity of electronic phases in the form of nanoscale spatial electronic separation is associated with many exotic properties of CEMs accompanying an insulator-metal transition (IMT). As a typical example, the coexistence of ferromagnetic metal (FM) and charge and orbital order insulator (COl) nanoscale phases in (La,Pr,Ca)MnO₃ through IMT has been observed. Electronic domain engineering, dynamics, and manipulation using the response to external stimuli are indispensable leading to realization of “Mottoronics”. In this study, an electric double layer transistor (EDLT) utilizing (La,Pr,Ca)MnO₃ film was fabricated and the gate voltage (V_G) dependence of the conductive properties was investigated.
(La_0.375Pr_0.1Ca_0.525)MnO₃ (LPCMO) thin films were grown on MgO(001) substrate using pulsed laser deposition technique. The LPCMO films were patterned into a Hall-bar structure and Au/Ti electrodes were deposited. By putting a small amount of ionic liquid (DRME-TGSI) on the channel and side gate, the EDLT structure was achieved. The V_G dependence of resistance (R) vs. temperature (T) was examined. In order to understand a single electronic domain to the response to the electric-field, we employed a simple model of the statistical first-order transition among domains, which illustrates the quantitative aspects of the mixed-phase regime in manganite [2].
At V_G = 0 V, R change corresponding IMT observed at the transition temperature T_IM of 120 K with the width of coexistence region (σ) of ~50 K in cooling process. With a positive bias of V_G= +3 V, corresponding to electron doping, T_IM decreased to 125 K and σ shrank to ~40 K, and reversely T_IM increased to 115 K and σ expanded to ~60 K under a negative bias of -3 V (hole doping). This result macroscopically agrees with an expanded diagram for chemical doped LPCMO at Ca concentration of around 0.525 [3]. In terms of microscopic phenomena, the coexistence of FM and COI phases can be satisfactorily interpreted by the T_MI distribution among single domains, and it can be thought that electric-field effect would induce the statistical IMT property change, implying the possibility of IMT property tuning. In the presentation, our detail results and analysis will be discussed.
[1] T. Hatano et al., Sci. Rep. 3 (2013) 2904.
[2] A. N. Hattori et al., submitted.
[3] M. Uehara et al., Nature 399 (1999) 560.

Due to recent trends in the development of environmentally benign materials, a number of new electronic ceramic devices have been reported that are based on bismuth-containing perovskite ceramics. Recent reports by Li et al. showed that (Bi_0.5Na_0.5)TiO₃-based materials exhibit oxide ion conduction with ionic conductivity values comparable to existing fluorite-based electrolyte materials.[1] In this work, the effect of cation non-stoichiometry and doping in a number of bismuth-based perovskite materials will be presented. Two binary solid solutions were investigated including compositions near the morphotropic phase boundary in (Bi_0.5Na_0.5)TiO₃-BaTiO₃ (BNT-BT) and Bi_0.5Na_05shy;TiO₃-Bi(Mg_0.5Ti_0.5)O₃ (BNT-BMT). Ceramic specimens were prepared using solid-state methods. In BNT-BT, acceptor doping resulted in higher dielectric losses. Impedance spectroscopy analysis indicated that the high dielectric losses were correlated to ionic conduction at low frequencies. In contrast, donor doping resulted in a decreased in dielectric loss most likely due to an oxygen vacancy compensation mechanism. In the BNT-BMT system, similar results were obtained where acceptor doping resulted in a significant increase in conductivity at elevated temperatures. The activation energy values derived from Arrhenius plots closely match those of the results of Li et al., which suggests a similar conduction mechanism. In addition, the influence of different electrode systems on the conduction mechanism will be reported.
References
[1] M. Li et al., Nature Mater., 13(1), 31-35 (2014).

Perovskite oxides with strong electron correlations have attracted considerable attention due to the potential applications as active electronic material for logic application utilizing the metal-to-insulator transition. The interest not only emerges from a fundamental point of view to explore correlated excitation in these systems, the successful monolithic integration of perovskite oxides with Si makes them an ideal material choice. However, electronic properties emerging from strong electron correlation may be masked by defects disrupting the periodicity of the underlying crystal and promoting carrier localization through disorder. Thus electronic phase transition materials require a superior level of material quality similar to those of conventional semiconductor material. New growth techniques are required that are able to produce these materials with high perfection.
In this talk we will present the growth of CaVO₃ and SrVO₃ films by hybrid molecular beam epitaxy (HMBE) where alkaline earth cations are supplied using a conventional effusion cell and the transition metal vanadium is supplied using the metal-organic precursor vanadium oxi-tri-isopropoxide. Oxygen is provided in molecular form. Growth temperature was fixed to 800 °C for (La_0.3Sr_0.7)(Al_0.65Ta_0.35)O₃ [LSAT], and LaSrAlO₃ substrates with film thicknesses ranging from 20 nm to 50nm.
Films have been characterized using reflection high energy electron diffraction (RHEED), X-ray diffraction, atomic force microscopy and Hall measurements. Optimized films exhibit high quality Kiessig fringes, with substrate limited rocking curve widths of 8 arc seconds in the case of CaVO₃ and 17 arc seconds in the case of SrVO₃.
Both materials grew in a step-flow mode with atomic steps visible after growth by AFM. In SrVO₃, the perovskite phase remained present with a gradual lattice expansion in all growth conditions. For CaVO₃, the distorted perovskite phase remained pure and with little change throughout a growth window that spanned a 30% changed in cation flux ratios. The Ca-rich limit comes in the form of an abrupt transition to a Ca-rich Phase. The V-rich limit is in the form of a gradual reduction in lattice parameter, in contrast to SrVO₃ where all defects have been shown to increase unit cell volume!
It becomes clear that the A-site cation plays a significant role in the growth window kinetics in HMBE of perovskite vanadates, as SrVO₃ displays a small window or point optimization. Further, relations of strain effect on growth window are discussed in the comparison of minimally strained CaVO₃ on LSAO and tensile strain on LSAT.
Scanning transmission electron microscopy of defects reveal the crystals compensation mechanisms for A-site and B-site rich films, which are then correlated to changes in electronic transport properties. SrVO₃ lattice expansion shows a strong correlation to residual resistivity ratio, a key proxy for defect level.
Support was provided by ONR Grant No. N00014-11-1-0665

Raman spectra of SrHfO₃ have been successfully obtained at temperatures from 300 to 1450 K. Raman spectroscopy studies show an abrupt and very large change in line width of some optical phonon modes at 673 and 1023K, which is interpreted as an order disorder phase transition from orthorhombic phase (Pnma) to Orthorhombic (or Pseudo Tetragonal) at 673K. In addition, we observed a continuous order disorder transition take place and the structure of SrHfO3 changes from Cmcm to tetragonal (I4/mcm) phase in the temperature range from 673 to 1023K. In this temperature range, all four instense Raman modes at 144, 166, 424 and 599 cm^-1 show anomalous change in frequency and line width and other phonon modes nearly disappeared revealing a dynamically unstable state of Cmcm and I4/mcm phases. Above 1023K, we observed only two instense Raman modes, suggesting that SrHfO₃ undergoes a first order displacive phase transition from tetragonal (I4/mcm ) to cubic (Pm3m). The primitive unit cell doubles below 1023 K in the orthorhombic (Cmcm) phase, and a twofold degenerate zone-boundary soft mode in the high-temperature phase becomes Raman-active and nondegenerate, splitting into a B_1g branch(107 cm^-1 at room temperature) and an A_g branch at 232 cm^-1 (ambient). Significant softening of both branches of the soft phonon modes, that becomes doubly degenerate in the tetragonal (I4/mcm) phase above 1023 K, is observed. It is found that the damping constant of the soft mode satisfies a universal scaling law , where, t is reduced temperature and n = ½, as in other pseudocubic displacive ferroelectrics

Bulk studies have found that AMn₇O₁₂ (A=Ca, Sr) quadruple perovskites possess several interesting features including multiferroicity and charge ordering transitions. We are interested in understanding the heteroepitaxial structure of these materials in thin films, a form in which they are relatively unstudied. Commonly X-ray diffraction is employed to determine the crystal structure of an unknown material, but when analyzing a heteroepitaxial film it is difficult to characterize the exact crystal structure of the film using non-synchrotron x-ray techniques. Transmission electron microscopy (TEM) is ideally suited to study thin films due to the ability to use the electron beam to select a small (>30nm) area for study. Traditionally, electron diffraction in the TEM is limited by the ability to interpret the dynamical behavior of electron scattering within a specimen, but through the use of precession electron diffraction one can decrease dynamical effects. Using the quasi-kinematical diffraction patterns produced by precession electron diffraction in the TEM one can solve the crystal structure of a film, in which the exact composition and crystal structure are unknown. By employing precession electron diffraction and obtaining diffraction patterns at myriad of sample tilts, we combine them to form a 3D diffraction tomogram covering all of reciprocal space and have solved the structure of a nominal SrMnshy;₇O₁₂ film using ab initio methods.
This work is supported by the Office of Naval Research (N00014-11-1-0664).

Morphotropic phase boundary relaxors have been intensely studied since the discovery in 1997 of their giant electromechanical performance [Park&Shrout, J.Appl. Phys. 1997]. Even before this finding, relaxors had already intrigued physicists for decades on account of their unusual dielectric behaviour. Relaxor ferroelectrics are also known to possess a surface “skin” layer that is disctinct from its interior. Yet while there is consensus about its existence, there are disagreements about its structure, phase diagram, and origin. We have used a combination of piezoresponse force microscopy, grazing incidence x-ray diffraction and cross-sectional transmission electron microscopy in order to establish that, unlike the bulk of the crystal, the surface remains ferroelectric hundreds of degrees above Tc. The stabilization of ferroelectricity at the surface is shown to be caused by a stoichiometric imbalance, whereby the loss of volatile lead is compensated by an excess of Nb⁺⁵, which is a small ion with higher charge than either Nb⁺² or Ti⁺⁴. The persistence of surface ferroelectricity far above the bulk Curie temperature has important implications for the interpretation of the polar behaviour of relaxor ferroelectrics.

Point defects are pivotal for charge and mass transport in oxides, their concentration and mobility strongly influence the physical, chemical and/or mechanical properties. In the past, numerous studies were performed to understand point defects, and various means to vary their concentration - aliovalent doping, micro-structure or grain boundary engineering and nano-size effects are a few examples. In addition to these well-known techniques, dislocations can potentially influence defect transport properties locally [1]. Owing to a lower defect formation energy and a higher defect segregation driving force [2,3], dislocation cores are expected to have excess charges with adjacent charge redistribution regions (electrostatic effects). Further, dislocations also result in a variation in local strain state (compressive or tensile) that can influence defect mobility [4]. Charged dislocations, with electrostatic and local strain fields, may render these defect sites electrochemically active for redox reactions for e.g., catalysis [4] and filament formation in resistive switching memories [5].
Even though the role of dislocations were studied in the past, to a great extent the focus has been on metals with fewer studies on oxides. A fundamental understanding of dislocation effects on the electronic and ionic defect transport properties in wide band gap oxides remains very limited. Our present work is aimed to bridge this gap by providing a more comprehensive understanding of dislocations in oxides. We will discuss these effects in various mixed ionic-electronic conducting (MIEC) wide band gap oxides, of relevance to solid oxide fuel cells, catalysis and memristive device research.
References:
[1] K.K. Adepalli, M. Kelsch, R. Merkle, J. Maier, Influence of Line Defects on the Electrical Properties of Single Crystal TiO₂, Adv. Funct. Mater. 23 (2013) 1798-1806.
[2] K.P. McKenna, A.L. Shluger, Electron-trapping polycrystalline materials with negative electron affinity, Nat. Mater. 7 (2008) 859-62.
[3] Z. Feiwu, A.M. Walker, K. Wright, J.D. Gale, Defects and dislocations in MgO: atomic scale models of impurity segregation and fast pipe diffusion, J. Mater. Chem. 20 (2010) 10445-10451.
[4] B.Yildiz, 'Stretching' the Energy Landscape of Oxides Inspired from Metals and Polymers: Effects of Elastic Strain on Surface Chemistry and Catalysis, MRS Bulletin, 39 (2014) 147-156.
[5] Ch. Lenser et. al., Identification of screw dislocations as fast-forming sites in Fe-doped SrTiO₃, Appl. Phys. Lett., 102 (2013) 183504

Novel double perovskite material, Nd₂MgRuO₆, crystallizes in a monoclinic structure with space group P2₁/n. Magnesium and Rutheniumions are allowed to occupy Wyckoff positions 2b(0, 0, 0.5) and 2a(0, 0, 0)and the Rietveld analysis indicates a B-site fully ordered structure. In general, the cationic ordering at B-site is an important issue owing to its correlation to electrical resistivity, dielectric and magnetic properties of ordered double perovskite. Weakly magnetized ordered double perovskites are interest currently because of the exotic magnetic behavior exhibited by many of them.
The temperature dependence of magnetization measurements shows a magnetic ordering around 6 K. From the Curie-Weiss fit, the calculated Curie temperature and m_eff are - 82 K and 6.52 m_B, respectively. The Curie temperature and field variation of magnetization around 2 K, indicates there exist an antiferromagnetic ordering of Nd and Ru. The temperature dependence of real part of dielectric constant (ε') exhibits similar behaviour that of CaCu₃Ti₄O₁₂, reaching a high value as 133 (around 300 K); with decreasing temperature, ε' display a step-like decrease to a lower value of about 33. The drop-off (step-like decrease) temperature shifts to higher temperature with increasing frequency, indicating thermally activated behaviour. The peak position in the frequency variation of Z'' and M'', corresponds to the condition omega;_mtau;_m=1, are calculated using Gaussian function and the calculated activation energy using omega;_m= omega;₀exp(-E_a/K_BT) is ~ 0.20 eV indicating that the relaxation mechanism mainly originates from hopping of polarons. The high value of ε' near room temperature is mainly due to Maxwell-Wagner mechanism.

Now a day considerable research interest in multiferroic composite materials has been growing due to number of applications in sensing, transducers, actuators, and memory elements. Multiferroic [Pb[(Zr_.52Ti_.48)_.60(Fe_.67W_.33)_.40]O₃]_0.80-[CoFe₂O₄]_0.20 (PZTFW-CFO) was synthesis by solid state reaction route, both the phases coexist separately without any impurity phases, interestingly addition of CFO purify the small trace of impurity present in PZTFW matrix. X-ray diffraction (XRD) patterns suggest polycrystalline nature of both phases and have tetragonal crystal structure of PZTFW (with space group P4mm) and cubic spinel of CFO (with space group Fd3m). It also revealed inbuilt compressive strain, small shift in dielectric maximum temperature, and two activation energy regions, respectively, in PZTFW-CFO composite compare to PZTFW ceramic. The PZTFW-CFO composite shows flat dielectric behavior and high dielectric constant near FPTT at high frequency may be useful for tunable dielectric capacitors. Polarization-Electric field graphs of PZTFW-CFO illustrated well saturated hysteresis confirming the ferroelectric nature. Magnetic susceptibility data indicates three successive magnetic phase transitions near 21 K, 172 K and 340 K which may relate ferromagnetic to antiferromagnetic, antiferromagnetic to superparamagnetic, superparamagnetic to paramagnetic phase transitions of PZTFW in PZTFW-CFO matrix. External inhomogeneous stress developed more magnetization and exhibited visible magnetic phase transitions due to flexomagnetic effect.
Magnetic hysteresis at 10 K and 350 K shows the presence of typical square loops, indicating ferromagnetic and ferromagnetic nature of the sample, respectively, the hysteresis at 350 K may due to presence of CFO which has high magnetic phase transition temperature. Irreversibility in the zero-field-cooled (ZFC) and field cooled (FC) magnetization vs. temperature (M vs. T) measurements indicate discontinuity (phase transition) in the M vs. T plots of PZTFW-CFO. The effect of grains (bulk), grain boundaries and electrode-electrolyte interface on dielectric and electrical properties of electro-ceramics have been discussed using impedance spectroscopy technique. Nyquist plot and modulus formalism show a poly-dispersive nature of relaxation, relate the activation energy (E_a) of oxygen vacancies, mainly responsible for the bulk capacitive conduction. Modulus spectra displayed a spiral structure for high temperatures and frequencies may be due to experimental artifacts (cable inductance). Flexomagnetic effect for this composite will be discussed.

We present the experimental demonstration of a thin film “thermal switch” in which the thermal conductivity of the film is reversibly controlled via the application of an external electric field. The switch operates on the principle of modulating the distribution of ferroelastic nanodomains in the top half of a bilayer PZT thin film, which affects the rate of phonon scattering at domain boundaries within the film. The bilayer film (30/70 tetragonal PZT top, 70/30 rhombohedral PZT bottom) results in a domain structure within the top layer that is not clamped by the underlying layers, thereby enabling tunability via an applied electric field. The domain structure within the top layer was characterized via Piezoresponse Force Microscopy (PFM) and Transmission Electron Microscopy (TEM), and the cross-plane thermal conductivity of several devices was measured via time-domain thermoreflectance (TDTR) while under applied electric fields of up to ~500kV/cm. We find that the thermal conductivity of the film can be tuned by up to ~12.5% and is shown to decrease in magnitude with increasing applied electric field. We attribute the decrease in thermal conductivity to an increase in the domain wall density that occurs when electric fields are applied across the film, which is corroborated by our PFM images. This work introduces several new ideas for future research frontiers, including electrothermal devices and thermal management technologies at the nano-scale.

Careful growth of LaAlO₃ thin films on SrTiO₃ by molecular beam epitaxy has shown that the La/Al ratio of the film is key to the formation of a two-dimensional electron liquid at the interface--metallic conductivity is only observed in Al-rich films.¹ The interfacial electron liquid forms due to the polar catastrophe, the diverging potential caused by the atomic layer arrangement at the interface when polar LaAlO₃ is grown on TiO_2-terminated non-polar SrTiO₃. The system eventually reconstructs, moving negative charges to the interface to screen the diverging potential. Our density functional calculations showed the defects that form in the film depend on the stoichiometry, and these defects lead to electronic reconstruction in Al-rich films and ionic reconstruction in La-rich films.
Here I extend those calculations to include the LaAlO₃ surface. I show the stoichiometric AlO₂ surface is unstable to phase separation into Al-rich and Al-poor regions. The surface structures control the chemical potentials of the film, determining the defects that form both in the film and at the interface. These defects determine whether a conducting interface forms.
1. Warusawithana, M. P., Richter, C., Mundy, J. A., Roy, P., Ludwig, J., Paetel, S., Heeg, T., Pawlicki, A. A., Kourkoutis, L. F., Zheng, M., Lee, M., Mulcahy, B., Zander, W., Zhu, Y., Schubert, J., Eckstein, J. N., Muller, D. A., Hellberg, C. S., Mannhart, J., and Schlom, D. G. “LaAlO₃ stoichiometry is key to electron liquid formation at LaAlO₃/SrTiO₃ interfaces” Nature Communications 4, 2351 (2013).

Electronic phase separation is present in many complex material systems and may contribute to high T_c superconductivity, multiferroicity, and colossal magnetoresistance. Here, nanometer to micron sized regions of vastly different electronic and magnetic properties coexist and compete within a single crystal structure. We will discuss recent work on fabricating single crystal wires of electronically phase separated manganites to a size comparable to the domains of the electronic phases residing within the material which allows finite mesoscale dynamics to dominate macroscale device characteristics. Specifically, we will present the transport properties of spatially confined [La_1minus;xPr_x]_5/8Ca_3/8MnO₃ (x = 0.3) (LPCMO), and report on the observation of compliance-dependent resistance states. Single-crystalline LPCMO films grown on SrTiO₃ (001) substrates are photolithographically patterned to wires of three different dimensions, i.e. widths of 200 µm, 10 µm and 2 µm. All structures show temperature-dependent resistance behavior typical of phase-separated manganites with a metal-insulator transition slightly decreasing with the wire size. However, while the widest wire essentially exhibits conventional ohmic behavior, the narrower wires, confined to length scales on the order of the inherent phase domains residing wihin, show non-linear hysteretic current-voltage curves and characteristics of memristive behavior. Further, the character of these non-linear responses is shown to be fully controllable with magnetic field.
Supported by the US DOE Office of Basic Energy Sciences, Materials Sciences and Engineering Division and under US DOE grant DE-SC0002136.

The conducting interface between the two band insulators LaAlO₃ and SrTiO₃ has drawn a large share of attention, as it presents a variety of exciting electronic properties that are tunable by an electric field [1]. At low temperatures, magnetotransport analysis has revealed a strong Rashba spin-orbit interaction originating from the breaking of inversion symmetry [2] and, in field effect devices, the ground state has been tuned from an insulating to a superconducting state.
In this presentation I will discuss field effect experiments to probe spin-orbit signatures in the normal and superconducting state of the 2DEG. A comparison with doped bulk SrTiO₃ will also be drawn.
[1] A. D. Caviglia et al., Nature 456, 624 (2008).
[2] A. D. Caviglia et al., Phys. Rev. Lett. 104, 126803 (2010); A. Fecirc;te et al., Phys. Rev. B 86, 201105 (2012).

A combined chemical pressure and substrate biaxial pressure crystal engineering approach was demonstrated for producing highly epitaxial Sm-doped BiMnO₃ (BSMO) films on various substrates. Highly strained BSMO films grown on (110) DyScO₃ with thickness of 20 nm show an enhanced magnetic transition temperature, T_C of ~ 150 K, 50 K higher than for standard BiMnO₃ (BMO) films. Strong room temperature ferroelectricity with piezoresponse amplitude, d₃₃ = 10pm/V and long term retention of polarisation were also observed in 170 nm thick BSMO film on Nb doped SrTiO₃ (Nb-STO). Furthermore, the BSMO films were much easier to grow than pure BMO films, with excellent phase purity over a wide growth window. This study represents a very effective way to independently control strain in-plane and out-of-plane which is important not just for BMO but for controlling properties of many other strongly correlated oxides.

Recently, metallic properties have been observed in thin films of the Mott insulators LaTiO₃ and LaVO₃ grown on SrTiO₃ [1]. While it has been shown that the conductivity in the LaVO₃ films seems to be constrained only to the substrate-film interface, the conductivity in the case of LaTiO₃ persists throughout the whole film and has been ascribed to substrate-induced epitaxial strain [1].
Generally, it is difficult to clearly distinguish between different potential sources for the observed conductivity (such as e.g. strain, oxygen vacancies, interface or surface effects, etc.) based on experimental observations alone. On the other hand, first principles electronic structure calculations allow to address each of these factors individually, and therefore can give valuable insights in the underlying mechanisms.
Here, we combine density functional theory (DFT) and dynamical mean-field theory (DMFT) to address the question of whether substrate-induced epitaxial strain can induce metal-insulator transitions in perovskite materials with d¹ and d² electron configurations, such as e.g. LaTiO₃, SrVO₃, or LaVO₃. In particular, we show that compressive strain indeed induces an insulator-to-metal transition in LaTiO₃ [2], while this is not the case in LaVO₃, consistent with experimental observations.
We compare cases with and without octahedral rotations (GdFeO₃-type distortion), and we show that the metal-insulator transition is controlled mostly by the crystal-field splitting within the t_2g orbital manifold, which in turn is due to the strain-induced changes in the crystal structure [2].
Our results provide a rigorous reference for the interpretation of experimental observations in thin films containing d¹ and d² perovskite materials, and also represent a sound basis for DFT+DMFT studies of interfacial effects in oxide hetero-structures.
[1] F. J. Wong et al., Phys. Rev. B 81, 161101 (2010); C. He at al., Phys. Rev. B 86, 081401 (2012).
[2] K. Dymkowski and C. Ederer, Phys. Rev. B 89, 161109 (2014).

Ferroelectricity in ABO₃ perovskites and related compounds has been a topic of intensive research over the last 60 years. Recently, the coupling of the ferroelectric lattice mode with other non-polar modes has attracted an increasing interest since it offers promising and still widely unexplored possibilities to couple ferroelectricity with other functional properties and even to produce unusual phenomena. In this context, the trilinear coupling between ferroelectric and oxygen rotational modes in naturally-occuring and artificially layered perovskites emerged as a practical way to produce unusual dielectric properties or to achieve enhanced magneto-electric coupling. Focusing on half-doped titanates, we will discuss here how even more interesting phenomena can appear when additional orbital and charge orders enter into play.

Materials exhibiting strong electron correlations are in the focus of research due to their rich electronic phase diagram emerging from various competing ground states. In particular electronic phase transformation, where the electronic properties undergo an abrupt change and carrier characteristics changes from predominantly itinerant behavior to localization at the lattice sites is of particular interest and not well understood. The case of strongly correlated itinerant electron systems in the vicinity of an MIT transition is of particular interest for thermoelectric application¹, however experimental transport data on high quality material beyond resistivity measurements are scarce.
In this talk we will discuss the transport anomalies of the correlated metal SrVO₃. High quality SrVO₃ thin film with varying thicknesses, from several unit cells (u.c.) to 45 nm were grown by hybrid molecular beam epitaxy (hMBE) on ((LaAlO₃)_0.3(Sr₂AlTaO₆)_0.7) LSAT (001) substrates². The excellent quality of the films were confirmed by measuring the residual resistivity ratio (RRR). A large positive magnetoresistance, exceeding 100% at 2K and 2.5T has been observed for the 45 nm films accompanied by an anomalous Hall effect, which disappeared for film thicknesses below 15 nm. Both transport effects will be explained using a two band model involving carriers with two different characteristics. The disappearance of the anomalous Hall effect for thinner films can be consistently explained by lifting the overlap of the current carrying bands through quantum confinement.
This work was supported by Office of Naval Research through Grant No. N00014-11-1-0665.
References
(1) Haule, K.; Kotliar, G., Thermoelectrics Near the Mott Localization—Delocalization Transition. In Properties and Applications of Thermoelectric Materials, Springer: 2009; pp 119-131.
(2) Moyer, J. A.; Eaton, C.; Engel#8208;Herbert, R., Highly Conductive SrVO3 as a Bottom Electrode for Functional Perovskite Oxides. Advanced Materials 2013, 25, (26), 3578-3582.

The RtiO₃ and RVO₃ orthorhombic perovskites (R = rare earth or Y) exhibit complex phase diagrams. The complexity reflects intrinsic distortions of the octahedral sites that change the orbital order with the size of the R³⁺ ion. In the RTiO₃ family, long-range magnetic order is suppressed by orbital disorder and magnetic order is completely suppressed at the crossover from anti-ferromagnetic order for larger R³⁺ to ferromagnetic order for smaller R³⁺. Nevertheless, ferromagnetic YTiO₃ exhibits a spin only paramagnetic Curie-Weiss law with, as is normal, a Weiss constant slightly above T_C, indicating the same spin-spin interactions in the orbitally disordered paramagnetic phase as in the orbitally ordered ferromagnetic phase. YVO₃ retains a glassy thermal conductivity even below a T_N, but not below a change of magnetic order below a T_CG < T_N. This magnetic behavior is rationalized by postulating an (001)-plane stacking wave propagating along the [001] axis to give a dynamic orbital order that determines the sign of the interatomic spin-spin interactions.

Emergent phenomena at complex oxide heterointerfaces suggest new possibilities for next generation electronics. In particular, the coupling between charge and spin degree of freedoms across the interfaces provides a new avenue for achieving electric field control of magnetism. BiFeO₃/La_0.7Sr_0.3MnO₃ heterointerface, which is one of the model systems, attracts much attention recently due to the electrically controllable exchange bias. Here we present a systematic study of magnetoelectric coupling across two different atomically engineered interfaces. Due to the advances in atomic-scale synthesis, two asymmetric polar interfaces (BiO-MnO2 and La/SrO-FeO2) have been successfully made. We found that the ferroelectric polarization controls reversibly the magnetization, exchange-bias and coercive fields of La_0.7Sr_0.3MnO₃ layer. More importantly, the electric control phenomena are dramatically different across the two kinds of interfaces. These results suggest a new strategy that should be considered in designing the magnetoelectric coupling heterostructures. It also clarifies some of the inconsistencies in recent studies of BiFeO₃/La_0.7Sr_0.3MnO₃ heterointerface.

Materials that exhibit simultaneous order in both their electric and magnetic ground states hold tremendous promise for use in next-generation memory devices. Full exploitation of these properties demands a material that has both a coherent orientation of the electronic dipoles—a ferroelectric—as well as alignment of the spin states in a ferromagnetic state. Current magnetoelectric multiferroics are limited by weak or anti-ferromagnetic alignments and/or properties that only emerge well below room-temperature, stymieing device applications. Here we use precise atomic layer-by-layer control to engineer a new strong ferrimagnet-ferroelectric with the highest-known simultaneous transition temperatures. These (LuFeO₃)_n(LuFe₂O₄)₁superlattices are constructed through integration of the ferroelectric, yet antiferromagnetic, LuFeO₃ and paraelectric, ferrimagnetic LuFe₂O₄. Notably the ferroelectricity of the LuFeO₃ layers, manifest by a buckling of the lutetium-oxygen planes as observed in high-resolution electron microscopy imaging, propagates throughout the structure at room-temperature for large n. This drives the nominally paraelectric LuFe₂Oshy;₄ layers into a ferroelectric ground-state. Moreover, the ferrimagnetism of the LuFe₂O_{4 shy;}is not only maintained with the integration of the LuFeO₃ layers yet enhanced beyond the transition temperature of either end-member to T_N= 260 K for the (LuFeO₃)₃(LuFe₂O₄)₁ compound. Extending and enhancing these properties through precise interfacial engineering provides a new framework for the construction of room-temperature multiferroics.

There is an increasing interest in multiferroics showing ferroelectricity coupled to magnetism. Multiferroic behavior is often found in frustrated magnets. I will discuss several materials where competing magnetic orders, soft magnon modes and large spin fluctuations, resulting from magnetic frustration, lead to an enhanced magnetoelectric response. One example is the hexagonal ErMnO₃, in which the spin re-orientation transition gives rise to a sharp peak in the magnetoelectric susceptibility near the critical point (1). Another example is the orthorhombic DyMn₂O₅ showing the giant magnetocapacitance effect at the transition to a multiferroic state. The strong dependence of the dielectric constant on an applied magnetic field is related to a soft electromagnon mode (2). Finally, I will report on our recent theoretical studies of frustrated triangular magnets showing a plethora of modulated magnetic phases, including the skyrmion crystal state, all of which carry electric polarization. The soft collective magnetic modes appearing in some of these phases give rise to large magnetoelectric susceptibilities. These results can be important for manipulation of skyrmions in magnetic insulators with an applied electric field and magnon currents [3].
References:
[1] Y. Geng et al., Nature Materials 13, 163 (2014).
[2] A. B. Sushkov et al., arXiv: 1403.3717.
[3] M. Mochizuki et al., Nature Materials 13, 241 (2014).

Controlling magnetism by electric fields is a key issue for the future development of low-power spintronics. Progress has been made in the electrical control of magnetic anisotropy, domain structure, spin polarization or critical temperatures. However, the ability to turn on and off robust ferromagnetism at room temperature and above has been remained elusive until lately. Very recently it has been found that in FeRh/BaTiO₃ a moderate electric field can produce a giant magnetization variation [1], arising from the electric-field induced transformation from an antiferromagnetic state to a ferromagnetic FeRh state. The effect occurs just above room temperature and it is mostly driven by voltage-induced strain effects from BaTiO3 but charge accumulation and depletion effects related to the ferroelectric polarization of BaTiO₃ possibly contribute as well. Here we use x-ray magnetic circular dichroism (XMCD) contrast in photoemission electron microscopy (PEEM) to show that sub-micron-sized ferromagnetic (F) regions are created in FeRh and coexist with the antiferromagnetic (AF) phase. Detailed ab-initio calculations show that the relative stability of the F and AF phases is tuned primarily by the changes in the pseudocubic lattice parameter of strained FeRh, despite the changes in bond angles that arise due to monoclinic distortions. Our results suggest new routes to engineer giant magnetoelectric effects by harnessing magnetic phase transitions in transition metal alloys.
[1] R. O. Cherifi et al, Nature Materials 13, 345-351 (2014).

The insertion of thin antiferromagnetic (AFM) films allowed spin-valves to exhibit memory. In spin-valves, two stable and switchable resistance states persist after the removal of external magnetic fields, owing to the presence of an AFM layer that pins the magnetic state. We show that, without the deliberate introduction of such an AFM layer, this functionality is transferred to Multiferroic Tunnel Junctions (MFTJ) based on Co/PbZr0.2Ti0.8O3 (PZT)/La0.7Sr0.3MnO3 (LSMO), allowing us to create a four-state resistive memory device. We observe that the upper ferromagnetic/ferroelectric interface (in the proximity of the Co electrode) is at the origin of the pinning of the Ti magnetic moment, similar to spin-valve architecture, which ultimately leads to four robust resistance states in the junction, of which two are electrically and two magnetically accessible.
Exhaustive transport, magnetic, and temperature-dependent characterization, and interface chemistry modification, univocally show the key role of the last pinned Ti layer in the observed effect. Synchrotron experiments show a clear presence of an induced magnetic momentum of Ti at the Co/PZT interface. This coupling would explain the observed magnetic anisotropy and the change of the spin polarization sign in similar junctions (Pantel. et al, Nat. Mat., 2012).

Flexoelectricity is electro-mechanical coupling, which generates an electric field by using a strain gradient. Although it was first reported as early as in the 1960s [1], there have been few studies on flexoelectricity in bulk solid materials. It is partly because the flexoelectric coefficients are quite small and partly because the amount of strain gradient should be quite small in the rigid bulk solids. In addition, it is quite difficult to control the strain gradient. Recently, we demonstrated that the strain gradient in epitaxial oxide thin films could be 6 or 7 orders of magnitude larger than the corresponding bulk values [2]. This important finding indicates that we could use the flexoelectric effects by controlling strain gradient.
In this talk, I will introduce our recent activities on flexoelectric effects of ferroelectric epitaxial thin films. Most epitaxial films are grown on substrates, which have lattice constants different from the films. Such large lattice mismatch will result in strain relaxation as the films become thicker. The strain relation, typically within tens of nanometers of the #64257;lm-substrate interface in epitaxial #64257;lms, will induce a very large strain gradient. By controlling growth conditions such as oxygen partial pressure or deposition temperature, we can obtain a series of samples with different strain gradients. Using ferroelectric films, such as HoMnO₃ and BiFeO₃, we could systematically investigate flexoelectric effects on their functional properties, such as polarization-electric field hysteresis curves [2], domain con#64257;gurations [2], local electronic transport properties [3], self-polarization direction [4], and defect formation [5]. I would like to show that flexoelectric effects should be properly included to control various physical properties of novel nanoelectronic devices.
References
[1] S. M. Kogan, Sov. Phys. Solid State 5, 2069 (1964).
[2] D. Lee et al., Phys. Rev. Lett. 107, 057602 (2011).
[3] D. Lee et al., Nano Lett. 12 6436 (2012).
[4] B. C. Jeon et al., Advanced Materials 25, 5643 (2013).
[5] D. Lee et al., Advanced Materials (2014).

Flexoelectricity describes the electric polarization that is linearly induced by a strain gradient, and is
being intensely investigated as a tantalizing new route to converting mechanical stimulation into
electrical signals and vice versa. [1] While several breakthough experiments have been reported in the past
few years, progress on the theoretical front has been comparatively slow, especially in the context of
first-principles electronic-structure theory.
The main difficulty with calculating the flexoelectric response of a material is the inherent breakdown
of translational periodicity that a strain gradient entails, which at first sight questions the very
applicability of traditional plane-wave pseudopotential methods.
In this talk I will show how these obstacles can be overcome by combining density-functional
perturbation theory with generalized coordinate transformations [2,3], gaining access to the full
microscopic response (in terms of electronic charge density, polarization and atomic displacements)
of a crystal or nanostructure to an arbitrary deformation field. (Earlier theories of the uniform
strain response are thus recovered as special cases.) As a practical demonstration, I will
present results on the full flexoelectric response of SrTiO₃, including atomic relaxations and
surface effects. [4] I will show that, upon bending a SrTiO₃ slab, one obtains a positive voltage if the
crystal lattice is terminated by a TiO₂ layer, a negative voltage if the termination is of the SrO type.
This result has profound implications, both for the interpretation of the experiments and for the
optimization of electromechanical devices based on the flexoelectric effect.
[1] P. Zubko, G. Catalan, and A. K. Tagantsev, Annu. Rev. Mater. Res. 43, 387-421 (2013).
[2] M. Stengel, Phys. Rev. B 88, 174106 (2013).
[3] M. Stengel, Nature Communications 4, 2693 (2013).
[4] M. Stengel, arXiv:1402.2121 (2014).

A primary objective in computational materials science is to identify promising compounds, including structure-types and compositions, for future synthesis with targeted properties out of an immense chemical-structural phase space. The launch of the Materials Genome Initiative (MGI) has especially reinvigorated the search for new routes to accelerate the discovery of advanced materials for rapid deployment—the aim being to evolve a “hunter and gatherer” discovery paradigm into the cultivation of materials by design. Within this setting, and motivated by the dearth of functional ferroelectric phases in layered A₂BO₄ (214) Ruddlesden-Popper (RP) oxides, I describe an ensemble computation materials protocol (ECMP) with predictive capability to design inorganic oxides without inversion symmetry by leveraging anharmonic lattice mode interactions at the atomic scale. The approach is based on solving an inverse Landau problem to identify plausible structures and uncover the underlying crystallographic symmetry restrictions that enforce inversion in this materials class, and hence limiting ferroelectric functionality. With these symmetry guidelines, we then formulate a data-driven model founded on Bayesian inference that allows us to rationally select combinations of A- and B-site elements satisfying the inversion symmetry lifting criterion, the stability of down-selected compositions are then explored with electronic structure calculations based on density functional theory [1]. This general methodology is applied to 214 manganates and iridates with A²⁺ cations, and we identify RP-structured LaSrMnO₄ and Ca₂IrO₄ as a potential multiferroic and acentric iridate, respectively. Working with experimental colleagues, I describe the validation of the ECMP in alkali metal RP titanates [2]. I conclude by suggesting more broadly how the approach of physical properties by atomistic structure designs is immediately amenable to other electronic functionalities.
This work was performed in collaboration with Drs. Prasanna Balachandran, Antonio Cammarata, and Danilo Puggioni, supported by DARPA (N66001-12-1-4224), ONR (N00014-11-1-0664), and ARO (W911NF-12-1-0133).
References:
[1] Crystal-chemistry guidelines for noncentrosymmetric A₂BO₄ Ruddlesden-Popper oxides, P.V. Balachandran, D. Puggioni, and J.M. Rondinelli, Inorg. Chem., 53 336 (2014).
[2] Inversion symmetry breaking by oxygen octahedral rotations in the Ruddlesden-Popper NaRTiO₄ family, H. Akamatsu, K. Fujita, T. Kuge, A.-S. Gupta, A. Togo, S. Lei, F. Xue, G. Stone, J.M. Rondinelli, L.-Q. Chen, I. Tanaka, V. Gopalan, and K. Tanaka, Phys. Rev. Lett., 112 187602 (2014).

The successful exploitation and deployment of complex materials in technological applications relies on the accessibility of their properties in realistic environments. In this regard, VO₂ is a very attractive functional oxide due to the proximity of the remarkable bulk metal-insulator transition (MIT) to ambient temperatures and pressures. In the bulk material, the MIT of VO₂ is well characterized as a cooperative atomic and electronic transition, consisting of a large structural distortion between monoclinic insulating and tetragonal metallic phases, concomitant with the electronic (conductivity) transition. However, recent experimental reports at high pressure and excited out of equilibrium suggest an unusual monoclinic metallic phase can be stabilized, providing new perspectives on the physical nature of the MIT, as well as having important consequences for material application.
Using electron and photoelectron microscopies and spectroscopies simultaneously, we directly show that this novel monoclinic metallic phase is easily accessible at ambient temperatures and pressures in strained VO₂ thin films. In particular, these measurements reveal the evolution of the structural and electronic transitions over separate temperature scales. Indeed, this separation is even observed in the same photoelectron measurements when the O 2p and V 3d states are analyzed independently, which correspond to the local bonding structure and metallic electronic structure respectively.
Since the techniques employed here (LEEM, LEED, PES and XPEEM) are sensitive to very similar volumes of the sample and are recorded simultaneously, we eliminate ambiguity due to variations and hysteresis in successive heating/cooling cycles, providing directly comparable snapshots of the evolution of the MIT in both structural and electronic channels.
Our results provide conclusive evidence that the new monoclinic metallic phase is accessible in the thermodynamic transition at ambient pressure. Based on our observations, we discuss the likely mechanism behind this important phase, as well as its implications on the nature of the MIT in VO₂.
Work supported by the Department of Energy under Grant No. DE-FG02-98ER45680.

The importance of domains in determining the properties of ferroics in general and ferroelectrics in particular cannot be overstated: the effective function of FeRAM memory, ferroelectric piezoelectrics, electroresistive memristors and positive temperature coefficient of resistance (PTCR) ceramics, for example, all rely on the existence and growth of domains. Awareness of the great potential of domain walls, as perhaps the most exciting and versatile 2D functional materials known to date, is also developing rapidly [1, 2]. Making the most of the advantageous properties of ferroelectrics often means mastering domain and domain wall behaviour. This is particularly true in nanoscale structures, where domain densities are much higher than in bulk. In this talk, the way in which domain states may be controlled in ferroelectric and multiferroic nanostructures will be described: how shape can be used to generate self-ordering through surface depolarizing fields [3]; how electron beams and associated fields can affect domain states [4]; how electrode and interelectrode patterning can generate local field hot-spots to control both the location and bias levels at which domains nucleate [5]; how thermal cycling and stress can control the formation of naturally charged domain walls which act as distinct planar conductors in improper ferroelectrics. The grand challenge of making domain wall based devices by controlling both domain configurations and domain wall properties will also be discussed. [1] J. Seidel et al., Nat. Mat. 8, 229 (2009) [2] G. Catalan et al., Rev. Mod. Phys. 84, 119 (2012) [3] A. Schilling et al. Nano Letters 7, 3787 (2007); R. G. P. McQuaid et al., Nat. Comm., 2, 404 (2011); L-W. Chang et al. Nano Letters, 13, 2553 (2013) [4] R. Ahluwalia, et al. Phys. Rev. Lett., 111 165702 (2013) [5] J. R. Whyte et al., Adv Mat, 26, 293 (2014)

Research into ferroic materials experienced a shift of focus lately with domain walls being considered as entities in their own right following results that showed enhanced electrical conduction in their vicinity [1] and further findings of numerous unexpected properties [2-4]. As domain walls can be created, displaced and annihilated under applied electric fields, much excitement comes from the possibility that they could provide changeable, compliant pathways that can be modified during device operation; a key element in the emerging field of adaptive electronics. While the investigations of properties of individual domain walls is intensifying, the ‘next essential step&’, namely control of their movement, one of their most highly prized features, is entirely missing. In the present work we demonstrate an ability to nucleate, move and control domain walls, over long distances in PbZr_0.1Ti_0.9O₃ thin films. Domains can be nucleated locally on the nanoscale and the associated domain walls can be subsequently propagated on the scale of several microns without the need for moving parts such as a PFM probe tip. This domain wall propagation can be easily controlled via selection of the appropriate voltage pulse duration and magnitude. In fact these domain walls can be positioned, reset and re-positioned many times with a high degree of precision while the manipulation of several domain walls at once is demonstrated, all from simple appropriate biasing of the top electrode within the thin film architecture. Instrumental in this implementation of domain wall control is the unique properties of the thin Pt top electrode (~10 nm) deposited by electron-beam induced breakdown of a precursor gas. Additionally we are able to accurately describe the system by analogy to the classical Stefan problem [5] which has previously been used to describe many diverse systems but is here applied to electric circuits for the first time.
[1] Seidel, J. et al. Conduction at domain walls in oxide multiferroics. Nat. Mater.8, 229 (2009).
[2] Yang, S. Y. et al. Above-bandgap voltages from ferroelectric photovoltaic devices. Nat. Nanotechnol.5, 143 (2010).
[4] Meier, D. et al. Anisotropic conductance at improper ferroelectric domain walls. Nat. Mater.11, 284 (2012).
[3] Sluka, T., Tagantsev, A. K., Bednyakov, P. & Setter, N. Free-electron gas at charged domain walls in insulating BaTiO₃. Nat. Commun.4, 1808 (2013).
[4] Wei X. -K. et al. Ferroelectric translational antiphase boundaries in non-polar materials. Nat Commun.5, 3031 (2014).
[5] Gupta, S. C. The classical Stefan problem: basic concepts, modelling and analysis. Elsevier, Amsterdam (2003)

Domain walls in ferroelectric materials can exhibit very different properties from the bulk, enabling the creation of mobile functional interfaces within these materials. If indeed domain walls are to become functional elements, their pattern has to be controlled minutely. The first step toward this goal is the ability to control the creation of domain structures in as-grown films in a predefined pattern. The confinements given by the epitaxial relationship of the film with the substrate might be conducive for such control: Depending on the orientation of the substrate, only certain domain variants are allowed. The degree of control can be further increased by avoiding elastic relaxation of the film, which additionally allows controlling the sign and degree of misfit strain while at the same time keeping the defect density at minimum.
Here we demonstrate how to control the domain structure of coherently grown tetragonal ferroelectric thin films in the as-grown state and self-assembled patterns induced by local switching. The key factor is the reduction of possible domain variants due to the epitaxial strain imposed by the (110) oriented SrTiO₃ substrates. Depending on the tetragonality of the ferroelectric, different domain states can be realized. In case of a strong tetragonal distortion, two domain states differing only by the in-plane phase form in as-grown films. The polarization can be switched by 180° using piezoresponse force microscopy. However, if the applied voltage is too low, a transient domain pattern of very dense 90° ferroelastic domain walls develops, which relaxes towards extended areas separated by 180° domain walls. In contrast, if the tetragonal distortion is weak, a completely homogeneous single-domain state is stabilized in the as-grown samples, which can be easily switched without the observation of any intermediate state. Interestingly, a further domain is created by this process, appearing outside the switched area. Similar to the as-grown walls observed in the strongly tetragonal material, this wall is characterized by its in-plane phase change.
Both the structural details and crystallographic origin of these findings and the properties of these domain walls will be presented during the talk

The observation of anomalous electronic transport at ferroelectric domain walls and its significance for nano-electronics triggered tremendous scientific interest. To date, the transport behavior and potential barriers at domain walls have been scrutinized by scanning probes. This, however, convolutes the intrinsic electronic properties with contact resistance and inhomogeneous probe fields, so that the detailed origin of the behavior remains to be determined.
Here, we report on the capability of high-resolution X-PEEM to image and characterize ferroelectric domain walls contact-free and with nanometer resolution. Using ferroelectric ErMnO₃ as a test system, we visualize ferroelectric domain walls by exploiting photo-induced charging effects and generate electronic conduction maps by analyzing the kinetic energy of photoelectrons. With this we open a pathway for non-destructive and element-specific studies of electronic and chemical domain-wall structures bypassing previous experimental limitations and significantly expanding the accessible parameter space.

Twin walls (TWs) in ferroelastic materials can be regarded as a distinct phase bridging two orientation states of the same crystal. Since a twin plane represents a mirror plane lost in the ferroelastic transition, the symmetry of the TW is expected to be enhanced relative to the adjacent domains at least by the action of that plane, thus approaching that of the prototype phase. In agreement with this view, TWs often exhibit distinct physical properties [1], particularly in strongly correlated oxides in which even slight perturbations may dramatically modify the subtle balance among various degrees of freedom. Here we report on the observation of enhanced conductivity and enhanced magnetization at TWs in La_2/3Sr_1/3MnO₃ thin films grown on SrTiO₃ substrates by conductive scanning force microscopy and magnetic force microscopy, respectively, and scrutinize its microscopic origin by electron microscopy. Moreover, scanning force microscopy reveals that the hardness is also enhanced at the DWs. The films, grown by magnetron sputtering, develop a perfectly arranged pattern of TWs aligned with the (100) and (010) planes normal to the substrate [2,3]. Z-contrast imaging and electron energy loss spectroscopy do not reveal neither any chemical or electronic structure variation. According to the double exchange model, these results strongly suggest that the observed behavior results from the reduction or complete suppression of octahedral tilts at the TWs, thus supporting the idea that TWs tend to preserve the physical properties of the prototypical phase [1].
[1] G. Catalan, J. Seidel, R. Ramesh, F. F. Scott, Rev. Mod. Phys. 84, 119 (2012); and references therein.
[2] J. Santiso, Ll. Balcells, Z. Konstantinovic, J. Roqueta, P. Ferrer, A.Pomar, B.Martínez, and F.Sandiumenge, CrystEngComm 15, 3908-3918 (2013).
[3] F. Sandiumenge, J. Santiso, Ll.Balcells, Z.Konstantinovic, J.Roqueta, A Pomar, J. P. Espinoacute;s, and B. Martínez, Phys. Rev. Lett., 110, 107206 (2013).

PbZr_1-xTi_xO₃ (PZT) is one of the most important and widely used piezoelectric materials. The study of its local and average structures is of fundamental importance in understanding the origin of its high-performance piezoelectricity. Neutron Pair Distribution Function analysis and Rietveld refinement have been carried out to study both the short- and long-range order in the Zr-rich rhombohedral region of the PZT phase diagram. The nature of the monoclinic phase across the Zr-rich and morphotropic phase boundary area of PZT has been finally clarified, and shows a transition from one type of monoclinic structure to another. In addition diffuse scattering studies of PZT crystals and pure single crystal PbZrO3 have been carried out and show the nature of the disorder in the high-temperature cubic phase.

Multiferroic layered structures of the ferroelectric (FE) perovskite Pb(Zr_0.52Ti_0.48)O₃ (PZT) with the ferromagnetic (FM) oxide La_0.7Sr_0.3MnO₃ (LSMO) have attracted considerable attention due to their potential application in magneto-electric (ME) memories. Incorporation of the hard-magnetic CoFe₂O₄ (CFO) in PZT/LSMO heterostructures could potentially enhance their intrinsically soft FM properties, and consequently the ME device performance. However, it introduces dielectric losses which degrade the FE polarization of the PZT layer, thus, limiting the long-sought mutual control of ferroelectricity and magnetism in these multiferroic composites. Here, we report the first evidence of simultaneous enhancements of polarization and magnetization in PZT/CFO/LSMO enabled by the ultrathin CFO sandwich-layers. Epitaxial PZT/LSMO and PZT/CFO(10-20nm)/LSMO heterostructures with 10-20 nm thick CFO sandwich-layers were grown on SrTiO₃ (STO) (100) substrates using pulsed laser deposition. Structural characterization using XRD, AFM, and TEM revealed their single-crystalline nature and epitaxial relationship between the layers, smooth surface morphologies and defect-free interfaces, respectively. Higher magnetic saturation and coercivity (M_sat = 536 emu/cm³, H_c = 1.2 kOe) were observed in PZT/CFO(10-20nm)/LSMO as compared to PZT/LSMO (M_sat = 395 emu/cm^₃, H_c = 0.1 kOe). This was attributed to the tetragonal distortion of the CFO lattice under compressive epitaxial strain as revealed from the XRD strain analysis. Crystal distortion of the interfacial CFO layer induced a reorientation of the magnetic anisotropy from the in-plane easy axis in PZT/LSMO to the out-of-plane in PZT/CFO(10-20nm)/LSMO as evidenced from the in- and out-of-plane magnetization and transverse susceptibility measurements. Surprisingly, FE measurements showed enhanced remanent polarization (P_r = 67-78 mu;C/cm²) in PZT/CFO(10-20nm)/LSMO as compared to that in PZT/LSMO (P_r = 52 mu;C/cm²). Leakage and charging current characteristics showed the occurrence of charge-injection from the LSMO electrodes into the dielectric CFO layer during voltage cycling. The entrapped charge at the CFO/PZT interface induced an internal built-in field in the heterostructures which caused the accumulation of higher switched charges during voltage cycling. This probably resulted in the enhanced polarization in PZT/CFO(10-20nm)/LSMO as compared to PZT/LSMO. The concurrent FM and FE enhancements in PZT/CFO/LSMO multiferroic thin films reported here provide a fundamental understanding of the structure-property relationships in these heterostructures; necessary for the coherent design of future ME memory devices based on these materials.

Sr_0.67Ba_0.33Nb₂O₆ (SBN) belongs to the tetragonal tungsten bronze (TTB) family of uniaxial ferroelectric relaxors, with a single component polarization vector pointing along the tetragonal c axis. High response coefficients and an enhanced width of the high response regime around the "ordering temperature", Tm, make relaxors popular systems for applications as piezoelectric/electrostrictive actuators and sensors [1].
The idealized structure of SBN, with the generalized formula A₂B₄C₄Nb₂Nb₈O₃₀ [2], has three types of structural channels along the c axis, formed by two types of crystallographically independent NbO₆ octahedra. Disorder due to presence of voids in A cation sites (occupied by Sr atoms) and random distribution of Ba and Sr in B cation sites are thought to be responsible for the relaxor behavior, via the formation of random electric fields and, subsequently, polar nanodomains oriented in the only easy polarization axis c, without a structural change occurring in the phase transition. In this work we present structural and chemical studies of a SBN single crystal by means of Selected Area Electron Diffraction (SAED) and High Angular Annular Dark Field (HAADF) along the [001] and [100] zone axes.
HAADF imaging in the [100] zone axis directly showed the presence of randomly distributed vacancies in the A cation position. SAED diffraction patterns showed the main reflections expected for the TTB structure and also additional superstructure spots due to a modulation of the structure. This modulation was found to be incommensurate, since the additional spots did not show equal separation in the superstructure planes. The presence of streaks along the [010] direction is also consistent with the expected lattice strains due to the presence of A site vacancies.
HAADF imaging along the [001] zone axis yielded structural information consistent with this modulation. Elongation of spots corresponding to B site cation columns (projection of displaced atomic positions) is in good agreement with previous works on XRD structure refinement on this system, where the modulation was attributed to displacement of atoms (Sr/Ba) occupying this B site.
In order to gain further insight into this modulation, Annular Bright Field (ABF) imaging experiments are planned. The sensitivity of this imaging technique to contrast arising from lighter atoms should allow obtaining a map of oxygen coordination in the structure along the [001] zone axis, which could be affected by the heavier B site atoms displacements. In order to check the remaining factor expected to give rise to the relaxor behavior, Sr/Ba random distribution in B cation position, EELS chemical mapping is also planned in both [001] and [100] zone axes.
References
[1]The relaxor-enigma - charge disorder and random fields in ferroelectrics, W. Kleeman, Journal of Materials Science, 41, 129-136 (2006)
[2]The modulated structure of Ba0.39Sr0.61Nb2O6, T. Woicke et al., Acta Cryst., B59, 28-35 (2003)

Designing of Curie temperature (T_c) and spontaneous distortion is the critical issues for the practical device applications of ferroelectric materials. This is because T_c determine the upper limit of the device operation temperature, while spontaneous distortion decides the spontaneous polarization. PbTiO₃ is one of the typical tetragonal ferroelectric materials having large tetragonal distortion, i.e. large spontaneous polarization, as well as high Tc. It is widely accepted that the large tetragonal distortion at room temperature related to the high T_c for PbTiO₃-based solid solution. In the present study, we discover the unusual feature of higher Tc with small tetragonal distortion in tensile-strained epitaxial PbTiO₃ films for the first time.
Epitaxial PbTiO₃ films having in-plane tensile-strain that have been hardly investigated were realized by using (100)KTaO₃ substrates. Films with various tensile strains were obtained by changing the film thickness to control the relaxation degree of the strain. T_c increased with increasing in-plane strain as theoretically predicted. In-plane c/a ratio spontaneous distribution along polarization direction, decreased with increasing in-plane strain. Resultantly, specific feature of higher Tc with small tetragonal distortion is realized in tensile-strained epitaxial PbTiO₃ films.

Interfaces between oxides and silicon are of great importance since oxide materials can provide versatile additional functionalities for semiconductive industry and can be exploited in micro-electro-mechanical systems, random-access memories, and other oxide-based nano-electronic devices. However, in numerous cases such integration requires high-quality growth of oxide, which is hindered by its intense reaction with the substrate.
SrTiO₃ (STO) is the most studied complex oxide on silicon (001) and can be fabricated with the highest degree of crystallinity. However, before the growth of STO is initiated, native SiO₂ layer has to be removed from the Si surface since its amorphous structure prevents epitaxial growth of deposited material. The resulting reconstructed Si surface is extremely reactive and has to be passivized in order to avoid reaction with oxygen during the deposition of STO. The passivation is normally performed by the formation of an appropriate buffer layer on a silicon surface that not only is chemically stable in the oxygen rich environment, but is also structurally compatible with silicon and the oxide.
In our study pulsed laser deposition (PLD) was used to prepare a buffer layer based on ½ ML of strontium (Sr), which has been so far prepared successfully only using molecular beam epitaxy method. We were able to monitor surface reconstruction changes during PLD deposition of Sr with reflection high-energy electron diffraction (RHEED) and control Si surface coverage at the atomic level. Subsequently, epitaxial layer of STO was grown on as-prepared buffer and analyzed using different analytical techniques. Results of our study revealed new PLD-based path towards epitaxial integration of STO and other functional oxides with Si.

Dielectric and ferroelectric properties of epitaxial thin films of perovskite oxides can be modified by synthesizing ferroelectric multilayer materials. The enhancement of dielectric properties in multilayers is a result of electrostatic and elastic interactions between layers with different polarizations and lattice structure parameters. Tail-to-tail and head-to-head polarization configurations of ferroelectric domains should be unstable assuming that electrostatic polarization coupling is strong. However, it has been reported recently that tail-to-tail and head-to-head polarization domains can be stable in BaTiO₃ thick film as a result of the electron density enhancement at the interface between ferroelectric domains.
We studied polarization switching in an epitaxial bilayer of BaTiO₃ and PbZr_0.2Ti_0.8O₃ films. We found that polarization coupling in this bilayer is weak that enables stable head-to-head polarization domain configuration. In order to explain weak electrostatic coupling, we analyzed the electronic structure of the BaTiO₃/PbZr_0.2Ti_0.8O₃ interface taking into an account strong depolarizing fields in coupled ferroelectric layers. We found that a thin layer with a high free charge carrier density can form at the interface between the ferroelectric layers. This free charge layer can screen depolarization fields and stabilize the head-to-head polarization domain configuration.

Optical response of ferroelectric materials is attracting attention nowadays, because switchable photocurrent by ferroelectric polarization and photo voltage above-bandgap is observed in BiFeO_3.[1]. Rare-earth manganites RMnO₃ (R=Ho, Er, Tm, Yb, Lu, or Y), which crystallize in a hexagonal phase with a space group of P63cm, should have an advantage for discussing the effect of polarization on the photo-induced phenomena, because it has uniaxial ferroelectric polarization.
In hexagonal YMnO₃, each Mn³⁺ ion is surrounded by three in-plane and two apical oxygen ions, which leads to the formation of the MnO₅ bipyramid with a symmetry of D3h. The crystal field of D3h splits the Mn 3d orbitals into two doublets, e_1g (xz, yz) and e_2g (xy, x²-y2), and one singlet, a1g (3z²-r²), in order of energy. The MnO₅ bipyramid plays a key role in the occurrence of the multiferroicity. The ferroelectric transition is induced by a structural transition accompanied with buckling of the MnO₅ bipyramids and displacements of Y ions in YMnO₃. In contrast, the antiferromagnetic transition is caused by geometrically-frustrated spin-spin interactions originating from an in-plane triangular arrangement of Mn³⁺ ions. Thus, probing the Mn³⁺ states is significant to investigate the multiferroicity in hexagonal RMnO₃.We have investigated the temperature dependence of the intra-atomic Mn³⁺ photoluminescence (PL) in a multiferroic YMnO₃ epitaxial film grown by pulsed laser deposition. The prominent finding is that the temperature dependence of the Mn³⁺ PL intensities highly correlates with that of the magnetic spin order originating from the antiferromagnetic transition [2]. In contrast, the decay times of the Mn³⁺ PL bands are independent of temperature; namely, the transition probabilities are not affected by the antiferromagnetic transition [2]. The above results suggest that the electron transfer process to the Mn³⁺ excited state from the conduction band is dominated by the magnetic spin order, that should be the origin of the photo-induced current in YMnO₃ films.
Recently, we successfully observed switchable photo-induced current in YMnO₃ films along the polarization axis. In this paper, we discuss the relationship between electron transfer process to the Mn³⁺ excited state from the conduction band and the photo-induced current, which can be controlled by the switching of the ferroelectric polarization.
[1] S. Y. Yang et al, Nat. Nanotechnol., 451 (2010) 143
[2] M. Nakayama et al., Appl. Phys. Express, 7 (2014) 023002

Recently, reports of spontaneous electronic polarizations arising as a result of multiple lattice distortions in perovskite superlattices and layered Ruddlesden-Popper phases have garnered considerable interest in the rational discovery of new ferroelectric oxides. Ferroelectricity is typically induced in complex oxides by having a d⁰ B-site cation which undergoes polar displacements via the second-order Jahn-Teller effect, whereas “geometric improper” ferroelectricity is usually independent of the chemistry of the cations, making it a promising mechanism by which to design novel ferroelectric materials with more functional cations. Here we describe a computational approach to discover new forms of ferroelectricity in cation ordered perovskites and identify plausible compositions and structures for experimentation.
We first systematically identify the structural chemistry features that must be satisfied in ABO₃ perovskite oxides with atomic scale A- or B-site cation ordering for non-polar metal-oxygen polyhedra to lift inversion symmetry. Through a materials informatics approach, we identified combinations of 500 cation ordering and BO₆ octahedral rotation patterns which result in polar crystal structures. Following the symmetry classification of the resulting space groups, we uncover two BO₆ rotation modalities in A-site ordered double perovskites supporting hybrid improper and improper ferroelectricity: in-phase and out-of-phase rotations, and only out-of-phase rotations, respectively. Density functional theory calculations were then used to validate each modality. First, in nine orthorhombic compounds ((La,Nd)Ga₂O₆, (Sr,Ca)Zr₂O₆, and (Sr,Ca)Hf₂O₆ ordered along [001], [110], and [111]), we find that although layered and rock salt ordered perovskites exhibit spontaneous polarizations (P) ranging from 1 to 6 mu;C/cm², the columnar structures remained centrosymmetric. We then examined nine multiferroic (A,A&’)(Mn,W)O₆ compounds with the same tilt pattern containing both layered A-sites and rock salt ordered B-sites, in addition to a G-type anti-ferromagnetic order. Through substitution of various alkali (Na, K, Rb) and rare earth (La, Nd, Y) cations on the A and A&’ sites, we were able to increase the polarization by 150%. Improper ferroelectricity derived from solely out-of-phase rotations was then explored in three aluminates with rock salt ordered A-site cations. While two were identified to be polar (P asymp; 2 mu;C/cm²), all were found to undergo a transition to a chiral phase accompanied by a large increase in piezoelectric response. From these results it is clear that ordered isovalent A-site substitutions at the atomic scale, combined with octahedral rotations, is a robust strategy to achieve polar oxides from centrosymmetric starting materials.
JY and JR were supported by the Army Research Office (W911NF-12-1-0133).

The rich properties of correlated oxides make this class of materials one of the most important for future technological applications. In correlated electron systems, the interaction between electrons is of the order of, or larger than, the electron kinetic energy, and the concept of a well-defined quasiparticle breaks down. In the last few decades, the discovery of unusual and promising behavior in strongly correlated materials has yielded effects as diverse as high temperature superconductivity, colossal magnetoresistance and multiferroics. Indeed, the functionalisation of strongly correlated materials, either in bulk crystalline form or as artificial layered heterostructures, is fast emerging as one of the most promising avenues for future advanced technologies, and key to unlocking the potential of such designed materials is a firm grasp of how electron correlations evolve at surfaces and interfaces.
Here, we investigate Sr_xCa_1-xVO₃ as a prototypical example of a strongly correlated material, exhibiting both strong Hubbard subbands and appreciable quasiparticle peaks. Using a variety of ultraviolet, soft, and hard x-ray spectroscopies, we present a detailed depth-sensitive study of the evolution in the effects of electron correlations from the sample surface to its bulk. Our results illustrate the intrinsic enhancement of the effects of electron correlations at the surface, which has important implications for the designed properties at the interface of heterostructures. Strong incoherent subbands are found to lie ~ 20% closer in energy to the coherent features in the most surface-sensitive measurements, accompanied by a ~ 10% narrowing in the overall bandwidth. Secondly, we demonstrate that resonant soft x-ray emission spectroscopy is a sensitive probe of correlated electron behavior, capable of providing complementary information to photoemission spectroscopy from a truly bulk perspective.
This work is supported in part by the Department of Energy Basic Energy Sciences under Grant No. DE-FG02-98ER4568.

Recently, above band gap voltage have been reported in BiFeO₃ (BFO) thin films having striped-domain structure with 71^o or 109^o domain walls.¹⁾ The possible mechanism of the abnormal photovoltage is polarization-related charge separation at domain walls, which is quite different from classical semiconductor photovoltaic effect, and allows unlimited photovoltage by energy band gap (E_g). The most recently, the abnormal photovoltage in an unexpected geometry in which electrodes on striped-71^o-domain-structured BFO thin film are fabricated perpendicular to the domain walls has reported.²⁾ It indicates that the abnormal photovoltage is due to bulk photovoltaic effect. As mentioned above, the possible mechanisms of the abnormal photovoltage are photovoltaic effects in domain walls and bulk. In addition, the abnormal photovoltages are observed in BFO on orthorhombic perovskite crystals of DyScO₃ (110) and TbScO₃ (110). In this study, we have demonstrated above band gap open circuit voltage of 26 V at room temperature in single-domain BFO thin films without domain walls on SrTiO₃ (STO) substrate.
A 300-nm-thick BFO thin film was grown on STO (113) substrate by RF planar magnetron sputtering. It has already reported that domain structure of BFO thin film can be controlled by vicinal direction of STO (001) substrate, and we have also reported the BFO thin films on vicinal STO (001) with a vicinal direction of <110> show single-domain structure. Therefore, the BFO thin film on the STO (113) is also expected to show single domain structure, because the STO (113) plane is tilted about 21.00^o from STO (001) plane along <110>. Rectangular-shaped Pt electrodes with width of 150 mu;m were formed on BFO surface by photolithography and lift-off processes. Inter-electrodes distance is 200 mu;m. To examine photovoltaic effect in the BFO thin film I-V measurements with/without illuminating collimated violet laser (lambda; = 405 nm) were performed.
X-ray diffraction reciprocal space mappings revealed the BFO thin film on STO (113) had only one in-plane orientation, because both BFO (113) and (123) diffraction spots are single-spots, respectively. In addition, vertical-piezoelectric-force-microscopy image was uniform contrast. These indicates the BFO thin film had single-domain structure. From I - V characteristic under dark condition, the current was below 1 × 10^-10 A in entire applied voltage region of -60 to +60 V. Under illumination of violet laser with a power density of 380 W/cm², open circuit voltage (V_OC) of 26 V along direction in parallel to in-plane component of Ps vector was observed with conductivity increase due to photo induced carriers. In addition, clear laser polarization dependence on V_OC was also observed. Therefore, the abnormal photovoltage is due to bulk ptotovoltaic effect.
References
S. Y. Yang et al., Nat. Nanotechnol.5, 143 (2010).
A. Bhatnagar et al., Nat. Commun.4, 2835 (2013)

The emergence of 2D electron gas at the interface of polar (LaAlO₃)/ non-polar (SrTiO₃) interfaces has potential for applications. For oxide electronics a few key challenges remain: 1) the oxygen affinity of the polar layer leads to oxygen vacancy induced carriers in the SrTiO₃ (STO) layer and 2) the high reactivity of silicon (when used as a substrate) with the metal atoms in the oxide overlayers, both of which affect device reproducibility and performance. Discovering a polar oxide that does not produce oxygen vacancies in STO and the optimum growth temperature where silicon reactivity can be minimized and yet have the 2D electron gas from polar discontinuity would be essential. The mechanism of two dimensional electron gas at ABO₃/STO interface was studied by the transport properties of crystalline and amorphous interfaces with different polar layers- LaAlO₃, PrAlO₃, NdAlO₃, NdGaO₃ and LaGaO₃. A robust 4 uc metal insulator transition thickness is seen in all crystalline interfaces, however the amorphous aluminate and gallate differ greatly in introducing conductivity at the interface. We show that NdGaO3 introduces only polar discontinuity electrons and none from oxygen vacancies. Also a minimum temperature of 550 0C is required to produce the 2D electrons which is adequate for the growth of such oxide heterostructures on silicon. Moreover, carrier mobility of 2D electron gas degrades for interfaces fabricated at higher temperature, we relate this trend by coherent strain reduces carrier mobility for samples fabricated at higher growth temperature.

Functional perovskite oxides are attractive for next generation electronic materials due to their versatile properties such as high Tc superconductivity, colossal magnetoresistance, giant thermoelectric and magnetocaloric effect, giant ionic conduction, high magnetic, ferroelectric, piezoelectric, and multiferroic behaviors. Nanocomposite oxides with heterogeneous structures can allow combinations of useful properties in one film. These films are made of columnar, epitaxial crystals of two immiscible phases formed by codeposition. A notable example is nanocomposites formed from spinel and perovskite which show both magnetism and ferroelectricity.
In this work we show the formation of perovskite-metal nanocomposites formed by the deposition of films from a target of Sr(Ti_0.78Cu_0.22)O₃ by pulsed laser deposition in vacuum on (001), (011), and (111) oriented SrTiO₃, Nb:SrTiO₃, and on Si coated with an 8 nm thick epitaxial SrTiO₃ film grown by molecular beam epitaxy. The resulting films consisted of a perovskite matrix containing vertical nanorods of Cu with spacings ~(10 ~ 20) nm and diameters ~10 nm, with a square cross-section. The film thickness was up to 100s of nm. Nanocomposite formation is attributed to the limited solubility of the Cu in the provskite matrix. In contrast, films grown in an oxygen atrosphere consisted of weakly magnetic Sr(Ti_1-xCu_x)O_3-d (STCu) perovskite films (D. H. Kim, G. F. Dionne, and C. A. Ross, J. Appl. Phys., 114, 113902 (2013)). When grown epitaxially on CeO₂/YSZ-buffered (001) Si, the oxygen-grown STCu perovskite consisted of a (001) oriented matrix with (110)-oriented crystals that formed to relieve strain.
The metal-perovskite nanocomposite grown in vacuum could further be etched in ammonium hydroxide to remove the Cu and leave a nanoporous perovskite film. High resolution transmission electron microscopy, x-ray diffraction, and x-ray photoelectron spectroscopy were used to investigate the Cu nanorod structure and etching behavior.
This process provides a general method to introduce porosity in a perovskite matrix which could subsequently be back-filled with other materials. The metal-perovskite nanocomposite was also used as a substrate on which BiFeO₃-CoFeO₄ (BFO-CFO) epitaxial perovskite-spinel nanocomposite was grown, producing a bilayer nanocomposite.

AMn₇O₁₂ quadruple perovskites exhibit many interesting phenomena including multiferroicity and charge ordering transitions. In these compounds, the transition metal resides on both the A- and B-site of the perovskite structure yielding the formula (AA&’₃)B₄O₁₂, where both A&’ and B are Mn. Previous reports have studied the bulk synthesis of quadruple manganites, but there remains little known about their thin film counterparts. Herein, we describe the thin film synthesis of two compositions of AMn₇O₁₂ (A= Ca, Sr) quadruple perovskites grown by molecular beam epitaxy on SrLaAlO₄ and YAlO₃ substrates. We use x-ray diffraction, transmission electron microscopy, atomic force microscopy, magnetization, Raman spectroscopy, and temperature dependent resistivity measurements to characterize as-grown films and films that are treated with high temperature post-growth anneals. The structural and electronic results are compared to density functional theory calculations to provide further insights into the physical behavior of the films.
This work is supported by the Office of Naval Research (N00014-11-1-0664).

Perovskite oxides have attained considerable interest in research due to their strong electron correlations and interplay between spin, charge, orbital and lattice degrees of freedom. Heterostructures of these materials can include abrupt interfaces between two compounds with different electronic and structural properties, creating a unique environment where new collective phenomena may occur. In this study, we investigate the electronic properties of digital superlattices which are cation-ordered analogues of the iron-based perovskite La_1/3Sr_2/3FeO₃ (LSFO). LSFO superlattices of LaFeO₃ (LFO), an antiferromagnetic (AFM) insulator, and SrFeO₃ (SFO)_, a paramagnetic conductor, were fabricated via molecular beam epitaxy (MBE). Using a combined experimental and computational approach, three isocompositional superlattices with repeat structures of LSSLSS, LSLSSS, and LLSSSS (L = LFO, S = SFO) were studied and compared to random alloy films. The superlattices were confirmed to have atomically sharp interfaces via synchrotron x-ray diffraction and corresponding simulations of (00L) crystal truncation rods. The superlattices were found to undergo an electronic phase transition similar to the random alloy, indicating that the superlattices do not behave as a simple combination of LFO and SFO. However, the details of the charge ordering phase transitions, as measured by a discontinuity in the temperature dependent resistivity, indicate that the electronic behavior depends on the number of LFO/SFO interfacial layers and non-interfacial SFO layers. The electronic structure was investigated using layer-resolved density functional theory (DFT), which reveals the emergence of a metallic ground state in non-interfacial SFO layers as the interfacial spacing is increased.

Rombohedrically distorted perovskite BiFeO₃ (BFO) oxide presents two types of ordering: antiferromagnetic with Niel temperature of 370 K and ferromagnetic with Curie temperature of 1100 K. The presence of antiferromagnetism at room temperature makes it appealing to different uses of the magnetic property when is combined with other materials with ferromagnetic property. For example, ferromagnetic phase of the Sr doped lanthanum manganese perovskite oxide, La_2/3Sr_1/3MnO₃, (LSMO) also has a perovskite structure, with Curie temperature of 350 K. To investigate multiferroic coupling effects, here we report the synthesis and properties BFO and LSMO thin film bilayers. We have grown epitaxially La_2/3Sr_1/3MnO₃ and BiFeO₃ thin films and LSMO/BFO bilayers on (100) oriented SrTiO₃ substrates by using DC and RF sputtering technique at pure oxygen pressures. We have structurally characterized samples by x-ray diffractometry Electrical properties were evaluated by resistivity measurements as function of temperature, showing how the transition temperature of manganite is affected by the presence of the BFO layer. From thermal demagnetization measurements at zero field cooling and field cooling at different magnitudes of the applied magnetic field, we found Curie temperature around 350 K for the LSMO ferromagnetic phase in thin film and in the bilayer. Finally, we are studying the influence of the BFO layers on magnetic properties and the effects of field cooling on the observed properties will be described.
Acknowledgments: This work was financially supported by CENM-Univalle, Patrimonio Autoacute;nomo Fondo Nacional de Financiamiento para la Ciencia, la Tecnología y la Innovacioacute;n Francisco José de Caldas (Colciencias) through 1106-56-933104 and1106-48-925531 projects and Universidad del Valle through CI “7917” project.

~SbVO₄ system, with rutile structure, is a key element in the catalysis of propane to acrylonitrile. This system exhibits a great structural flexibility [1,2] involving cation vacancies, changes of the oxidation state of vanadium while antimony remains in 5+ oxidation state, long range ordering, short range ordering, structural modulations, etc... This structural flexibility is possibly linked to its catalytic properties and a proper and exhaustive characterization of the subtle structural variations that happen along the wide non-stoichiometry range of its existence limits is required in order to understand its functionality. In this work we study the existence limits of this phase, close to the binary oxide VO₂. The samples have been prepared by heating SbVO₄ and VO₂ in a 1:8 molar proportion at 800-850 C under Ar atmosphere. Powder X-ray diffraction was carried out in a Powder Diffractometre Bruker D8 Advance with Cu Kα radiation. Electron diffraction experiments were accomplished in a JEOL 2000FXII transmission electron microscope. The sample has also been characterized by neutron diffraction experiments, magnetic susceptibility measurements, DSC calorimetry and Raman spectroscopy. VO₂ is monoclinic at room temperature due to the pairing of vanadium cations, with a reversible semiconductor-metal phase transition VO₂ (M ) ® VO₂ (T) (monoclinic ® tetragonal) associated with drastic changes in the optical properties such as a rapid decrease in optical transmittance in the near-IR region. However, Sb_0.1V_0.9O₂ sample produce powder X-ray diffraction patterns that indicate already tetragonal symmetry at room temperature. Electron diffraction shows the presence of intense layers of diffuse scattering between the diffraction maxima due to Short Range Ordering (SRO). Besides, careful heating of the crystals with the electron beam gives rise to a structural transition where the diffuse scattering condenses in two-fold superlattice maxima. This transition is reversible and very quick since repeated expanding and condensation of the electron beam reproduces the transition at one single "click" of the "brightness" knob. This transition has been measured by DSC at 510C (670C for VO₂) being reversible with almost no hysteresis. Magnetic susceptibility suggests a probable spin-Peierls pairing of vanadium atoms at the low temperature phase which are related with the SRO observed below 51 C. The Raman spectrum of the Sb_0.1V_0.9O₂ sample is similar to the one for the tetragonal phase of VO₂. The overall intensity is much weaker than for the monoclinic phase of VO₂, indicating that Sb doping produces free charges that screen the Raman intensity.
[1] A.R. Landa-Cánovas, J. Nilsson, S. Hansen, K. Staahl and A. Andersson. J. Solid State Chem. 116, 369-377 (1995). [2] Angel R. Landa-Cánovas, F. Javier García-García, Staffan Hansen. Catalysis Today 158 (2010) 156. [3] Authors thank Spanish Government (project MAT2011-27192) for financial support.

Interest in the one-dimensional ferroelectrics such as nanowires, nanorods and nanotubes has increased greatly from the view point of not only practical applications but also basic physics. To date, they have been synthesized by the sol-gel process, hydrothermal process, electrospinning process and pulsed laser deposition (PLD) process. We have developed a fabrication technique for PbTiO₃(PTO) and Pb(Zr,Ti)O₃(PZT) nanowires by metalorganic chemical vapor deposition (MOCVD) using ZnO nanowires as a positive template. MOCVD process is one of the most promising processes because this process is suitable for mass production and is compatible with current Si-based process.
In this study, we demonstrate selective growth of ferroelectric nanowires and fabrication of ZnO/PZT/ZnO structure for ferroelectric capacitors in FeRAMs.
In the first stage of our experiments, ZnO nanowires were prepared by MOCVD. VS (Vapor-Solid) growth process was used. In the next stage, PbTiO₃ and PZT were deposited on ZnO nanowire positive template at 550^oC by MOCVD. PbTiO₃/ZnO and PZT/ZnO core-shell heterostructured nanowires were successfully fabricated. The diameter and length of PZT/ZnO core-shell nanowires were 120-140nm and 5-7mu;m, respectively.
Optimizing growth condition, ZnO nanowires were selectively grown on patterned Pt (40x40mu;m²) on SiO₂/Si. When Pt dots with a diameter of 60nm was used, several ZnO nanowires were grown on those Pt dots. This may be caused by the polycrystalline nature of Pt with grain boundaries which play an important role of nucleation site. On single crystalline Pt dot, single ZnO nanowire was selectively grown.
ZnO/PZT/ZnO nanostructures with an average diameter of 46nm and length of 3.3mu;m for nanowire capacitor were successfully prepared.

Multiferroic phenomena, where magnetic and electric ordering are correlated, open exciting opportunities for designing microelectronic devices with multifunctional nature. Although bulk BiFeO₃ is a well-known multiferroic compound with high non-linear magnetoelectric coupling, little is known about the influence of different magnetic structures on the electrical properties of nanostrucutured samples. We have performed an electrical property comprehensive study on two sets of BiFeO₃ nanoparticles obtained with different sizes (73 and 54 nm) by employing a wet chemical method with slightly different route. The analysis of the magnetic properties indicates that the magnetic moment decreases for the sample with smaller crystallite size which is in contrast with expected. This result along with the behavior observed for the coercive field and saturation magnetization suggest that the magnetic ground state of each sample is different. The lager particles exhibit ferromagnetic-like properties whereas smaller particles show antiferromagnetic-like behavior. At room temperature, where the charge carriers are strongly localized, electrical polarization has low values for both samples while the coercive electrical field is higher in the antiferromagnetic sample. As the temperature is increased, the electrical resistivity drops significantly almost 7-order of magnitude leading to huge values in the electrical polarization which is in agrement with colossal values observed on dielectric constant for both samples. It is also observed that at low frequency region, where intergrain contribution dominates the physical properties of the system, the dielectric constant of smaller particles is higher than that of lager particles. However, at high frequency region, where intragrain contribution dominates, different values of the dielectric constant have been observed only at T = 573 K. This result suggests that the intragrain capacitance contribution is approximately the same revealing that it is little influenced by different magnetic exchange interactions observed in these samples. The combined results suggest that the delocalization character of the charge carriers plays the major role in the electrical properties. The different magnetic ground state observed in these samples has little influence on electrical properties suggesting weak coupling between magnetization and electrical polarization physical properties.

Vertically aligned two phase multiferroic nanocomposites have been widely studied for their potential application in future memory and logic devices since they enable control of both electric and magnetic order parameters. These composites show strong magnetoelectric coupling through strain at the interface between the ferri/ferromagnetic pillar phase and the ferroelectric matrix phase. Variation of pillar composition and control of its shy;position can enable manipulation of the electric and magnetic properties of the composites. In previous studies, composites of magnetostrictive CoFe₂O₄ (CFO) - ferroelectric BiFeO₃ (BFO) have shown strong magnetoelectric coupling. In this work, the pillar composition is varied between that of high anisotropy, hard magnet CFO and lower anisotropy, soft magnet NiFe₂O₄ (NFO). These intermediate compositions would enable better control of magnetization of pillars on application of potential in the ferroelectric phase, due to dominant magnetostatic interactions. This can enable magnetic quantum cellular automata devices where the pillars interact magnetostatically but can still be switched by applying a local electric field to strain the matrix.
Co_xNi_1-xFe₂O₄/BFO nanocomposites were grown on single crystal SrTiO₃ (STO) (100) substrate by combinatorial pulsed laser deposition. These composites were also successfully patterned on Niobium doped STO by self-assembly using the focused ion beam to guide pillar growth. The films are epitaxial with the substrate and also at the interfaces, which results in an out-of-plane strain that is compressive in the spinel pillars and tensile in the BFO matrix. The saturation magnetization of NFO from the hysteresis curve is around 270 emu/cm³ which agrees well with theoretical values. The coercivity values decrease from 1300 Oe to 200 Oe from pure CFO to pure NFO pillars. Theoretical calculations based on the experimentally obtained coercivity values of the composites reveal that for certain compositions, suitable growth conditions can be obtained such that the nearest neighbor magnetostatic interactions between pillars is sufficient to switch the magnetization between them. Magnetic force microscopy images show clear magnetic contrast confirming the magnetic nature of the single domain pillars. The large magnetostatic interactions could result in a checkerboard pattern of the magnetic domains, which is desirable in magnetic logic devices.

Nanoscale memories, especially memristors, rely on migration of vacancies or defects in oxide systems to store information. Designing the defects and the ability to control them dictate memristive performance, especially aspects of switching ratio, durability, and scalability of devices.
Memristive devices rely on systematic resistance transitions as a function of voltage and time, thereby enabling the programming of device resistance. Most recent advances in this field have seen the development of memory cells that are generally based on a capacitor-like structure with an insulating layer sandwiched between two metal electrodes forming a metal-insulator-metal (MIM) structure. Perovskite oxides such as strontium titanate (SrTiO₃) are particularly attractive as the insulating layer due to their inherent dislocations and the ability to harbor oxygen vacancies. The bipolar resistive switching in STO-based MIM structures arise due to nanoionics transport processes that are triggered in the oxide layer during an electroforming process that is used for the creation of extended defect structures.
The key research question here relates to the ability to control and stabilize the defect chemistry of complex oxides.
In this work, we present such control in amorphous STO (a-STO) thin films synthesized at room temperature. It is shown that as#8209;grown oxygen deficiencies through the control over the synthesis parameters, significantly improve the memristive behavior of a-STO devices. Further, room temperature co-doping with Nb results in the creation of native oxygen vacancies and defect structures at considerably lower electric field stimulus, which plays a significant assistive role in the electroforming and the subsequent nanoionics-based switching processes.
Rigorous compositional characterizations reveal a classic nanoionics-based switching mechanism in the a-STO-based nano#8209;switches. The introduction of Nb dopant significantly lowers the energy threshold required for the creation of conduction dislocations during the electroforming process and lowers the read current compared to previously reported a-STO based memristive devices.
Structural and nano#8209;contact studies (with in situ electrical nanoindentation) on switching a#8209;STO cells reveal the formation of large area defect structures, responsible for the exceptional memristive performance of these devices. Uniformly distributed nano-filaments are found to be isolatedly switchable with bi#8209;stable characteristics while the non#8209;deformed regions in a-STO cells retain their highly insulating properties. These observations also highlight the ultimate scalability of a#8209;STO based devices.

The perovskite material Pr_0.67Ca_0.33MnO₃ (PCMO) combined with SrTi_0.998Nb_0.002O₃ (STNO) is presently explored as a model system for manganite-based pn-heterojunctions where PCMO is p-doped and STNO is n-doped. In this system, it is expected [1] that the rapid thermalization of low-energy photoexcited charge carriers can be quenched by making use of hot polaron-type correlated states with long lifetimes [2] and/or long diffusion lengths. This would open the possibility of converting the photon energy of a broad range of the solar spectrum into electric energy. PCMO can be epitaxially grown on STNO and atomic scale PCMO-STNO interface design is expected to be crucial for controlling the charge transfer across the interface and thus photovoltaic properties.
Illumination- and temperature-dependent photovoltaic response has already been measured in the system. [1, 3] Electron beam induced current (EBIC) measurements provide a method for mapping the short-circuit current, from which the carrier diffusion length can be extracted. [4] To investigate the carrier diffusion length in PCMO-STNO samples, we measure EBIC in plan view, in a bevel geometry as well as in cross-section by combining SEM-based EBIC with Focused Ion Beam preparation in a dual beam instrument.
EBIC is resolution-limited because of the SEM probe size and the carrier generation volume, increasing with the electron beam energy. This is important especially in cross-section EBIC measurements around the PCMO/STNO interface as all relevant scales are in the order of 1 to a few 10nm and, consequently, usual approximations do not hold. To encounter this situation, we interprete our results by comparison to Monte Carlo simulations. Furthermore, we perform EBIC measurements in thin TEM lamellae of the junction in order to reduce the generation volume. This allows simplifying the data evaluation and increasing the spatial resolution.
We thank the German Research Society for funding through CRC1073.
1 G. Saucke et al., Phys. Rev. B 85 (2012) 165315.
2 P. Grossmann et al., Rev. Sci. Instrum. 83 (2012) 053110.
3 A. Sawa et al., App. Phys. Lett. 86 (2005) 112508.
4 H. J. Leamy, J. App. Phys. 53(6) (1982) R51-R80.

Vanadium dioxide (VO₂) is well-known as a typical material invoking an electronic phase transition between high temperature metallic state and low temperature insulating one accompanied by orders of magnitude changes in resistance at 340 K. In the vicinity of the metal-insulator transition (MIT) temperature, VO₂ has mixed electronic phases consisting of metal and insulator domains. The coexistence states naturally forms a non-traditional interface consisting of competing electronic phases with different type of physical properties in nanoscale, which is entirely different from conventional rigid hetero-junction composed of two different materials. Recently, it has been found that the unique interface provides interesting physical effects such as enhancement of photocurrent [1] and the local Peltier cooling [2]. In the contrary, these effects would potentially have an ability to manipulate the correlated electron phases in a local area. In this research, we demonstrate the control of the MIT in a domain using a local thermoelectric cooling and heating effects generated at the interface between metal and insulator phases having largely different Peltier coefficients. This operation indicates reversible and memristive switching in VO₂ nanowires on Al₂O₃ (0001) single-crystal substrates, which can be realized by one dimensional domain configuration in VO₂ nanowires because of one straight current path through the domain interface enabling us to theoretical control of thermoelectric cooling and heating to induce the transition. Our demonstration gives recognition that the interface of electronic phases including peculiar physics is an attractive frontier and opens opportunities for functionally designing and controlling the configuration of nano-domains in a thin film, leading to realize memristive and switching devices using correlated electron phases.
[1] T. S. Kas#305;raga et al. Nature Nanotech. 7, 723-727 (2012).
[2] T. Favaloro et al. Nano Lett. 14, 2394-2400 (2014).

Lower dimensional structures of high-T_c superconductors have been achieved by a wide range of synthetic strategies, such as CVD, RABiTS and crystal pulling; yielding high quality films, tapes and wires. Time, energy and financial costs for yielding high performance materials are often technologically inhibitory. In contrast; cheaper, quicker synthetic protocols; such as sol-gel or solid state syntheses, yield polycrystalline products of low density, poor grain connectivity and no macroscopic structural design, hence poor performance. Here, a generalised sol-gel strategy for higher-ordered architectures of superconductor YBa₂Cu₃O_7-x is presented. Superconducting, self-supporting spheres, wires, tubes and spirals are generated through thermal decomposition of salt-impregnated polysaccharide-rich Sephadex beads or durum wheat pasta. Inorganic replicas of templates are characterised by SEM, TEM, SQUID and pXRD. Sufficient product purity, grain connectivity and density was obtained with the addition of AgNO₃ prior to calcination and sintering to yield immeasurably low resistance across bulk spaghetti-derived wires at 22 K determined by 4#8209;point probe measurements. This highly demonstrative study presents an interesting direction for sol-gel processing of complex functional materials into higher dimensional, bespoke macromorphology via a very simple, rapid, cost-effective method.

Apatites represent a class of phosphate based minerals with the general formula Ca₁₀(PO₄)₆[Cl,F,OH]₂. They constitute a major component of bones and teeth and are under consideration as a nuclear waste form for halide bearing waste-streams. As a result the transport of halides within apatites is relevant when evaluating their performance for waste-form applications. The present work uses atomic scale simulation to predict a new migration mechanism for Cl^- within chlorapatite.
Using molecular dynamics in conjunction with classical pair potentials, a highly anisotropic concerted vacancy mediated mechanism was identified within the temperature range 1000 - 1400K. For stoichiometric chlorapatite this had an effective activation energy of 2.37 ± 0.07 eV. In addition hypostoichiometric chlorapatite was considered and exhibited a lower activation energy of 0.54 ± 0.16 eV. This has been attributed to the availability of Cl^- vacancies within the crystal structure. Finally, a transport process involving the concerted migration of two to four Cl^- ions directly along the c axis halide channels within apatite has been identified and is described .

Single-phase LuCrO₃ was successfully synthesized via an auto-ignition method (1). Air heating of the resulting amorphous powder shows an intermediate LuCrO4 phase before LuCrO₃ crystallization takes place above 1100 K. Refined lattice paramenters of LuCrO_3, in the orthorhombic (Pbnm) space group showed a deviation of the [Cr-O₆] octahedra from the ideal -regular- shape with the Cr-O(1)-Cr and Cr-O(2)-Cr chains tilted away as a consequence of the small radius of Lu(III) in twelve-fold coordination.
Magnetization measurements showed a canted antiferromagnetic transition at 116 K confirmed by a magnetic hysteresis loop at 5 K. The relaxor ferroelectric behavior, confirmed by a broader peak, shifted toward a higher temperature with increasing frequencies; this can be attributed to the Lu-cation displacements as was inferred by a greater distance. Electrical conductivity behavior in LuCrO₃ showed lower activation energies than those reported for YCrO3 in detriment of the biferroic properties. The tilt angle in the [Cr-O₆] octahedra in LuCrO₃ seems to have a great influence in the activation energy with respect to YCrO₃ (2).
Acknowledgments
A.D. thanks to DGAPA-UNAM Project No. IN103213 and C.O. to the CODI-UDEA IN633CE.
References
1:- A Durán, C. Meza F., E. Morán, M.A. Alario-Franco, C. Ostos: Biferroic LuCrO3: Structural characterization, magnetic and dielectric properties. Materials Chemistry and Physics 143 (2014) 1222e1227.
2:- A. Duran, A.M. Arevalo-Lopez, E. Castillo-Martínez, M. Garcia-Guaderrama, E. Moran, M.P. Cruz, F. Fernandez, M.A. Alario-Franco, Magneto-thermal and dielectric properties of biferroic YCrO3 prepared by combustion synthesis: J. Solid State Chem. 183 (2010) 1863.

With the proliferation of wireless communication, there is an increasing demand for widening the range of frequencies accessible to devices. At gigahertz frequencies, the thin film dielectric material, Ba_xSr_1-xTiO₃, commonly used for frequency tunable microwave circuit elements, experiences large attenuation arising from dielectric loss. The attenuation is commonly attributed to non-stoichiometry related to point defects. We utilize oxide molecular-beam epitaxy (MBE) as a controlled way to fabricate the next generation of thin-film, high-frequency tunable dielectrics via precise atomic layering and epitaxial strain. Using this approach we recently demonstrated record tunable dielectric performance at gigahertz frequencies for strained Sr_n+1Ti_nO_3n+1 Ruddlesden-Popper (RP) phases [1]. In this work we use MBE to grow related RP phases containing Ba_xSr_1-xTiO₃, specifically Sr_n+1Ti_nO_3n+1-(BaTiO₃)₁, in hopes of accommodating point defects by the (SrO)₂ faults, allowing the rest of the dielectric material to remain stoichiometric. An advantage of these Ba-containing RP phases is lower epitaxial strain so films can be grown thicker without loss of the desired low-temperature ferroelectric instability. The first three phases of this RP series (n=1, 3, and 5) have been grown on DyScO₃ (110) using MBE, and characterized with x-ray diffraction. Rocking curve measurements have shown all films are of extremely high quality, with the full width at half maximum (FWHM) less than 16 arcsec. Characterization of the films by dielectric measurements, second-harmonic generation, and scanning transmission electron microscopy will be presented.
[1] C.H. Lee, N.D. Orloff, T. Birol, Y. Zhu, V. Goian, E. Rocas, R. Haislmaier, E. Vlahos, J.A. Mundy, L.F. Kourkoutis, Y. Nie, M.D. Biegalski, J. Zhang, M. Bernhagen, N.A. Benedek, Y. Kim, J.D. Brock, R. Uecker, X.X. Xi, V. Gopalan, D. Nuzhnyy, S. Kamba, D.A. Muller, I. Takeuchi, J.C. Booth, C.J. Fennie, and D.G. Schlom, “Exploiting Dimensionality and Defect Mitigation to Create Tunable Microwave Dielectrics,” Nature502 (2013) 532-536.

Calcium-barium niobate (Ca_xBa_1-xNb₂O₆, CBN) thin films present a unique combination of a relatively high electro-optic (EO) coefficient (~ 130 pm / V) and a high thermal stability (Curie temperature above 260°C), which makes them very promising for the development of high speed EO devices. Previous studies reported epitaxial growth of CBN thin films in the (001) orientation using Pulsed Laser Deposition (PLD) with good EO properties. However, such films were grown on small surfaces (typically 1 cm²), with a roughness of 7 nm RMS and the presence of particles on the surface, which is unsuitable for optical devices fabrication.
In this work, we report the growth of high-quality CBN thin films using PLD and radio frequency (RF) magnetron sputtering with a view to fabricating high-performance EO modulators. More specifically, we first use PLD with optimized target rastering to grow epitaxial CBN thin films on MgO substrates. We demonstrate epitaxy for CBN thicknesses up to 2 µm and thickness homogeneity better than 10% on 1 inch² MgO substrates using a deposition temperature of 800°C and a slow cooling rate. We also show a refractive index of 2.2 at 1550 nm and low propagation loss in the thin film (below 5 dB/cm). In a second step, we study the RF magnetron sputtering of CBN thin films on MgO. We show that the careful control of the stoichiometry of the as-deposited CBN thin films is crucial to achieve the appropriate crystallization of CBN. Highly (001)-oriented CBN thin films are deposited on 1 inch² substrates by choosing a fraction of 5% O₂ in the deposition plasma followed by rapid thermal annealing at 1000°C for 60 s. The resulting refractive index is about 2.1 at 1550 nm, which is close to the bulk crystal. Moreover, the sputter-deposited CBN films are quite homogeneous and display low roughness (below 3 nm RMS) with a particle-free surface.
The high crystalline quality of such CBN films grown over large surface substrates together with their good surface quality promise great potential to develop high-performance electro-optic devices with low loss and power consumption.

The discovery of conductive behaviour in domain walls of insulating ferroelectric Perovskites triggered intensive research as it opened a new possibility: use the ferroelectric materials as electronic devices in which the information is carried by rewritable nanoscale conductive paths [1],[2]. The conduction was identified in two types of domain walls: Neutral domain walls, the most common, where the polarization bound charges from one domain to the next cancel each other out, and charged domain walls, where the polarization bound charges at the walls add up requiring additional free carriers for compensation. The neutral domain walls have the advantages to be easy to create and manipulate with an electric field but the mechanisms at the origin of their conduction remain unclear. Numerous experiments performed with a conductive-tip atomic force microscope (CTAFM) revealed a defect-assisted conduction with a hardly-controlled and weak amplitude [3]. On the contrary, charged domain walls present a high, quasi-2DG conduction, which makes them more suitable candidates for ferroelectric-domain-wall-based devices. The challenge comes from their creation, which is practically unknown. In bulk materials they were very recently successfully created with a poling procedure through their Curie temperature, which is hardly suitable for a device [4]. In thin films, unstable ones were created in proper ferroelectrics [5], and stable appear in improper ferroelectrics [6] but their control is unknown.
We have grown by pulsed laser deposition La-doped BiFeO₃ (BFO) thin films in which the as-grown domain structure as well as the type of neutral domain walls, 71°, 109° or 180° are controlled. We propose a novel approach using piezoresponse force microscopy that allows the controlled switching of both the out-of-plane and the in-plane components of the polarization without requiring a planar-capacitor-like structure. This way we show that it is possible to create at will stable arrays of charged domain walls with sizes ranging from micrometres down to 150 nanometers. CTAFM experiments revealed a stable conduction nearly 3 orders of magnitude higher than the one of the neutral domain walls. This technic can be potentially applied to all ferroelectric materials. These results pave the way towards fully reconfigurable devices based on highly conductive charged domain walls.
[1] Seidel et al., Nat. Mater. 8 229 (2009)
[2] Sluka et al., Nat. Comm. 4 2839 (2013)
[3] Seidel et al., Phys. Rev. Lett. 105 197603 (2010)
[4] Bednyakov et al., (unpublished)
[5] Maksymovych et al., Nano Letters 12 209 (2012)
[6] Meier et al., Nat. Mater. 11 284 (2012)

Transition metal perovskites have attracted huge interest over the last decades since the seminal discoveries of colossal magnetoresistance and high-temperature superconductivity. Vanadium based perovskites (A3+V3+O3, A=La to Y), although not widely studied, are a fascinating playground for modern physics. These systems are Mott insulators and crystalize in the well-known Pnma phase at room temperature. With decreasing temperature, they develop “Jahn-Teller” distortions modifying the degeneracy of the d orbitals and producing different orbital-ordered phases, which can be exclusive or coexisting. Additionnaly, those orbital ordered phases are intimately connected to particular antiferromagnetic orderings. Based on a symmetry mode analysis, we identify new relevant couplings between structural, electronic and magnetic degrees of freedom in vanadates and clarify the unusual coexistence of Jahn-Teller distortions, providing new insight on the latter and their role in perovskites in general. Using first-principles calculations, these new lattice mode couplings are proven to enable an unprecented Jahn-Teller and orbital ordering induced ferroelectricity in vanadate superlattices. Due to the intimate connection between orbital ordering and magnetism, an electric field control of both magnetic state and orbital ordering is predicted. These findings enable potentially large magnetoelectric effects and open new routes for the design of magnetoelectric multiferroics and functional oxides in general.

Bulk La_0.4Sr_0.6MnO₃ shows a 4-point phase boundary where ferromagnetic metal, paramagnetic metal, canted antiferromagnetic (AFM) metal, and A-type antiferromagnetic metal phases meet at 240K. Below this temperature, the material straddles a canted AFM and an A-type AFM phase boundary. The strong spin-charge-lattice coupling inherent in complex oxides make this a prime candidate for exploring the impact of interfacial strain effects on phase behaviour since even slight perturbations to the underlying order parameters should have dramatic effects on the magnetic and resistive character of a material at a phase boundary. We use pulsed laser deposition to grow single crystal thin films of this composition on several different substrates to induce a range of epitaxial strains. We will present data showing a high level of tunability in resistive and magnetic properties depending on applied lattice strain. Of particular interest, is the fact that anisotropic magnetoresistance varies greatly depending on strain type. These behaviors are attributed to epitaxy induced changes in orbital occupation driving different magnetic ordering types. This suggests that AFM ordering type has a profound impact on the AMR magnitude and character which would be a critical finding in the quests for antiferromagnetic alternatives to ferromagnets in spintronic applications.

VO₂ has become an important material system and the metal insulator transition (M Phase) is being exploited in energy and electronic applications. However VO₂ exhibits other interesting phases such as semi-metallic B and insulating A which have not been demonstrated as single phase crystalline films. We show here that by controlling the arrival rate (laser frequency) and the gas phase oxidation of the V atoms one can produce high quality A and B phases. In this paper we develop a phase diagram (oxygen pressure versus laser frequency) for the growth of the various phases of VO₂ and also compare their electronic properties. At low oxygen pressures the M phase is preferred while the A and B phases are dominant at higher oxygen pressures. The B phase is semi-metallic, the M phase exhibits a metal-insulator transition (MIT) and the A phase is almost insulating. Raman and IR measurements show an evidence of dimers in the M and A phases. XPS measurements show increased O2p hybridization with V3d for the A and B phases with respect to the M phase. At higher pressures oxygen atoms screen the vanadium from forming structures with V-V dimers and effectively increase the average V-V bond lengths promoting the A and B phases. The role of oxygen atoms is crucial in regulating this V-V interaction which in turn determines the structural and electronic properties of these phases. Thus relative control of cationic and anionic atomic arrival rates during film growth is a powerful processing step for the growth of novel functional polymorphic materials.

Rare earth iron perovskites, RFeO₃ where R is a rare earth cation, have been of great interest in recent years due to their interesting magnetic, catalytic, optical and electrochemical properties. The most studied of these is LaFeO₃, which displays a band gap of 2-2.6 eV, is insulating, and displays G-type antiferromagnetism. Substitution of smaller R cations, for instance Eu for La, does not alter the fundamental magnetic structure or insulating behavior, but instead will have a great effect on the atomic structure. The focus of this work is on understanding the sensitivity of the optical band gap to the structural distortions in rare earth ferrites.
To this end, we demonstrate the growth of EuFeO₃ thin films by molecular beam epitaxy, the first demonstration of physical vapor deposition of this material. The quality of the films is determined by a combination of x-ray diffraction, x-ray reflectivity, Rutherford backscattering spectroscopy, and scanning transmission electron microscopy. X-ray absorption was used to confirm nominal 3+ valence states of Eu and Fe. Variable angle spectroscopic ellipsometry was used to measure the optical absorption of EuFeO₃ from 1.25 to 5 eV. Determination of the optical band gap of EuFeO₃ resulted in a band gap of 2.75 eV, a value blue-shifted from that of LaFeO₃. The EuFeO₃ and LaFeO₃ absorption data is compared in the context of the structural differences of the two materials. #8203;X-ray absorption data was collected on beamline 4-ID-C at the Advanced Photon Source, Argonne National Laboratory, and a portion of the research was performed using EMSL, a national scientific user facility sponsored by the Department of Energy's Office of Biological and Environmental Research and located at Pacific Northwest National Laboratory.

The electron density in 2DEGs at oxide interfaces can be very effectively modulated by electric field effect [1]. Such possibility has been successfully exploited to tune the properties of the LaAlO3/SrTiO3 system, e.g. spanning the superconducting phase diagram [2] or tuning the spin-orbit coupling [3,4]. The effect of an applied electric field on the transport properties is typically interpreted in terms of a standard field effect transistor model, in which charge is transferred between the 2D electron gas and the electrode as in a capacitor charge/discharge process.
We present a detailed study of the effect of a back-gate voltage application, also complemented by photodoping [4], on a number of 2D electron gases found at oxide heterostructures, including standard (001) oriented LaAlO₃/SrTiO₃, (110) oriented LaAlO₃/SrTiO₃, amorphous-LaAlO₃/SrTiO₃ and Al2O₃/SrTiO₃. We show that, beside the previously observed “standard” field effect, which is reversible and only limited in speed by electronic time constants, other more complex effects take place under the application of an electric field. These further transport features, whose magnitude compares with, or sometime exceeds, the standard field-effect modulation, include non-reversible carrier depletion effects, low-temperature hysteretic ferroelectric-like response and very slow high-temperature dynamics presumably related to ionic effects. Our data point to a complex scenario, in which the applied electric field can perturb the properties of the 2DEG in oxide heterostructures in different and complementary ways.
References:
[1] S. Thiel et al., Science 313 1942 (2006)
[2] A. D. Caviglia et al., Nature 456, 624 (2008)
[3] M. Ben Shalom et al., Phys. Rev. Lett. 104, 126802 (2010)
[4] A. D. Caviglia et al., Phys. Rev. Lett. 104, 126803 (2010)
[5] E. Di Gennaro et al., Advanced Optical Materials, 1, 834 (2013)

Ultrathin layers of ferroelectric perovskites form striped nanodomains to minimize the electrostatic energy of the system. Superlattices consisting of alternating ferroelectric and dielectric atomic layers form striped nanodomains that are more resistant to surface contamination and the resulting compensation of the depolarizing field by the ambient charge species than a single layer of ferroelectric thin film. We have studied nanodomain structures in a PbTiO₃/SrTiO₃ superlattice using a focused synchrotron coherent x-ray nanobeam. The superlattice nanodomains produce diffraction patterns with intensity speckles that correspond to the spatial frequency spectrum of the disordered domain pattern. Variations of the domain pattern as a function of elapsed time can therefore be evaluated by the decorrelation of the corresponding speckle patterns. We measured two superlattice samples with the same nominal repeating units (8 PbTiO₃ layers and 3 SrTiO₃ layers). Both samples have a highly disordered domain pattern with a short in-plane coherence length of a few domain periods. For the first sample, the domain patterns remain highly correlated for the duration of the experiment. For the second sample, the domain pattern decorrelates as a function of elapsed time tau; and such temporal decorrelation can be described using a compressed exponential function exp[-(tau;/tau;_s)^p] with p=1.5. The characteristic decorrelation time tau;_s ranges from 910 s to 1440 s for a series of measurements. We hypothesize that the disordered spatial structure of the nanodomains corresponds to one of the various metastable states of the sample with nearly degenerate energy levels, and the temporal decorrelation of the domain pattern is caused by the thermally driven transition between metastable states at room temperature.

We demonstrate a method to determine the intrinsic effective piezoelectric coefficient using dynamic X-Ray Diffraction (XRD) on a PbZr_xTi_(1-x)O₃ (PZT) thin film. Additionally we gain insight into the behavior of the domains and domain walls during actuation. Pulsed Laser Deposition (PLD) is used to grow a 5mmx5mm epitaxial 1 µm thick PZT film between two conductive SrRuO₃ layers on top of a single crystal SrTiO₃(001) substrate. XRD measurements were done during the actuation of the device, which enabled the study of induced intrinsic changes in the unit cell parameter and domain fractions. From these intrinsic material properties we were able to extract its effective dshy;₃₃ piezoelectric coefficient and compare it to commonly used vibrometer results. It was observed that during the application of an increasing out of plane electric field the out of plane unit cell size of the domains was decreasing . This phenomenon can be explained by an interplay between the strain, unit cell size and domain fraction within the material. It was also found that domain wall movement is minimized when applying electric fields below the coercive field, thus avoiding polarization switching. Once the coercive field is exceeded, domain wall movement initiates a reconstruction of the PZT domains. This reconstruction differs in a positive and negative applied field, giving new insight into the actual domain wall movement and polarization switching mechanisms.

Thermotropic phase boundaries (TPBs), as thermal analogues of morphotropic phase boundaries (MPBs), are associated with the thermal inter-ferroelectric phase transitions. Similar to an MPB, a TPB exhibits a characteristically flattened energy profile which favors polarization rotation, thus giving rise to a structurally bridging low-symmetry phase. We report on the kinetic process of thermal inter-ferroelectric phase transitions in BaTiO₃ and KNbO₃ using the phase-field method. The domain structures are found to play key roles in stabilizing the monoclinic phase. In simple domain structures, the monoclinic phase is an intermediate phase and cannot be stabilized into its neighboring phase regimes. While by introducing structural inhomogeneity (orthogonal in-plane domain twins), we found that the monoclinic phase can be stabilized over a range of over 100 K across the transition. As a result, the piezoelectric properties are enhanced due to the stabilized monoclinic phase. Besides the emergence of new piezoelectric components with monoclinic symmetry, most of the original components present in the tetragonal symmetry also show substantial enhancement with the rotation of polarization.

The best known example of a dissipative contribution to the piezoelectricity of ferroelectric materials is motion of domain walls. This contribution, which persists to driving fields in gigahertz range, can lead to large enhancement of the macroscopic piezoelectric response and can account for as much as 70% of its total response. Another, much less studied contribution to the dissipation of apparent piezoelectric properties is electric conductivity. The most straightforward way in which the conductivity affects electro-mechanical response is through the piezoelectric Maxwell-Wagner (MW) mechanism. It requires boundary between two sufficiently conducting materials with different piezoelectric coefficients - a condition that is easily met between layers in a thin film heterostructures or between grains in a polycrystalline material. The theoretical models show that this mechanism can lead to a large enhancement of the piezoelectric effect at low frequencies.
In this presentation we report on a very interesting and unusual situation that arises when domain walls themselves exhibit conductivity that is larger than in the domains. The effect is reported here for BiFeO₃ polycrystalline samples. The dynamics of domain walls together with the local conductivity of the walls results in so-far unreported nonlinear MW effect. The resulting macroscopic piezoelectric response shows rich dispersive behavior. At high frequencies (above 10 Hz) domain walls are pinned and move reversibly around the pinning sites. At these frequencies the effect of conductivity is small. At medium frequencies (0.1 to 10 Hz) the conductivity of domain walls becomes significant and helps depinning of the domain walls, which start to move irreversibly, dominating the piezoelectric response. At frequencies below 0.1 Hz, the conductivity through domain walls dominates the piezoelectric effect through the MW mechanism. Since domain walls move irreversibly, the combination of the two effects leads to nonlinear Maxwell-Wagner mechanism and a very large enhancement of the apparent piezoelectric coefficient.
The method used here for the separation of the different contributions (reversible and irreversible domain wall motion, linear and nonlinear Maxwell-Wagner effects, conductivity) to the dispersion of piezoelectric and dielectric responses is in part based on the analysis of the scaling behavior of the subswitching hysteresis area with the driving field.
This work was financed by the Slovenian Research Agency (programme P2-0105 and project J2-5483). The work of D.D. was supported by the Swiss National Science Foundation (PNR62 project no. 406240-126091).

The switching of the spontaneous strain, otherwise called ferroelastic switching, induced by an electric field has received significant interest due to the resulting enhancement of the piezoelectric response in ferroelectric thin films as well as its potentially critical role in coupling the electric field and the magnetic order in magnetoelectric materials and heterostructures.^1-7 Still, in conventional ferroelectrics (PbTiO₃, Pb(Zr_0.2Ti_0.8)O₃ (PZT) etc.), it has remained elusive primarily due to the suppression of the ferroelastic switching and the immobility of the ferroelastic domain walls in continuous thin films.^1,5,7 In this work, we show that the suppression of ferroelastic switching can be removed by epitaxial strain tuning in continuous films of an archetypal ferroelectric like PZT.⁸ We report for the first time that an electric field applied in the opposite direction of the c-domain polarization can ensue a ferroelastic c-to-a switching in tensile strained PZT films as thin as 40 nm grown on GdScO₃ (GSO) substrates. We further show that such voltage controlled ferroelastic c-to-a and a-to-c reversible switching can be achieved without a concurrent ferroelectric 180⁰ switching of the surrounding c-domain matrix. Such a ferroelastic switching is also accompanied by an enhancement of the piezoelectric response compared to that of a mono-domain PZT film.
Together with the enhancement of the piezoelectric response, the reversible creation and annihilation of a-domains with applied voltages show that a high mobility of the domain walls qualitatively analogous to that in the high aspect ratio islands can be achieved,¹ albeit in a continuous and much thinner film, by epitaxial strain tuning. When a ferroelastic switching is accompanied by a concurrent ferroelectric switching, most of the energy is dissipated towards the ferroelectric switching. Hence the ferroelastic switching without a concurrent ferroelectric switching as reported herein could turn out to an order of magnitude less dissipative. Thus, in addition to applications where nano-scale control of strain is needed, such as in nano-scale MEMS, multiferroics and straintronics,^9-11 the voltage controlled ferroeleastic switching scheme could also lead to high energy efficiency in these applications.
References:
1. Nagarajan, V. et al. Nature Mater. 2, 43 (2002). 2. Baek, S. et al. Nature Mater. 9, 309 (2010). 3. Lahtinen, T. H. E., Tuomi, J. O. & van Dijken, S. Advanced Mater. 23, 3187 (2011). 4. Eerenstein, W., Wiora, M., Prieto, J., Scott, J. & Mathur, N. Nature Mater. 6, 348 (2007). 5. Gao, P. et al. Nature Comm. 4, 2791 (2013). 6. Gao, P. et al. Nature Comm. 5, 3801 (2013). 7. Ouyang, J. et al. Adv. Func. Mat. 17, 2094 (2007). 8. Khan A. I., Marti X., Serrao C., Ramesh R. & Salahuddin S. (under review) 9. Baek, S. H. et al. Science 334, 958 (2011). 10. Cherifi, R. et al. Nature Mat. 13, 345 (2014).11. Newns, D. et al. J. Appl. Phys. 111, 084509 (2012).

The lead-free ferroelectric BiFeO₃ (BFO) has attracted much attention due to its superior properties in both epitaxial and polycrystalline thin films for potential applications to multiferroic-based devices. The remnant polarization P_rand out-of-plane converse piezoelectric coefficient d₃₃are comparable to those of the tetragonal, Ti-rich PZT system. However, the BFO films exhibited large coercive fields and a large leakage current, which might limit the applicability of BFO in devices. Here, we report a reduced leakage current behavior of ferroelectric BFO films as we insert an insulating layer like SrTiO₃ with nanometer-scale thickness in the middle of BFO layer. The BFO/STO nanolaminate films show still high remnant polarization with reduced leakage current. We will discuss about the mechanism of high polarization and low leakage current behaviors of nanolaminate BFO/STO/BFO structure, and the potential application of BFO-based piezoelectrically actuated MEMS devices for biomedical applications (e.g., biosensors and drug delivery systems), based on the biocompatibility of BFO components.

Transition metal oxides (TMOs) are frequently studied due to their many applications in fields such as high T_c superconductivity, catalysis, sensors and magnetic storage devices. The key aspects of TMOs which make them technologically relevant are the tunable nature and flexibility of their chemical composition and structure under various experimental conditions. In particular, manipulating the oxygen stoichiometry and strain-state of epitaxial TMO films has significant impacts upon their structural and physical properties. Our goal was to apply this methodology to epitaxial films grown by pulsed laser epitaxy of the K₂NiF₄ structured compound, La_2-xSr_xCuO_4±δ. This material is isostructural with the highly studied cathode material La₂NiO_4±δ which contains ABO₃ perovskite and AO rock-salt layers along the c-direction which provide ample space for enhanced oxygen mobility. We have investigated the electrochemical behavior of the oxygen-reduction reaction at high temperatures and the transport properties of La_2-xSr_xCuO_4±δ films grown under different strained states. Through different post-annealing experiments, we observed drastic changes within the structure and transport properties of the films under compressive and tensile strain. The high sensitivity of the films to different post-growth conditions and strain reveals an intricate coupling between strain and oxygen content control which will be discussed in detail.
The work was supported by the U.S. Department of Energy, Basic Energy Sciences, Materials Sciences and Engineering Division and by the LDRD Program of Oak Ridge National Laboratory.

Engineering nanocomposite oxides via precise control over the interface relies upon the fundamental understanding of their structure and its correlation with various properties. A detailed understanding of the structure of complex oxide heterointerfaces is challenging as they can have significant mismatch between the two constituent phases wherein the atomic structure is strained or altered, which results in a number of competing effects between e.g. electrostatics and elastic strain, the outcome of which has significant ramifications on the behavior of defects and on material properties. More importantly, the influence of strain and misfit dislocations at oxide heterointerfaces on properties pertinent to energy applications is not well understood.
In an attempt to understand the dislocation structure and its influence on the overall behavior of oxygen vacancies at heterointerfaces, we have conducted atomistic simulations of the experimentally observed cube-on-cube orientation relationship in a model SrTiO₃/MgO heterointerface. We demonstrate a strong dependence of the dislocation structure at oxide heterointerfaces on the termination chemistry, wherein SrO- and TiO₂-terminated SrTiO₃/MgO interfaces exhibit notably different dislocation structures with varied dislocation spacing and Burgers vector. These differences are the consequence of local electrostatic interactions across the interface, which differ for the two terminations. These very different dislocation structures, in turn, impact the behavior of oxygen vacancies at the interface, having important repercussions on the transport properties at the interface and on radiation damage evolution of oxide nanocomposites. Such a relationship between termination chemistry structure and properties in oxide heterointerfaces has not been established in the past, and offers novel avenues for designing oxide nanocomposites with applications spanning from fast ion conductors and ferroelectrics to advanced nuclear materials by controlling the termination chemistry at the interface.
This material is based upon work supported as part of the Center for Materials at Irradiation and Mechanical Extremes, an Energy Frontier Research Center funded by the U. S. Department of Energy, Office of Science, Office of Basic Energy Sciences.

Several members of the ordered (A⁺A3⁺)BB&’O₆ family of alkali-based mixed A-site perovskites have attracted considerable attention due to their ability to form a variety of unusual periodic nanoscale compositional and structural modulations. Examples include the (Nd_2/3-xLi_3x)TiO₃ system and (A⁺A3⁺)B²⁺W⁶⁺O₆ tungstates with A⁺ = Na or K, A³⁺ = La or RE³⁺ and B²⁺ = Mn, Mg. A common characteristic of all these systems is the ordered occupancy of the two different A-site cations in alternate layers along the [001] direction. While there are now several examples of these unusual modulated systems, many aspects of their local structure and composition, and the driving forces responsible for their formation are not fully understood.
In this paper we will present the results of a study of the nanocheckerboard NaNd(MgW)O₆ system using high angle annular dark field (HAADF) imaging conducted on an aberration corrected STEM. This system had been reported to form a 14a_px14a_p superlattice in the a-b plane with a periodic arrangement of tilt-twin boundaries and a possible associated compositional modulation [1]. Analyses of the HAADF images collected in this work reveal the structure is more complex than previously reported. Intensity variations in the Z-contrast images point to localized segregation of Nd to intersections of octahedral tilt-twin boundaries arranged periodically in the (100) and (010) directions. The structure also comprises a family of periodic antiphase boundaries oriented along <110>. By mapping the locations of the A and B-site columns, the images reveal the A-site cations undergo small collective displacements from their centered position to produce a local dipole oriented along <110>. These nanopolar domains are periodically twinned to produce what appears to be an ordered polar domain structure comprised of periodic 90° and 180° domain walls. Many aspects of this system are similar to the improper ferroelectric behavior recently reported for the non-modulated NaLa(MnW)O₆ perovskite system.[2] Details of these complex structures and their relationship to other polar systems will be discussed.
1. M.W. Licurse and P.K. Davies, Applied Phys. Lett., 97, 123101 (2010)
2. J.M. Rondinelli and C. J. Fennie, Adv. Mater, 24, 1961 (2012); T. Fukushima et al., Phys. Chem. Chem. Phys. 13, 12186 (2011).

Bi₂WnO_3n+3 complex oxides are members of the Aurivillius phase family of compounds which on top of their technological interest are also attractive at the fundamental level in view of the complexity of their phase transitions. W-based Aurivillius phase oxides ((Bi₂O₂)⁺² (B_nO_3n+1)^-2) appear as naturally stable superlattice structures, in which A-cation deficient perovskite blocks (n=1,2,3,4,5,6,8) alternate with fluorite-like bismuth oxide layers, giving components such as Bi₂WO₆, Bi₂W₂O₉, Bi₂W₃O₁₂,...
In this work we will report first-principles study of the first two members of this family, which exhibit unusual behaviors amongst the Aurivillius family. First, in order to understand the unexpected reconstructive phase transition of Bi₂WO₆ (BWO) at high temperature, we have characterized two highly-symmetric paraelectric structures: the idealized tetragonal phase of I4/mmm symmetry, common to all other Aurivillius phases, and the experimentally observed monoclinic phase of A2/m symmetry, specific to BWO. We will discuss the energetics of many different metastable phases appearing as small distortions of these two parent phases.
The dynamical, elastic, piezoelectric and non-linear optical properties of BWO in its P2₁ab ferroelectric ground state will also be presented. Second, we will characterize Bi₂W₂O₉ and discuss why the ground state of this component is antiferroelectric while other Aurivillius phases like SrBi₂T₂O₉ are ferroelectric.

The electronic structures of hole-doped 3d transition metal complex oxides have been extensively studied for certain B-site transition metal cations in an effort to elucidate the microscopic origins of high-T_c superconductivity, colossal magnetoresistance and metal-insulator transitions.¹ Strong electron correlation, resulting from localized 3d orbitals, is believed to underlie these phenomena. Density functional theory (DFT) based on one-electron wavefunctions usually fails to accurately predict the electronic structure. Moreover, difficulties associated with achieving stoichiometric material free of oxygen vacancies makes determination of the intrinsic properties a challenge.² Here we present a systematic study of the evolution of the electronic and optical properties of Sr-doped LaCrO₃ (La_1-xSr_xCrO₃), a relatively unexplored material. All films were deposited using state-of-the-art molecular beam epitaxy (MBE) and were characterized by means of x-ray and UV photoemission spectroscopy (PES), x-ray absorption spectroscopy (XAS), transport measurements, optical absorption and DFT calculations. Pure LaCrO₃ (Cr 3d³) is a Mott insulator with bandgap of ~3.0 eV.³ Substitution of La³⁺ by Sr²⁺ effectively introduces holes at the top of valence band, leading to Cr⁴⁺ (3d²) states. Core-level and valence-band features shift uniformly to lower binding energy with increasing x, indicating a downward shift of the Fermi level toward the valence band maximum with increasing hole doping. Transport reveals that the material is semiconducting for x up to 0.5 and becomes metallic for x ge; 0.65. O K-edge XAS reveals the development of a new unoccupied state above the Fermi level which grows in intensity with increasing x, and excitations from the top of the valence band to this state are clearly seen in optical absorption. These results indicate a pronounced redistribution of the density of states by hole doping and failure of the rigid-band model based on DFT to accurately describe the electronic structure. We interpret the observed behavior in terms of strong electron-lattice and/or electron-electron interactions, which produce localized in-gap states.
1. M. Imada, A. Fujimori and Y. Tokura, Rev. Mod. Phys. 70, 1039 (1998).
2. L. Qiao, K. H. L. Zhang, M. E. Bowden, T. Varga, V. Shutthanandan, R. Colby, Y. Du, B. Kabius, P. V. Sushko, M. D. Biegalski and S. A. Chambers, Adv. Funct. Mater. 23, 2953 (2013).
3. P. V. Sushko, L. Qiao, M. Bowden, T. Varga, G. J. Exarhos, F. K. Urban, D. Barton and S. A. Chambers, Phys. Rev. Lett. 110, 077401 (2013).

The development of atomic scale growth techniques for complex oxide heterostructures has introduced a wide range of opportunities and issues at their interfaces. Here we focus on the formation and spectroscopy of electric and magnetic dipoles at interfaces. The electrostatic boundary conditions can be designed to form electric dipoles of remarkably large magnitude, reflecting the relatively ionic nature of the constituents. By contrast, magnetic dipoles can also be induced at interfaces, but their origin and predictive control is yet unclear. Here we present our studies of the properties of interface dipoles using transport and x-ray spectroscopies in a number of representative oxide heterointerfaces.

The discovery of a two-dimensional electron gas (2DEG), formed at the interface between the two band insulators LaAlO₃ and SrTiO₃, has generated significant interest partly because of the opportunity to realize oxide nanoscale devices. The 2DEG oxide interface has revealed intriguing electronic properties, tunable superconductivity and a rich phase diagram. This interface naturally provides a versatile system to artificially build stacks of multiple 2D superconductors that would allow coupled 2D superconducting layers to be studied.
However, one main obstacle is the realization of a fully metallic two-dimensional electron gas (2DEG) at the interface between artificially-grown LaAlO₃ and SrTiO₃ thin films. We present here the successful realization of a superconducting 2DEG at interfaces between artificially-grown LaAlO₃ and SrTiO₃ thin films [1]. Our results highlight the importance of two factors - the growth temperature and the SrTiO₃ termination. By adopting an extremely high SrTiO₃ growth temperature, we demonstrate a way to realize metallic, down to the lowest temperature, and superconducting 2DEG at interfaces between LaAlO₃ layers and SrTiO₃ thin films. This study paves the way to the realization of functional LaAlO₃/SrTiO₃ multilayers and we also demonstrate for the first time the interesting superconducting properties of these LaAlO₃/SrTiO₃ multilayers.
[1] D. Li et al., APL Materials 2, 012102 (2014).

Complex oxides offer a wealth of novel behaviours such as high temperature superconductivity, colossal magnetoresistance, and multiferroicity. A fundamental understanding of how these properties arise from the overlapping charge, spin, orbital, and lattice degrees of freedom is lacking in part due to the fact that it is often impossible to fully explore predictive models that require crystal structures outside of what can be experimentally fabricated. We will present recent work using helium ion implantation as a means to finely control the out-of-plane crystal axis in an epitaxially locked manganite thin film to create artificially sized unit cell volumes without modification to other order parameters. Monte Carlo simulations on these artificial structures predict an increase in resistivity and a decrease in transition temperature as the out-of-plane axis is elongated due to a shift in orbital occupancy from controlled Jahn-Teller distortion. The experimental results confirm the predicted resistive behaviors. The process is also shown to be reversible by removing the helium through an anneal process. This new method of strain doping creates a unique way to adjust a single lattice parameter and should have substantial implications in allowing for experimental exploration of predictive models across many material classes.
Supported by the US DOE Office of Basic Energy Sciences, Materials Sciences and Engineering Division.

Our ability to control the electronic properties of materials, for instance at semiconductor interfaces, has had enormous scientific and technological implications. Recently, this concept has been extended to complex transition metal oxides which possess inherently strong quantum many-body interactions, such as correlated transition metal oxides, allowing us to synthesize artificial heterostructures which can harbor novel electronic or magnetic properties. I will describe some examples of our recent work in creating such heterostructures using oxide molecular beam epitaxy and using knobs such as dimensionality or biaxial strain to control their properties, while employing high-resolution angle-resolved photoemission spectroscopy (ARPES) to probe the effects on their electronic structure. In particular, I will describe how dimensional confinement at interfaces can be used to drive a metal-insulator transition in both superlattices of colossal magnetoresistive manganites and ultrathin films of correlated nickelates, and describe our recent work using epitaxial strain to control the electronic structure and properties of other correlated oxides.

Recently, a variety of new physical properties and phenomena have been discovered to emerge at atomically engineered interfaces of complex oxide systems. One example is the two-dimensional electron gas (2-DEG) forming at the interface between two insulating perovskite oxides, LaAlO₃ (LAO) and SrTiO₃ (STO). Theoretically, the electron concentration at this atomically-controlled interface can be manipulated by means of the polarity-induced electric field, which is facilitated by simply changing the film thickness of LAO. The resulting conducting “interface material” is known to be localized within a few nm from the interface. Although the existence of the 2-DEG has been proved and utilized in many prototype devices, there are still compelling debates related to its origin, spatial distribution, and concentration. Here we show direct imaging and quantitative analysis of the 2-DEG forming at the LAO/STO interface by using inline electron holography. In inline electron holography the phase of transmitted electron beam is obtained through exit-wave reconstruction of a through-focal series of transmission electron microscopy images, which enables the determination of local internal field and charge density with sub-nm spatial resolution. From two-dimensional charge density map, we unambiguously determined that the 10 unit cell (u. c.) sample contains the negative charges beneath the interface. In order to extract a correct density of 2-DEG from the total charge density map, one has to take account of a change of the dielectric constant of STO near the interface due to an intrinsic electric field associated with 2-DEG. The calibration of the dielectric constant using Landau theory yields a 2-DEG density close to the theoretically expected value corresponding to the transfer of 0.5 e per u. c. Our results indicate that the possible origin of this interfacial 2-DEG is most likely related with the oxygen vacancies formed at the LAO surface. By contrast, the charge density map of the 3 u. c. sample, which is below the critical thickness, does not show any significant charge density near the interface. In the absence of 2-DEG, the electric field in the 3 u. c. LAO film is partially compensated by the depolarization field induced by atomic displacement.

The epitaxial interface between LaAlO₃ (LAO) and SrTiO₃ (STO) has been found to exhibit a two-dimensional electron gas (2-DEG) with sufficiently high mobility to exhibit Shubnikov-de Haas oscillations. Electrons were confined near the interfaces for epitaxial LAO films grown by either pulsed laser deposition (PLD) or molecular beam epitaxy (MBE) on TiO₂-terminated STO substrates. Devices based on this phenomenon have already been proposed. One explanation for 2-DEG formation at the LAO/STO interface is based on electronic reconstruction, in which electrons move to the interface to avoid the potential divergence known as the polar catastrophe. The electronic reconstruction yields Ti³⁺ ions that arise from electrons in the STO conduction band.
It has also been discovered that 2-DEGs could be formed by growing amorphous LAO films on TiO₂-terminated STO substrates. Recently, 2-DEGs have also been observed in Al-based amorphous oxide/SrTiO₃ interfaces. The Al-based oxides were grown by atomic layer deposition (ALD) using trimethylaluminum. The formation of the 2-DEGs was explained by oxygen vacancies at the oxide/STO interfaces created during film growth. More remarkably, 2-DEGs with extremely high electron mobility at 2 K (> 100,000 cm²V^-1s^-1) have been found at the spinel γ-Al₂O₃ (GAO)/STO interface. Oxide 2-DEGs might provide opportunities for new generations of all-oxide electronic devices. For device manufacturing applications, ALD has advantages over PLD and MBE due to its high step coverage, significantly low thermal budget, scalability, and low cost.
This presentation will summarize our recent growth of GAO on STO and TiO₂-terminated STO substrates at temperatures higher than 300 °C by ALD using trimethylaluminum and water as co-reactants. In-situ reflection high-energy electron diffraction and transmission electron microscopy are used to determine the crystallinity of the GAO films. As-deposited Al₂O₃ films on STO grown at 200 °C were amorphous, while as-deposited Al₂O₃ films on STO grown above 300 °C were crystalline in the GAO phase. We demonstrate the formation of 2-DEGs at the GAO/STO and GAO/TiO₂-terminated STO interfaces that can be controlled by changing the deposition temperatures. In-situ x-ray photoelectron spectroscopy reveals the presence of Ti³⁺ at the GAO/STO interface. Conductivity at the interface was found to be proportional to the amount of Ti³⁺ species. A GAO (2.1 nm)/TiO₂-terminated STO heterostructure exhibits a room temperature sheet resistance of about 20 kOmega;/sq. Electrical transport properties at the GAO/STO interface will be presented. The effect of STO surface termination on conductivity and carrier mobility will also be discussed.

The interfaces between transition metal oxides show astonishing properties: tunable insulator-superconductor-metal transitions, large magnetoresistance, coexisting nano-domains of ferromagnetism and superconductivity, etc. More recently, 2DEG was discovered also at the (001) surface of SrTiO₃ (STO) single crystals. Therefore, a complete understanding of the LDEGs becomes a basic step for a clear interpretation of the electronic properties of interfaces obtained by combining STO and other oxides. Here we show, using Angle-Resolved Photoemission Spectroscopy (ARPES) in UV and Soft X-ray ranges, Spin- Angle-Resolved Photoemission Spectroscopy (SARPES) and Resonant Inelastic X ray scattering (RIXS), a comprehensive depiction of the LDEGs electronic structure at STO surface and LAO/STO interface.
References:
1. A. F. Santander-Syro, F. Fortuna, C. Bareille, T. C. Rodel, G. Landolt, N. C. Plumb, J. H. Dil, and M. Radovic, Giant spin splitting of the two-dimensional electron gas at the surface of SrTiO3, under review (2014).
2. N. C. Plumb, M. Kobayashi, M. Salluzzo, E. Razzoli, C. Matt, V. N. Strocov, K.-J. Zhou, C. Monney, T. Schmitt, M. Shi, J. Mesot, L. Patthey, M. Radovic, From the SrTiO3 Surface to the LAlO3/SrTiO3 Interface: How thickness is critical, arXiv:1304.5948 (2013).
3. N. C. Plumb, M. Salluzzo, E. Razzoli, M. Maring;nsson, M. Falub, J. Krempasky, C.Matt, J. Chang, J. Minar, J. Braun, H. Ebert, B. Delley, K.-J. Zhou, C. Monney, T.Schmitt, M. Shi, J. Mesot, C. Quitmann, L. Patthey, M. Radovic, Mixed dimensionality of confined conducting electrons in the surface region of SrTiO3, under review, arXiv:1302.0708 (2014).
4. R. Di Capua, M. Radovic, G. M. De Luca, I. Maggio-Aprile, F. Miletto Granozio, N. C. Plumb, Z. Ristic, U. Scotti di Uccio, R. Vaglio, and M. Salluzzo, Observation of a Two-Dimensional Electron Gas at the Surface of Annealed SrTiO3 Single Crystals by Scanning Tunneling Spectroscopy, PHYSICAL REVIEW B 86, 155425 (2012).
5. Kejin Zhou, Milan Radovic, Justine Schlappa, Vladimir Strocov, Ruggero Frison, Joel Mesot, Luc Patthey, and Thorsten Schmitt, Localized vs. delocalized character of charge carriers in LaAlO3/ SrTiO3 superlattices, PHYSICAL REVIEW B Rapid Communication 83, 201402(R)(2011).

The interplay between the structure and functional properties of transition metal complex oxides allows for the engineering of electronic and magnetic properties via tuning of the atomic-scale structure using interfacial coupling. We apply this principle to achieve two-dimensional electronic conductivity and orbital polarization in rare-earth nickelate heterostructures. [1, 2] Using a combination of synchrotron-based experiment and first principles theory, we identify key structural features in LaNiO3 thin films grown by molecular beam epitaxy, allowing one to correlate differences in physical structure with electronic transport properties and x-ray absorption spectroscopy measurements.
Using general design principles, such as inversion symmetry breaking and charge transfer, which lead to degeneracy breaking of the Ni 3d orbitals, we design and fabricate tri-component superlattices to control the electronic and orbital properties of rare-earth nickelate compounds to achieve a two dimensional, single band electronic structure at the Fermi energy.
[1]Chen et. al. Phys. Rev. Lett. 110, 186402 (2013)
[2]Kumah et. al. Adv. Materials 26, 1935 (2014)

Multiferroic order in functional perovskite oxides enable novel physics as well as memory and logic devices that cannot be achieved using conventional semiconductors. Although a number of single-phase multiferroic materials such as TbMnO₃, BiFeO₃, doped TiO₂ and doped-BaTiO₃ have been demonstrated, most of those materials are not multiferroic at room temperature. Here, we demonstrate room temperature ferromagnetism and ferroelectricity in Cobalt-substituted SrTiO₃ thin films (SrTi_0.70Co_0.30O_3-δ, STCo30) on conducting (Si, Nb-doped STO) and insulating (STO (100), STO (110) and thick SiO₂ films on Si) substrates. Ferroelectric hysteresis and magnetoelectric coupling measurements, electrical resistivity, current and capacitance vs. voltage, PUND tests, structural characterization (x-ray diffraction and photoelectron spectroscopy, transmission electron and atomic force microscopy) and density functional theory simulations indicate that STCo30 films are ferromagnetic and ferroelectric. Ferromagnetism has been found to originate from mixed-valence states of Cobalt ions and oxygen vacancies. Ferroelectricity is due to anisotropic lattice expansion.

Neodymium doped ceria (NDC) is a promising candidate for use as an electrolyte for solid oxide fuel cells operating at low to intermediate temperatures and we as in thin film applications due to its high ionic conductivity, low electronic conductivity, and good sinterability and stability. However, interfaces, defects, and strain have been shown to hinder ionic movement and decrease conductivity. These effects are typically magnified as the electrolyte dimensions decrease into the nanometer scale. To separate the effects of interfaces from strain and defects, single crystal and multigrained lamellae of NDC were cut using a focused ion beam, fashioned into coplanar platinum electrode structures, and tested using broadband impedance spectroscopy. Using this fabrication technique, valuable insight into interfacial dielectric dead layers which deleteriously affect the permittivity of ferroelectrics has been uncovered. Inspired by these results, here we gain equally valuable insight into the ionic conduction of doped ceria systems. The separate influence of electrode interfaces and grain boundaries on localized and non-localized conduction will be presented. Based on these results a set of guidelines for the development of enhanced electrolytes will be discussed.

Multiferroic materials have been the focus of significant research and development during the past decade owing to their inherent multi-functionality applicable to energy generation, tunable communications, as well as data storage and processing. Of the multiferroics, none has garnered more attention than BiFeO3 because of strong coupling between its primary order parameters—ferroelectricity, anti-ferromagnetism, and a canted ferromagnetic moment—which are robust over a wide thermal range including room temperature [1]. Several functional properties of BiFeO3 have been linked to the types of domain walls (71°, 109° or 180°) present and their configuration. Additionally the domain walls themselves exhibit intriguing transport and magnetic behavior very different from the bulk [2], and each type of domain wall has its own signature properties [3]. Periodic arrangements of domain walls open a pathway to controlling functional properties in BiFeO3 and adjacent materials, and they readily lend themselves to be studied by resonant soft x-ray scattering (RSXS).
RSXS directly probes charge and orbital degrees of freedom, providing chemical and structural information not obtainable by standard x-ray or neutron scattering techniques [4]. We have used RSXS at beamline 4.0.2 of the Advanced Light Source to study the domain configurations of epitaxial BiFeO3 thin films, and the influence of those domain walls on the surrounding material. We demonstrate how RSXS can be used to identify statistical properties of domain configurations across hundreds of microns laterally, such as average domain size, distribution, orientation, and even domain wall type. Notably, we have used Q-dependent scattering to recognize commensurate structural order shared between BiFeO3 domain walls and neighboring correlated electronic materials whose transport properties are readily influenced by their immediate environment. Moreover, we reveal how periodic domain structures (40-150 nm wide) modulate magnetic order [5] and change the electronic structure of iron 3d-like states within BiFeO3 using polarization-dependence of diffraction patterns near the iron L-edge. Insights into the interplay of domain configurations, electronic structure, and magnetic order are fundamental to understanding and engineering the functionality of multiferroic materials like BiFeO3.
References
[1] S. V. Kiselev et al., Sov. Phys. Dokl. 7, 742 (1963); J. R. Teague et al., Solid State Commun. 8(13), 1073 (1970).
[2] H. Béa et al., Phys. Rev. Lett. 100, 017204 (2008); L. W. Martin et al., Nano Lett. 8(7), 2050 (2008).
[3] J. Seidel et al., Nat. Mater. 8, 229 (2009); Q. He et al., Phys. Rev. Lett. 108(6), 067203 (2012).
[4] S. B. Wilkins et al., Phys. Rev. Lett. 90(18), 187201 (2003); S. S. Dhesi et al., Phys. Rev. Lett. 92(5), 056403 (2004); J. Schlappa et al., Phys. Rev. Lett. 100(2), 026406 (2008).
[5] H. A. Dürr et al., Science 284, 2166 (1999); E. Benckiser et al., Nat. Mater. 10, 189 (2011).

Recent advancements in thin film epitaxy have enabled single crystalline oxides to be grown on semiconductors, opening a pathway to introduce new functionalities into semiconducting devices. Critical to integrating the functionalities of oxides into semiconducting devices is controlling the band alignment at heterojunctions between the two materials. Crystalline SrTiO₃, which has been investigated as a gate dielectric and exploited as an epitaxial buffer for the growth of other oxides, has a type-II band alignment with respect to Si, Ge, and GaAs. However, a type-I band alignment is needed in order to integrate the dielectric, ferroelectric, and spintronic functionalities of oxides into semiconducting devices. Here we present a band engineering approach to modify the band gap and achieve a type-I alignment between a crystalline oxide and a semiconductor. Single crystalline SrZr_xTi_1-xO₃ is grown on Ge (100) using reactive molecular beam epitaxy. The band gap of SrZr_xTi_1-xO₃ and band offset with respect to Ge is tuned through Zr content x. Scanning transmission electron microscopy shows that atomically abrupt and structurally coherent interfaces between SrZr_xTi_1-xO₃ and Ge can be obtained through careful control of deposition conditions. A type-I band offset is demonstrated through current-voltage and capacitance-voltage measurements, in which the former shows significantly reduced gate leakage with Zr content, and the latter indicates that accumulation and inversion in SrZr_xTi_1-xO₃-Ge heterojunctions can be achieved.

We have demonstrated that unusual ferroelectric polarizations and characteristic domain structures emerge in PbTiO₃ nano-metamaterials with Archimedean lattices using a sophisticated real-space phase-field modeling, where the intrinsic effect of surfaces are explicitly included through the careful parameterization based on the quantum-mechanics data. In the thermodynamic equilibrium, the ferroelectric nano-metamaterials consist of both normal domains with a rectilinear polarization and unusual domains with a ferrotoroidic polarization where polarization forms several vortices. As a result of the formation of complicated normal and unusual domains, the spontaneous polarization forms a “Labyrinth-Like” polar structure, in which the polarization almost-randomly walks around the “road” of metamaterials. The polarization labyrinth was found to strongly depend on the microstructure of metamaterials, indicating that the shape of metamaterials control the polar labyrinth structures. Furthermore, the coexistence of ferroelectric and ferrotoroidic domains enables us to tailor the toroidal moments by the external electric fields through the nonlinear interaction between rectilinear polarization and toroidal moment. These results indicate that the novel order parameters of polar labyrinth consisting of ferroelectric polarization and ferrotoroidic moments can be controlled by microstructure of metamaterials and can be switched by external electrical loading. The labyrinth order parameter emerged in nano-metamaterials can open a new avenue for realization of novel functional device paradigms.

Preferential crystal growth of one-axis-oriented bismuth ferrite (BiFeO₃) film was accomplished on versatile substrates using metal-oxide nanosheets with pseudo-perovskite crystal structure. This research aims for aligning the polar axis of BiFeO₃ crystals on versatile substrates without epitaxial surface of perovskite structure, such as silicon, glass or metal wafers, that can result in enhanced polarization behavior as frequently reported in the previous researches for epitaxial BiFeO₃ films on specific single crystals such as SrTiO₃, DyScO₃, etc. The key concept of the present research mainly lies on the application of Ca₂Nb₃O₁₀ nanosheets (ns-CNs) [1,2] because they consist of single crystal platelets with pseudo-perovskite structure that can provide epitaxial crystal surface similar to (001) perovskite plane on the versatile substrates independent of substrate species. We believe that this technique will be of great help for constructing thin-film electronic devices with enhanced ferroelectric components integrated on certain substrates.
In this research, therefore, we attempt to fabricate one-axis oriented BiFeO₃ thin films on various substrates buffered with ns-CN layers. The ns-CN layer was supported on substrates (i.e., platinized Si or SUS316, etc.) by dip-coating of aqueous ns-CN dispersion prepared from chemical exfoliation of KCa₂Nb₃O₁₀ precursors. [1,2] The ferroelectric BiFeO₃ films were subsequently grown by chemical solution deposition (CSD) method on these substrates. Both the BiFeO₃ films crystallized at 600^oC on ns-CN-buffered Pt/Si and Pt/SUS316 substrates exhibited preferential crystal orientation of BiFeO₃(100) plane normal to the substrate surface, whereas the films on bare Pt/Si and Pt/SUS316 substrates consisted of randomly-oriented BiFeO₃ crystals. These results indicated that the ns-CN buffer layer on surfaces would induce the “cube-on-cube” growth of BiFeO₃ crystals to the preferential crystal orientation. The BiFeO₃(100) on ns-CN/Pt/SUS exhibited larger out-of-plane lattice parameter, 0.3990 nm, than that on ns-CN/Pt/Si, 0.3897 nm, due to the difference in thermal stress derived from the thermal expansion/shrinkage of film / substrate bilayers. P-E measurements for these films showed enhanced polarization (~50 mC/cm²) for the BiFeO₃ film on ns-CN/Pt/SUS substrate, compared with moderate ones (~30 mC/cm²) for the film on Pt/SUS, Pt/Si and ns-CN/Pt/SUS substrates. We concluded that enhanced polarization on BiFeO₃ films could be achieved by preferential crystal growth using ns-CN buffer layer together with compressive stress application from the substrates.
[1] K. Takahashi, et al., Appl. Phys. Lett. 89 (2006) 082901., and many other articles.
[2] Y. Minemura, et al., Jpn. J. Appl. Phys. 52 (2013) 09KA04.

~SbVO₄ plays a key role in the catalysis for the ammoxidation of propane to acrylonitrile. It exhibits an amazing structural flexibility involving cation vacancies, changes in oxidation states and different degrees of order-disorder, ranging from short range order (SRO) to superstructures and structural modulations [1,2]. In this work we have studied the rich structural and magnetic phenomena that occur in the solid solution Sb_xV_1-xO₂ (0 le; x le; 0.5) and their evolution with x. We have synthesized the samples by heating the stoichiometric amounts of Sb₂O₃, V₂O₅ and VO₂ at 800°C under argon atmosphere according to the following reaction stoichiometry:
x [Sb₂O₃ + V₂O₅] + (1 - 2x) VO₂ → 2x SbVO₄ + 2(1 - 2x) VO₂ → 2Sb_xV_1-xO₂
During the reaction all Sb³⁺ cations are oxidized to Sb⁵⁺ while all V⁵⁺ cations are reduced to V³⁺. This implies the substitution 2V⁴⁺ → Sb⁵⁺+ V³⁺ in the basic rutile-type VO₂ matrix. In the Sb-richest phase, ~SbVO₄, most of the vanadium is V³⁺, as confirmed by EELS spectroscopy, magnetic susceptibility and neutron diffraction, showing the latter antiferromagnetic ordering at T_N < 50K. Electron diffraction shows the presence of intense SRO in the form of wavy two-dimensional sheets of diffuse intensity in the reciprocal space, running perpendicularly to c* between the diffraction maxima. Very weak SRO can be observed at Sb_0.33V_0.67O₂ and magnetic ordering happens at lower temperatures (T_N~6K). For Sb_0.25V_0.75O₂ a different type of SRO appears and it can be observed by electron diffraction as very intense two-dimensional sheets of diffuse intensity forming a three-dimensional net of edge-sharing octahedra in reciprocal space. For Sb_0.1V_0.9O₂ the same SRO can also be observed by electron diffraction but this phase presents a structural transition similar to that of VO₂ though at lower temperature (51°C). This transition can be observed by electron diffraction when heating with the electron beam, from SRO to a two-fold superlattice and back to SRO as the temperature is lowered. The magnetic susceptibility curve of this phase is very similar to that for VO₂, with magnetic phase transitions above room temperature, indicating the presence of spin#8208;Peierls distortion due to V⁴⁺ (S=½) pairing, giving rise to the SRO. For VO₂ (x=0) the Raman spectrum is strong and the peaks correspond to the monoclinic phase. For x > 0 the spectrum becomes very weak and broad, due to the transition to the semiconducting phase and the electrostatic screening by free charges, as in the tetragonal phase of VO₂. For x=0.5 the Raman spectrum becomes strong again due to the transition to an insulating monoclinic phase.
Authors thank the Spanish Government for financial support (project MAT2011-27192).
[1] A. R. Landa-Cánovas, J. Nilsson, S. Hansen, K. Staahl and A. Andersson, J. Solid State Chem. 116, 369 (1995).
[2] A. R. Landa-Cánovas, F. Javier García-García, S. Hansen. Catal. Today 158, 156 (2010).

The typical strongly correlated (SC) electron oxide (La,Pr,Ca)MnO₃ (LPCMO) shows the insulator - metal transition (IMT) with the change in conductivity of several orders of magnitude from the charge ordering insulator (COI) phase at a high temperature to the ferromagnetic metal (FMM) phase at a low temperature through the transition temperature T_c [1,2]. In the vicinity of the T_c, the so-called coexistence region, the FMM and COI phases are both present. The domain dynamics, i.e., the temperature-dependent competing population (X(T)) between the FMM and COI phases governs the dc conductivity (σ_dc(T)) of an SC material. To realize a simultaneous investigation of σ_dc(T) and X(T) related to IMT, THz time domain spectroscopy (THz-TDS) has been employed in this study. The temperature-dependent conductivities at dc and THz frequency range from 0.5 THz to 2.4 THz, σ_THz(omega;,T), have been obtained for a 350-nm-thick LPCMO film in the temperature region from 10 K to 250 K. The conductivity increased with decreasing temperature, which corresponded to the IMT in LPCMO. To describe the σ_THz(omega;,T) behavior corresponding to the coexistence regime and estimate THz conductivity in the vicinity of 0 THz, σ₀(T), and X(T) concurrently, we have proposed a insulator-metal composite model by incorporating the Austin-Mott model, which characterizes the hopping of localized electrons and the Drude model, which explains the behavior of free electrons: σ_THz(omega;,T)=(1-X(T))(σ₀^I(T)+Aomega;^0.5)+X(T)(σ₀^M(T)/(1+tau;²omega;²)), (1) where σ₀^I(T) and σ₀^M(T) are the THz conductivity at 0 THz for the insulator and metal phases, respectively. The power parameter in the Austin-Mott part is fixed at 0.5 for all temperatures, which characterizes a system with strong electron-electron interaction [3]. Equation (1) showed the good agreement with the observed σ_THz(omega;,T) curves at all temperatures within the fitting error of about 10%. The σ₀(T) was numerically determined by extrapolating σ_THz(omega;,T) curve to 0 THz. The obtained σ₀(T) changed by about five orders of magnitude through the Tshy;_c, approximately 130 K during the cooling process, closely reproducing the σ_dc(T) curve. Simultaneously, the evaluated X(T) revealed the phase condition, namely the pure insulator phase at temperatures above 130 K with X = 0, the coexistence of metal and insulator phases at 90 K < T < 130 K where 0 < X < 1, and the metal phase at temperatures below 90 K with X = 1. Our composite model illustrates the potential for the investigation of dc transport dynamics for SC materials in a contact-free manner using the THz-TDS technique which can be effectively applied for various samples with any shape and size. In the presentation, our detail analyses and results will be discussed. References: [1] L. Zhang et al., Science 298 (2002) 805 [2] Y. Yanagisawa et al., Appl. Phys. Lett. 89 (2006) 253121 [3] M. Dressel, G. Grüner, Electrodynamics of Solids, Cambridge University Press, Cambridge (2002) p.329

Single phase Bi₃Fe₅O₁₂ (BIG) thin films were grown on (111)-oriented gadolinium gallium garnet (GGG) substrates by using mist chemical vapor deposition (CVD) method. Mist CVD method is an emerging method known as a potential candidate for low-cost, large-area, and industrial-scale process. The apparatus is simple and inexpensive, and so long as reagents can be dissolved in solvents and ultrasonically atomized, thin film can be grown efficiently and homogeneously under atmospheric pressure. The precursor solution was prepared by dissolving tris(acetylacetonato)iron(III), Fe(C₅H₈O₃)₃, and 2-ethylhexanoic acid solution containing bismuth(III) 2-ethylhexanoate (Bi:25%), Bi(C₈H₁₅O₂)₃, in N,N-dimethylformamide. The precursor solution was ultrasonically atomized by using a 2.4 MHz transducer, and the mist particles were transferred to the reaction area with nitrogen gas. The substrate temperature and process time was set as 300 °C and 30 min, respectively. Because as-deposited thin films were amorphous, post-annealing was carried out at higher temperatures. Various molar ratios of Fe(C₅H₈O₃)₃ / Bi(C₈H₁₅O₂)₃ were examined, and as a result, when the ratio of [Fe] / [Bi] of the precursor solution is equal to 2.162, the single phase of BIG can be obtained. X-ray diffraction (XRD) measurements reveal that only the 444 line of BIG and GGG appears in the out-of-plane X-ray diffrac-tion pattern and only 211 and 422 diffraction lines are observed in the in-plane XRD pattern. The measurements indicate that the present BIG thin film is epitaxially grown on the GGG substrate. From the analysis of Rutherford backscattering spectrometry signals, it is found that the distribution of elements is homogeneous in depth, and the chemical composition of the thin film is determined as Bi : Fe = 0.600, which agrees well with the stoichiometric composition. The atomic force microscopy (AFM) image reveals that the present BIG thin film has a relatively flat surface compared with BIG films prepared by physical vapor deposition methods. The arithmetic average surface roughness and root mean square surface roughness calculated on the basis of AFM measurement is 1.416 and 1.795 nm, respectively. The wavelength dependence of Faraday rotation angle of BIG thin films annealed between 450 and 550 °C suggests that the BIG phase grows above 450 °C and begins to be converted into stable phases at 550 °C. The optimal post-annealing temperature deduced from the wavelength dependence of Faraday rotation angle is around 530 °C. The maximum Faraday rotation angle is theta;_F = 38.9 deg/mu;m at 413 nm and the theta;_{F 633nm} is -6.6 deg/mu;m. Consequently, the mist CVD method is a cost-effective technique to obtain BIG thin films.

The sol-gel process is one of the effective and popular methods to fabricate one-dimensional ferroelectric nanotubes (1D FE-NTs). Generally, the sol-gel process consists of five steps, i.e. synthesis of sol-gel solution, solution deposition, drying, pyrolysis, and crystallization. Among the steps, the pyrolysis may significantly affect the morphology, perovskite phase formation, structure, crystalline orientation, and tetragonality. Thus, it is expected that the pyrolysis step plays an important role in governing ferroelectricity and piezoelectricity of 1D FE-NTs. In this work, we report the effects of pyrolysis temperature on morphology, structure, and tetragonality of PbTiO₃ nanotubes (PTO-NTs). The PTO-NTs were fabricated by spin coating of PTO sol-gel solution in porous anodic alumina followed by pyrolysis at 400-600 °C for 30 min. Then, all PTO-NTs were crystallized at 700 °C for 30 min in oxygen. All PTO-NTs had a 420 nm outer diameter, 5 nm wall thickness, and 20 mu;m length. X-ray diffraction patterns showed that all PTO-NTs had polycrystalline tetragonal PTO structures. We found that the tetragonality of PTO-NTs increased from 1.0316 to 1.0419 as the pyrolysis temperature increased from 400 to 600 °C. Note that the tetragonality of bulk powder is ~ 1.06. Dielectric constant of PTO-NTs pyrolyzed in 400 °C is about 200 which are similar with that of bulk PTO materials. The correlation between tetragonality and ferroelectricity as pyrolysis temperature increases will be discussed.

We report on the growth and characterization of the epitaxial layered Ruddlesden-Popper La_n+1Ni_nO_3n+1 on LAO substrate using Laser MBE as the growth technique. La₂O₃ and NiO targets have been used as the ablated materials. Here the focus was on reflection high energy electron diffraction as the in-situ control of the growth rate as well as studying the dynamics of atomic layers during the growth. Better understanding of the growth dynamics and correlating it with RHEED intensity behavior can be a powerful technique for the control of the interface in more complex engineered materials as well as ultra thin films. In addition to RHEED, we used atomic force microscopy for the control of growth mode and surface quality. X-ray diffraction, reflection and absorption spectroscopy also have been used for characterization.

Oxide ionic conductivity in the laminated film with nanometer thickness is attracting much interest because of the possibility of improved performance of high temperature devices such as SOFCs. In this study, layered thin film using Cu- and Ga-doped Pr₂NiO₄ and Sm-doped CeO₂ (PNCG/SDC) was fabricated by pulsed laser deposition (PLD) method, and changes in oxide ion diffusivity and charge carrier were studied using Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS).
It was found that the SDC in prepared laminated films have strains, and that the strain is strongly related with the roughness of the substrate. Moreover, tensile SDC layer and compressive PNCG layer were localized especially on the grain boundary of MgO substrate. In our previous report, the electric conductivity improved with decreasing the thickness of SDC layers, and this change in conductivity seemed to be assigned to the contribution of oxide ion conductivity. The lateral oxide ionic diffusivity in laminated film was measured using the isotope oxygen exchange technique to confirm the effect of localized strain. There is no enhancement of oxide ionic diffusivity compared with the bulk sample. However, the oxygen diffusion behavior across the thin film shows the difference between the tensile part and non-stress part. This report suggests that improved oxide ionic diffusion part is non-homogeneity, and local strain formed increased oxide ion diffusivity effectively.

The market for piezoelectric actuators is currently dominated by the ubiquitous lead zirconium titanate (PZT) for its outstanding properties. However, the main element in PZT, lead, is highly toxic and increasing regulation of materials and products containing lead is occurring in Europe, Japan, and a growing number of other countries seeking to ban the use of lead. Therefore, research in lead-free piezoelectric materials is increasing. The lead-free solid solution of (Bi_0.5Na_0.5)TiO₃-(Bi_0.5K_0.5)TiO₃ (BNT-BKT) has been intensively studied and the morphotropic phase boundary (MPB) composition (80BNT-20BKT) exhibited high T_C, high d₃₃, and high k. It was found that the addition of Bi(Zn_0.5Ti_0.5)O₃ (BZT) or Bi(Mg_0.5Ti_0.5)O₃ (BMT), increased the high field piezoelectric strain coefficient (d₃₃*) in 60BNT-40BKT compositions. High electrical fatigue resistance was also observed in both BZT and BMT ternaries. Because of the high d₃₃* values and high fatigue resistance, these lead-free compositions were looked at for piezoelectric actuator applications such as stack actuators, buzzers, and cymbal transducers.
Three compositions of xBZT-(0.60-x)BNT-0.40BKT, where x = 0.025, 0.05, and 0.10, and three compositions of yBMT-(0.60-y)BNT-0.40BKT, where y = 0.01, 0.05, and 0.10 were looked at for stack actuators containing three piezoelectric elements. It was found that bonding piezoelectric elements together with a silver conductive epoxy degraded displacement performance contrary to expected increases in displacement from observation of strain displacement loops. Applying conductive tape to bond piezoceramic elements may improve linear displacements of the stack actuators. Greater success was observed in buzzers where two compositions: 0.025BZT-0.575BNT-0.40BKT and 0.01BMT-0.59BNT-0.40BKT were bonded to brass discs with silver conductive epoxy. An audible chirp was observed around 17 kHz using an analog function generator with a sinusoidal function applied to 0.025BZT composition. Impedance spectroscopy confirmed resonance in this buzzer around this frequency. In the 0.01BMT buzzer, an audible chirp was heard around 8 kHz using the same settings as the 2.5% BZT buzzer.

The interface properties of heteroepitaxially grown polar materials have received much attention due to the observation of quantum confined 2D carrier gasses and other emergent phenomena. Epitaxial integration of lattice-matched rocksalt oxides with wide-bandgap semiconductors offers a path toward 2D interface conductivity at an oxide-nitride interface. It becomes conceivable, then, to envision devices combining the wealth of non-linear dielectric, magnetic and optical properties hosted in complex oxide materials, with the technologically relevant nitride semiconductors. Unfortunately, a smooth, commensurate, single domain oxide-nitride interface remains elusive due to the structural and chemical dissimilarity of the components. Recently, however, a new surfactant-assisted MBE growth technique has been used to stabilize the (111) rocksalt facet in MgO and CaO films on GaN and promote 2D layer-by-layer growth. In this case, water serves as both the oxidant and surfactant, hydroxylating the (111) surfaces and lowering the free-energy of the (111) planes relative to the (100) planes. While this method can eliminate the natural faceting tendency of (111) rocksalts, oxide films still possess two in-plane rotation domains related to the step and terrace morphology of the GaN substrate. The substrate 6₃ screw axis results in bilayer terraces of ½ the unit cell height (c/2), which are chemically identical but rotated 60° with respect to each other. TEM and HRSEM indicate that oxide films grown on stepped GaN substrates are locally coherent within a terrace but separated by disclinations at step edges. Production of single domain oxides requires modification of the substrate to remove the bilayer fluctuation and produce step-free GaN. We demonstrate that selected area epitaxial (SAE) growth of GaN on native substrates can provide the necessary step-free template. In this case, epitaxial GaN growth is confined to finite regions patterned by a SiO₂ hardmask. At low Ga supersaturations, adatom surface migration is enhanced, terrace width increases and on native GaN substrates, which possess dislocation densities < 10⁵ cm^-2, bilayer steps may be effectively swept out of the selected-area. These SAE GaN substrates represent the ideal template for commensurate oxide heteroepitaxy and HRSEM images indicate that rocksalt thin films nucleate and grow single domain on large-area terraces without disclination defects. Collectively these techniques elucidate the nature of structural defects at the polar rocksalt - nitride interface and more generally any epitaxial system that exhibits a transition from wurtzite to FCC crystal structure. We believe that the marriage of surfactant-assisted oxide PVD with SAE grown GaN represents a transformational approach to oxide-nitride heteroepitaxy, offering heretofore unachievable interface perfection.

The role of A-site acceptor (1mol%Na) and donor (2mol%Bi) doping in lead-free piezoelectric ceramics based on 1-x(Bi_0.5Na_0.5TiO₃)-xBaTiO₃ where x=0.055, 0.06 and 0.07 (i.e. near the morphotropic phase boundary) was investigated in this study. All ceramics showed a single perovskite phase with pseudo-cubic symmetry. A significant change in dielectric and piezoelectric properties was observed in all donor-doped and acceptor-doped compositions, respectively. Compared to the stoichiometric composition, the acceptor-doped compositions displayed an increase in coercive field (E_c) with the observation of a small but measureable internal bias (E_i), which is an indication of domain wall pinning in typical hard Pb(Zr_xTi_1-x)O₃. This result was further confirmed via PUND tests and remanent P-E hysteresis measurements where the same trend was still observed. Moreover, all A-site acceptor-doped compositions in this study also exhibited an increase in mechanical quality factor (Q_m), a decrease in d₃₃ value, a lower remanent polarization (P_r) and a lower dielectric permittivity, which are all characteristics of the effects of hardening that is typically seen in PZT. To the best of our knowledge, this is the first time that the hardening was observed in BNT-based systems when compared to B-site acceptor doped counterpart where a moderate or no hardening was observed at all.

The epitaxial growth of crystalline oxides on compound semiconductors offers the possibility of incorporating unique multifunctional properties of oxides into various existing semiconductor technologies. The successful growth of epitaxial SrTiO₃ (STO) film on Si substrate using Molecular Beam Epitaxy (MBE) has proved that it is feasible to monolithically integrate the functional oxides with high mobility compound semiconductors [1-3]. While STO has been also deposited on GaAs without amorphous interfacial layers [4], the exact interface structure has been controversial; while Scanning Transmission Electron Microscopy (STEM) analysis indicates As/Sr interface layers [5], X-ray diffraction (XRD) measurement shows signs of Ga/SrO interface [6]. Using ab initio density functional theory calculations, we determine the interfacial structure of the SrTiO₃/GaAs heterostucture. We demonstrate that forming a fully oxidized layer directly on top of GaAs substrate is thermodynamically unstable. Instead, an oxygen-depleted Sr metal layer stabilizes the SrTiO₃/GaAs interface, in accordance with STEM measurement. We also show that the interface structure observed by XRD is possible under oxygen-rich conditions. The identification of different interface structures and the corresponding growth conditions can be useful for development of growth processes of oxide/semiconductor heterostructures.
This work was supported by US AFOSR through Contract No. FA9950-10-1-0133.
References
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High carrier density oxide heterostructures have attracted significant interest owing to their rich physical phenomena and potential for technological applications. Typically, oxide heterostructures are grown on single crystal ceramic substrates. Eventual application of these oxide heterostructures will involve integration with common semiconductor substrates, such as silicon, and epitaxial growth of high carrier density oxide conductors will allow for integration of their diverse functionalities with conventional silicon-based electronics. Moreover, using silicon as a substrate allows for higher operating powers of oxide devices due to its higher thermal conductivity.
Here, we demonstrate conducting oxide interfaces between SrTiO₃ (STO) and GdTiO₃ (GTO) grown on silicon using reactive molecular beam epitaxy (MBE). To grow these oxides with high carrier concentration, the growth conditions must avoid the oxidation of silicon, the formation of silicates, and silicides. An additional challenge is the tendency of rare-earth titanates to form the pyrochlore phase, which further narrows the growth window for the heterostructure.
Within this growth window, the GTO-STO heterostructures we have grown have atomically abrupt interfaces on silicon, as confirmed using reflection high-energy electron diffraction, x-ray diffraction and reflectivity, and atomic force microscopy. The electrical transport and structural properties of these hetersotructures grown on (LaAlO₃)_0.3(Sr₂AlTaO₆)_0.7 (LSAT) substrates are similar and feature a high sheet density of electrons. Current densities as high as 1 A/mm are observed on silicon heterostructures, with low thermal degradation of the mobility, opening possibilities for high power applications for conducting oxide interfaces grown on silicon.

Ceramics materials, exhibit superconducting properties important for technological applications. The main property, magnetic response strongly depends on the route of synthesis, processing method, and particularly the stoichiometry of the material. In this work, structures were studied nanoparticulate compound YBa₂Cu₃O_7-δ synthesized by sol-gel electrospun by electrospinning method. It also sought to better understand the physical-chemical processes present in the sample preparation steps. Was used as the source of yttrium nitrate and copper acetate, and barium nitrate. The viscosity of material was obtained by addition of the polymer polyvinylpirrolidone (PVP). The resultant gel and treated nanowires were formed at temperatures of 900 and 800 °C, then were characterized using, X-Ray Diffraction X (XRD) for displaying the crystallographic phases and Scanning Electron Microscopy (SEM) to obtain the morphology of the samples.

We report on the improved multiferroic properties of BiFeO₃ (BFO) through atomic substitutions of rare-earth La and higher valence transition metal Ta at Bi (A) and Fe (B) sites, respectively. Bi_0.9La_0.1Fe_0.97Ta_0.03O₃ (BLFTO) ceramic was prepared by conventional solid-state reaction method. The single phase formation of BLFTO was confirmed by X-ray diffraction analysis. This co-substitution leads improved multiferroic properties in terms of higher remanent magnetization (M_r) and polarization (P_r) of 3.50 emu/g and 1.25mu;C/cm² respectively as compared to pure BFO. The possible explanations of as obtained higher values of magnetization and polarization can be explained in terms of suppression of spiral spin structure because of La doping and reduced leakage current by eliminating the oxygen vacancies due to high valence Ta doping, respectively. The current -voltage characteristics of the Pt/BLFTO/Pt metal-insulator-metal capacitor structure confirmed the reduction in leakage current about one order of magnitude lesser than the pure BFO. Further, the dielectric measurements showed low dielectric loss and higher value of real part of dielectric permittivity especially due to the Ta doping which suppressed the motional charge densities created by oxygen vacancies. Hence, the improvement in the multiferroic properties has been obsereved in co-doped BLFTO.

A laser MBE-based atomic layer-by-layer growth technique has been studied. By monitoring the reflection high-energy electron diffraction (RHEED) intensity, various oxide thin films and heterostructures can be assembled one atomic layer at a time, forming the desired structure with the expected thickness and composition. This technique has been applied to the homoepitaxial growth of SrTiO₃, using SrO and TiO₂ targets, and the epitaxial growth of LaAlO₃ on SrTiO₃, using La₂O₃ and Al₂O₃ targets. Stoichiometry and structures of the films are confirmed by x-ray diffraction and Rutherford backscattering spectrometry. UV Raman spectroscopy was used to probe the symmetry breaking due to the cation off-stoichiometry. Similar stoichiometry control as shown by reactive MBE has been demonstrated. It has been shown that both polar and nonpolar materials can be grown by this method.

In the present work, an electric-field-induced giant strain response and excellent photoluminescence-enhancement effect was obtained in a rare-earth ion modified lead-free piezoelectric system. Pr³⁺-modified 0.93(Bi_0.5Na_0.5)TiO₃-0.07BaTiO₃ was designed and fabricated by a conventional fabrication process. Temperature and composition dependence of the ferroelectric, dielectric, piezoelectric, and photoluminescence properties were systematically investigated and a schematic phase diagram was established. The Pr³⁺ substitution was found to induce a transition from ferroelectric long-range order to relaxor pseudocubic phase with short-range coherence structure. Around a critical composition of 0.8 mol% Pr³⁺, a giant reversible strain of ~0.43% with normalized strain S_max/E_max up to 770 pm/V was obtained under an electric field of ~5 kV/mm. Furthermore, the in-situ electric field induced an enhancement of the photoluminescence intensity by ~40% in the modified system. The present findings have great potential for actuator and multifunctional device applications which may also open up a range of new applications.

Classical pyrochlore oxides have the general formula A₂B₂O₇ where A is generally a trivalent cation and B is a smaller teravalent cation. These materials exhibit unique structural characteristics due to their structure, which can be considered an ordered defective fluorite structure with anion vacancies. Pyrochlores normally show high chemical stability, high catalytic activity, excellent ionic conductivity [1]. Depending on the identity of A and B, some pyrochlores can be used in upconversion processes [2]. Several rare earth pyrochlores also exhibit magnetic frustration, leading to spin-ice materials [3].
The conventional solid state route is carried out by mixing the precursor oxides and submitting the mixture to several high temperature treatments (above 1400°#8203;C) followed by grinding, to ensure elemental dispersion, resulting in high energy consumption.
The formation of pyrochlore structures depend heavily on the ratio between A and B ionic radii, as well as the synthetic route.
In this contribution we report the synthesis of Y_xGd_2-xTi₂O₇ by a modified sol-gel polymeric precursor citrate method, where an acidic Gd(NO₃)₃/Y(NO₃)₃ solution is added to an alcoholic titanium alkoxide/citric acid solution, followed by calcination at different temperatures.
High temperature in situ x-ray diffractograms show that the pyrochlore phase forms at about 725°C. Low temperature magnetization experiments reveal that the magnetic ground state is highly influenced by the oxigen deficiency in the samples. A further heat treatment at 1100°C in an O₂-rich atmosphere is necessary in order to eliminate O^2- vacancies and completely oxidize magnetic Ti³⁺ to non-magnetic Ti⁴⁺, a lower temperature in comparison to conventional solid-state routes.
X-ray diffractometry also shows that the lattice parameter decreases as the larger Gd³⁺ ions are substituted by smaller Y³⁺ ions in the A sites.
Morphologically, our materials exhibit aggregates of spherical nanoparticles with diameters in the 25-50 nm range, without signs of phase segregation.
Using this route, we were able to prepare different pyrochlore phases, such as Nd₂Ti₂O₇, Er₂Ti₂O_7, Dy₂Ti₂O₇ and Ho₂Ti₂O₇. However, attempts to prepare RE₂Zr₂O₇ (RE = Gd, Nd, Er, Dy) using this route led to fluorite phase materials.
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The 2D electron gas observed at the interface of a polar (LaAlO₃) and non-polar (SrTiO₃) interface has potential for oxide electronics. Controlling and enhancing the carrier density and electron mobility, critical for device applications, would require a detailed understanding of these interfaces. This interface two dimensional electron gas (2DEG) exhibits a metal insulator transition between 3 to 4 unit cells of LaAlO₃. Using reflection high energy electron diffraction (RHEED) we show that the insulator to metal transition of the LaAlO₃/SrTiO₃ (LAO/STO) interface after the third layer of LAO occurs at 65% LAO surface coverage. At this critical value of surface coverage we further show that the electron mobility is highly sensitive to the surface coverage, carrier density (varied by back gating) and Kondo scattering (seen at high carrier concentrations). We relate these effects to interface strain, carrier-carrier scattering and magnetic scattering from possible Ti³⁺ defects created by the back gating.

RNiO3 perovskites (R: rare-earth) have been extensively studied in the last few years since the report of metal-to-insulator (MI) transitions as a function of temperature that systematically vary with the rare-earth size. These strongly correlated electron systems often show metal-to-insulator transitions, whose microscopic origin is of on-going interest.
Neodymium nickelate (NdNiO₃) is a correlated oxide that exhibits a metal-insulator transition (MIT) close to 200K and is of interest for advanced electronics and optoelectronics. In this work, we have synthesized NdNiO₃ nanotubes using a template-assisted method. For this, precursor solution with stoichiometric amounts of Nd-Ni was prepared by polymeric precursor route, than this solution was deposited in commercial anodized aluminum mesoporous membranes (AAO) under vacuum for different times (0.5, 1 and 2 h). Additionally, these depositions were performed using polymeric resins with different viscosity values, ranging from 5 to 100 cP, to verify the influence of viscosity in the filling of the pores of the AAO template. All samples were heat treated at temperatures ranging from 350 to 750°C under oxygen pressure of 1 and 100 bar. The samples were characterized by X-ray diffraction, scanning electron microscopy (SEM), differential scanning calorimetry (DSC) and magnetization as a function of temperature (M(T)). The structural characterization revealed that samples crystallize in orthorhombic-distorted perovskite structure (Pbnm space group symmetry). Images of SEM revealed ultrafine grains assembled in NdNiO₃ nanotubes, the size of those grains vary from 20 to 30 nm. Those tubular samples present external diameter close to 250 nm and length ranging from 1 to 10 microm. The paramagnetic/antiferromagnetic transition could not be verified through M(T) measurements, although DSC measurements revealed a phase transition close to 200 K for both samples. Such transition could be related to a metal-insulator transition that occurs close to 200 K for NdNiO₃ bulk samples. In order to verify these results, these samples are going to be characterized through transport properties measurements. The synthesis and properties of the NdNiO₃ nanotubes was not related in literature yet.

Currently we explore multiferroics and dependence of their properties on thickness and growth conditions. These ferroelastic layer combinations, which hold the unique capability of being piezoelectric, ferroelectric, and ferromagnetic, have the ability to be polarized by both electric and magnetic fields. Barium Titanate (BTO) and Lanthanum Strontium Manganite (LSMO) layers are utilized on Strontium Titanate (STO) and Lanthanum Aluminate (LAO) substrates grown by pulsed laser deposition (PLD) using two methods: traditional partial pressure oxygen and radio-frequency (RF) oxygen plasma. The films grown by both methods show epitaxial layers of both BTO and LSMO. Although LSMO thickness are constant for each sample, BTO thicknesses were varied for exploration of BTO effects on the magnetization of the LSMO as well as the ease of polarization of the BTO layer. Mechanical properties were explored using X-Ray reciprocal space residual stress measurements and X-Ray topography. These results were compared to that of nanoindentation measurements. Surface topography for structural differences of the two growth methods are explored using atomic force microscopy. Results show a dramatic difference in the structural organization of films grown in RF oxygen plasma, which can be attributed to the more complete filling of oxygen vacancies within the oxide films. A link between mechanical and electrical properties is drawn using ferroelectric measurements and is used to show the connection between growth conditions, structural properties, and electrical properties.

Transition metal oxides display a rich variety of solid state phenomena that arise from a coupling of structural degrees of freedom to strong electron and magnetic interactions on the transition metal sites. The ability to fabricate transition metal oxide thin films and heterostructures with atomic layer control using molecular beam epitaxy provides a route to induce new electronic phases in oxide layers with thicknesses on the order of a unit cell. Here, we demonstrate that electronic transport in nickelate films can be manipulated through changes in the ionic charge of the surface termination due to a strong coupling of the surface electrostatic properties of (001) oriented LaNiO₃ thin films and the structural properties of the Ni-O bonds that govern electronic conduction. Using a combination of transport measurements, first principles density functional theory, and synchrotron-based x-ray absorption measurements, we study the electronic and structural properties of LaNiO₃ thin films. We observe ionic buckling in the film planes, with the direction of buckling dependent on the net ionic charge of the surface terminal layer. Films terminated with positively charged planes result in less distorted NiO₂ planes, resulting in bulk-like Ni-O bond properties and higher conducctivity compared to films terminated with negatively charged planes. The coexistence of metallicity and symmetry-breaking polar distortions in these nickelate films provides an ideal system for investigating the properties of polar metals.

Interfaces between transition metal oxides exhibit a variety of novel exciting properties as a result of the complex reconstruction of the charge, spin, orbital and lattice interactions. With the recent advances in deposition techniques, interface engineering of oxide compounds has become a recognized strategy for tailoring the properties of these materials. In this context, we recently reported on the observation of exchange bias in superlattices composed of nominally paramagnetic metallic LaNiO₃ and semiconducting ferromagnetic LaMnO₃ ultrathin layers [1], which exemplifies how interface engineering can induce a complex magnetic structure in the non-magnetic material LaNiO₃.
Here, we emphasize the critical role of structural characteristics on the resulting functionalities of oxide heterostructures. Particularly, we investigate the properties of LaNiO₃-LaMnO₃ bilayers, i.e. (001)-SrTiO₃//8 u.c. LaNiO₃/8 u.c. LaMnO₃. Transmission electron microscopy (TEM) measurements have revealed that the interface LaNiO₃/LaMnO₃ is much sharper than the LaMnO₃/LaNiO₃ one, which displays two monolayers intermixing. Surprisingly, this structural asymmetry results into very distinct transport and magnetic properties. Particularly, enhanced conductivity and extremely reduced magnetization is observed for the “sharp interface” samples in comparison to the rougher ones. State-of-the-art synchrotron techniques (XAS, XMCD, XLD, resonant reflectivity) enable us to reproduce the magnetic profile of the systems as well as to elucidate differences in interfacial charge transfer and induced magnetic moment in the Ni atoms. The role of orbital occupation will also be explored.
[1] Gibert et al., Nat. Mater. 11, 195 (2012).

In any material, the defects can strongly perturb the properties, either electrical or mechanical. Point defects in ferroelectric materials are very important and often control performance and device limitations of capacitors and piezoelectrics. As we look forward into the future, the applications pulls on ferroelectric and dielectric material will demand higher electric fields and higher temperatures. Here we will discuss new insights into oxygen vacancy behavior in perovskite-based dielectrics. First, we will consider the defects that are established under equilibrium conditions that can be quenched in and are determined by partial Schottky reactions. Second, we will consider defect complexes that are resulting from acceptor and acceptor-donor pairs in the formulation. Third, we will consider the degradation process in multilayer ceramic capacitors and a new equation that predicts meantime to fail. Finally, we consider ferroelectric materials with very high oxygen vacancy concentrations and consider ferroelectrics under the universality of the Mott Criterion, and the possible new applications such as thermoelectrics.

Dielectric materials designed for high operating temperatures are needed for electric vehicle systems, geothermal drilling applications, and SiC and GaN-based power electronics technologies. New materials are required because existing ceramic capacitor technologies are not compatible with extreme environments, especially at temperatures higher than 200 0C. In recent years, several research groups have developed new BaTiO₃ - based materials that exhibit promising dielectric properties for high temperature applications. This presentation will focus on materials based on BaTiO₃ - Bi(Zn_1/2Ti_1/2)O₃ (BT-BZT) that have demonstrated that a temperature-stable permittivity can be obtained over a broad temperature range. The crystallographic structure of BT-BZT was investigated through high resolution X-ray diffraction and neutron pair distribution function studies. These results revealed that with the addition of BZT, the structure underwent a transition from tetragonal symmetry to pseudo-cubic symmetry. However, the data also showed clear evidence of tetragonal distortions at the local scale even in the compositions with apparent cubic symmetry. The relaxor characteristics seen in this solid solution and many similar compounds are likely due to the chemical disorder introduced through the addition of Bi³⁺ onto the A-sublattice and Zn²⁺ onto the B-sublattice of the perovskite structure. The temperature dependence can be manipulated through the addition of other compounds such as BiScO₃, BiInO₃ and NaNbO₃, for example. For example, the dielectric characteristics of 50BT-25BZT-25NaNbO₃ vary less than ±15% over the temperature range from -55 to 200 0C. Another critical property for high-energy capacitor applications is the high temperature resistivity which ultimately underpins the breakdown strength. While the resistivity of pure BT is dominated by ionization of oxygen vacancies, the addition of BZT causes a change to an intrinsic conduction mechanism along with a significant increase in resistivity. In conclusion, BT-BZT relaxor dielectrics show promising dielectric properties with a high resistivity and a high permittivity over broad temperature ranges which is of interest for high energy capacitor applications.

The structural phases and lattice parameters of many perovskite-based oxide epitaxial thin films are sensitive to external stimuli such as electric field, temperature, and chemical environment. These structural changes often signify a change in ionic/electrical conduction, ferroelectric domain structure, or catalytic activity. In traditional coherent epitaxial thin films, structural changes perpendicular to the growth direction are limited because the in-plane lattice parameters are fixed to those of the growth substrate in order to maintain good crystalline quality. This in-plane clamping effect can significantly change the threshold for phase transformations in such systems. Materials integration through epitaxy also restricts the types of materials that can be grown on one another; in general, the crystal structure and lattice constants need to be nearly matched. We show here that we can remove the substrate constraint in oxide thin films and combine dissimilar materials with oxide nanomembrane (NM) synthesis through the release of single-crystalline oxide thin films from the original growth substrate. By measuring structural changes in ferroelectric NMs with in-situ synchrotron x-ray techniques, we can monitor changes in polarization, domain structure, and structural phase as a function of chemical environment and temperature.
We use elastically-relaxed, single-crystalline SrRuO₃ (SRO) nanomembranes as growth templates for Pb(Zr, Ti)O₃ (PZT) thin films. SRO films are grown on SrTiO₃ (001) (STO) substrates where the lateral size and shape is defined by a selective epitaxy process. The SRO NMs are then released from the STO growth substrate via selective etching. The freestanding SRO NMs are very thin (~50 nm) with large aspect ratios (length:thickness ge; ~10⁴), and readily bond to most surfaces because they are flexible and can conform to a new host substrate. This includes substrates that allow a portion of the SRO to remain freestanding (i.e., the SRO NM can be transferred over a hole in the new host substrate). Once transferred, we use the SRO NM as a template for growth of epitaxial PZT. We measure structural changes (in-plane and out-of-plane) in the PZT/SRO heterostructure as a function of temperature and oxygen partial pressure with in-situ synchrotron x-ray techniques. Our oxide NM synthesis technique represents a significant advancement in materials integration and provides a useful way for probing fundamental materials properties in freestanding thin films.

In recent years, Na_0.5Bi_0.5TiO₃ (NBT) based solid solutions have emerged as potential replacements for conventional lead-based piezoelectric materials. NBT is a relaxor ferroelectric that forms the basis of a number of lead-free piezoelectric systems. Perovskites that are relaxors are characterised by the presence of nano-sized polar regions and an apparently pseudo-cubic structure on the macroscopic scale. NBT is perhaps unusual in that it has, in addition to relaxor behaviour, a series of macroscopic phase transitions [Jones & Thomas, 2002] and planar defects of some type [eg Kreisel et al., 2003] that make a full description of the structure and identification of its individual changes particularly challenging.
Functional electroceramics are poled by applying an external electric field. In relaxor materials this can cause a significant change in the macroscopic structure. In classical ferroelectrics such as Pb(Zr, Ti)O₃ (PZT), this polarisation is lost when the material is heated through a phase transition, but NBT ceramics spontaneously depole when heated above T_D ~ 150 °C [Davies et al., 2011] via a mechanism that is not well-understood but does not coincide with a change in symmetry [Gorfman et al., 2012] and cannot be thought of as a conventional phase transition. This depolarisation temperature is observed in many NBT-based systems and has been frequently mistaken for a conventional phase transition.
Data from a variety of methods will be presented to give an insight into some of the structural changes in NBT-based materials that take place on heating up to and through T_D, both with and without electric field. Particular attention is paid to the role of defects in promoting symmetry states that are not attained in fully stoichiometric material.
GO Jones & PA Thomas (2002). Acta. Cryst. B58, 168.
J Kreisel et al., (2003). Phys. Rev. B68, 014113.
M Davies, E Aksel & JL Jones (2011). J. Am. Ceram. Soc. 94, 1314.
S Gorfman et al., (2012). J. Appl. Cryst.45, 444.
*Visiting student from School of Material Science and Technology, Indian Institute of Technology, Varanasi

We have performed in-situ x-ray diffraction during the growth of a number of finely layered BaTiO₃/SrTiO₃ superlattices using synchrotron radiation at X21 at the NSLS, BNL. The high quality superlattices were grown using an off-axis RF magnetron sputtering techniques and, due to the large strain exerted on BaTiO₃ by the SrTiO₃ substrates on which they were grown, were ferroelectric during the growth process. Through the use of a novel scanning techinque and a Pilatus area detector, snapshots which contained information about lattice parameters, superlattice repeat distance and ferroelectric domain structure were captured at short time intervals while the superlattices grew, enabling the construction of "movies" of the evolving structural parameters of the growing superlattices.
Analysis of the data shows several interesting features. Firstly, the evolution of the average out of plane lattice parameter of the superlattice suggests that the strain polarization coupling of the interfaces in the superlattices is quite different from the interior of thicker layers and the bulk materials. Secondly, the domain structure of superlattices grown on SrRuO₃ bottom electrodes evolves differently from those grown on SrTiO₃ alone. While the fact that electrostatic boundary conditions have an effect on ferroelectric domain structure is hardly surprising, being able to monitor the evolution of this structure during the growth presents an excellent opportunity to monitor how this structure evolves as a function of the individual layer thicknesses and the total thickness of the evolving structure. Finally, besides the fundamental knowledge gained from these studies, being able to monitor the structural parameters of a growing ferroelectric superlattice at this level of detail, provides numerous insights which can guide the growth of higher quality ferroelectric superlattices in general.

Functional oxide films and their interfaces can lead to exciting and new research areas due to their tunable emergent physical and chemical properties, such as magnetoresistance, superconductivity, two-dimensional electron gas behavior, memristive behavior (programmable resistive switching), and electrocatalytic functionality. New types of tunable electrocatalysts could provide opportunities to overcome the current limits of energy storage and conversion systems, e.g., water dissociation and formation. However, understanding of the electrochemical reaction mechanisms on complex oxide surfaces and interfaces is far from complete, particularly with regards to the structure-function relationships of surfaces in aqueous and non-aqueous solutions (ionic liquids) as well as under applied electric fields. Furthermore, since most of functional perovskite oxides systems are strongly correlated electron systems, the carrier concentration and oxygen stoichiometry can be effectively controlled with ionic liquid gating.
To elucidate the physical and electrocatalytic properties of oxide surfaces and interfaces, it is necessary to build a model system and employ in-situ experimental tools. We chose model perovskite-structured SrRuO₃ (SRO) ultrathin films, grown on Nb-doped SrTiO₃ (001), (110), and (111) substrates and determined their structure-function relationships during the oxygen evolution reaction, using a variety of in-situ synchrotron x-ray techniques [1]. Interestingly, we found that for the case of (001)-oriented SRO, the c-axis (out-of-plane) lattice parameter could be reversibly changed with at different potentials in aqueous and non-aqueous solutions (ionic liquid). We will discuss the kinetics and the evolution of the electron density profile of SRO ultrathin films under an applied gate voltage, where the full out-of-plane structure was determined with Coherent Bragg Rod Analysis (COBRA). Understanding the interrelationships among activity, stability, and the structural properties of complex oxide surfaces provides new strategies for the creation of highly stable and active electrocatalysts. Work at Argonne, including use of the Advanced Photon Source, is supported by the U.S. Department of Energy, Office of Science, and Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357.
[1] S. H. Chang et al., Nature Communications5, 4191 (2014).

There has been significant interest in utilizing ferroelectrics for next generation non-volatile memory storage, taking advantage of the spontaneous polarization state to encode information. However, a fundamental understanding of the switching dynamics and associated speed limits does not currently exist. Here we apply intense quasi-single-cycle terahertz (THz) pulses of duration ~1 picosecond to BFO and BTO thin films in order to drive large amplitude modulations of the ferroelectric polarization, which is measured using both second harmonic generation and femtosecond x-rays as time-dependent structural probes. We use metallic slit structures as field enhancement devices to apply peak THz fields greater than 1 MV/cm and show that the structural and nonlinear optical properties of ferroelectric and multiferroic oxides can be dramatically modulated. X-ray scattering measurements with 100 femtosecond resolution in BTO thin films show long-lived THz-driven strains scaling quadratically with the electric field amplitude when the applied field is perpendicular to the ferroelectric polarization, with additional transient modulations in the x-ray structure factor reflecting the positions of the atoms within the unit cell. Nonlinear optical studies in Sm-doped BFO show large amplitude THz-driven modulations in the second harmonic generation efficiency up to 15,000% near the morphotropic phase boundary. These measurements define new photoferroelectric effects in nanoscale ferroelectric thin films, and define novel opportunities for dynamic control of their functional properties.

Perovskite oxides have long been studied for use in electronic devices due to their novel electronic and magnetic effects. Of particular interest are perovskites exhibiting ferroelectricity, particular in thin films. Ferroelectric perovskites such as BiFeO₃ (BFO) show promise for use in memory storage devices due to their excellent power efficiency and speed. Additionally, ferroelectric effects can be coupled to both ferro- and antiferromagnetism either in a heterostructures or through inherent multiferroic coupling of a single phase film. In order for perovskite oxides to be incorporated into modern device structures, the mechanisms behind ferroelectric switching must be explored and understood. In particular, the ferroelectric domain wall kinetics in the presence of defects must be accounted for and the effects of one-, two- and three-dimensional defects with domain walls must be characterized. While recent advances have been made in the study of domain wall-defect interactions, these studies are still in their infancy and must be further expanded in order to be understood.
We use a biasing holder to apply DC voltage to BFO thin films in the TEM. Alterations to the bias magnitude, polarity and electrode geometry allow for control of the internal electric field and thus the ferroelastic domains. Epitaxial strain has been shown to have a profound effect on the switching dynamics of ferroelectric perovskite films with various defects functioning as nucleation or pinning sites. Changes in epitaxial strain lead to varying defect conditions at the substrate/film interface. By limiting and controlling both the initial nucleation and propagation and the relaxation of ferroelastic domains, defects are an important factor in determining the overall domain kinetics in ferroelectric oxides. We perform a full ex-situ and in-situ characterization of defect densities and interactions with domains under an applied electric field in order to quantify the effects of defects on domain motion. As a result of these combinations of techniques, we are able to determine the effects of both line and point defects on the switching dynamics of ferroelastic domains.

The orthorhombic perovskite structure is very popular for the formula ABO₃ with the geometric tolerance factor t = (A-O)/[ radic;2(B-O)] < 1. The octahedral tilting system is developed in order to accommodate the bonding mismatch due to t < 1. The structure of GdFeO₃ is exemplified for the structural distortion since both Gd³⁺ and Fe³⁺ are not Jahn-Teller active and the orthorhombic distortion is relatively large. What is missing in the GdFeO₃-type distortion is that octahedra are not rigid; the local distortions evolve as the buckling angles M-O-M change. In this talk, I will show first how to extract the local distortions and how they influence the physical properties. In RTiO₃ and RVO₃, the change of local distortions biases the orbital ordering therefore the spin-spin interaction. In La_1-xSr_xMnO₃, a sharp change of magnetic properties can be triggered by a modest uniaxial pressure applied along the a or b axis. These results can be interpreted in the context of a competition between the external stress and intrinsic site distortions of the structure. The local distortion is also important to explain the anomaly of bulk modulus found universally in families of the orthorhombic RMO₃, where M is a 3d transition metal. The primary pressure effect on the structure is to reduce the bonding splitting in octahedra. How the M-O-M bond angle changes under pressure can be well-explained in connection with this primary pressure effect.

To achieve the dream of designing theoretically new materials with desired properties, we must first describe existing materials with accuracy consistent with technological specifications. There are materials that can be predicted qualitatively and sometimes quantitatively with standard approximations of density functional theory (DFT). However, there is also a large class of materials in which there are outstanding quantitative and often qualitative problems. This challenging class includes the transition metal oxides (TMOs), where errors in DFT approximations, presumably due to self-interaction errors, have precluded reaching agreement with experiment without readjusting the theory to match experimental properties. This kind of approach, while valuable to support and explain experiment, lacks of predictive power. With these limitations, theory often follows experiment rather than lead. TMOs present a variety of properties that range from transparent conductors, super-conductors, magnetic, multiferroic etc. TMOs may thus have significant technological impact, facilitating energy generation, transport and storage. The development of theoretical methods that can describe ab-initio these properties accurately, without any experimental input, is key to design better TMOs. Our team focuses on many-body ab-initio methods that go beyond DFT. In this talk, we describe some ongoing calculations using the highly accurate diffusion Monte Carlo methods for a) the formation energies of point defects in ZnO b) the magnetic coupling constants in cuprates and c) the quasi-particle energies in superconducting cuprates. We also discuss how existing theoretical methods can be corrected using benchmarks coming from DMC to improve model Hamiltonians and density functional approximations.
Research supported by the Materials Sciences & Engineering Division of the office of Basic Energy Sciences, U.S. Department of Energy (ERKCS92). PRCK was supported by the Scientific User Facilities Division, Office of Basic Energy Sciences, U.S. Department of Energy. Computational time used resources of the Oak Ridge Leadership Computing Facility at the ORNL, which is supported by the Office of Science of the U.S. Department of Energy under contract No. DE-AC05-00OR22725.

Many electronic devices utilize functional oxides that contain lead, such as zirconia lead titanate, Pb(Zr,Ti)O₃,or PZT. Thin films of PZT are typically in contact with metallic connecting electrodes composed of Pt, Au, or Ir. Over time, the formation of new intermetallic alloys, such as Pt₃Pb, for example, has been observed at the PZT/Pt interface. Density functional theory (DFT) calculations are used to quantify the migration barriers of Pb atoms across the Pt/PZT and Pt₃Pb/PZT interfaces to determine the way in which this transport depends on the composition and orientation of the interface. The interfacial energies and work of adhesion are also determined. The DFT calculations are additionally used to optimize the properties of lead-free functional oxides that may ultimately replace PZT in electronic devices. This work is supported by the National Science Foundation through grants DMR-1207293 and DMR-1307840.

Strain mismatch has been used to effectively control the electronic ground states of epitaxially-grown complex oxides. Recent interest has focused on incorporating strain to alter the e_g¹ orbital degeneracy of Ni³⁺ in LaNiO₃. Differences in orbital occupation (e.g. polarization) due to strain-induced splitting can influence electrical transport, magnetic, and possibly superconductive properties. Since it is known that compressive strain will increase the orbital polarization out-of-plane, there is also the possibility for increased orbital overlap with surface adsorbents, which is a common feature of many electrochemical catalysts. We therefore explore the effects of strain-induced polarization on the catalytic activity of LaNiO₃ for the oxygen reduction reaction (ORR), which is essential for the next generation of energy storage materials, including fuel cells and metal-air batteries. While bulk LaNiO₃ is a known catalyst for the reaction, the effects of strain on modulating the room-temperature activity are hitherto unexplored. By epitaxially depositing (001) LaNiO₃ thin films on lattice-mismatched substrates (LaAlO₃, NdGaO₃, SrTiO₃, and DyScO₃), we could cohesively tune the strain from -1.2 to 2.7% .These strained films were characterized in a modified rotating disk electrode (RDE) setup specifically designed for use with complex oxide thin films. We found that the catalytic activity of LaNiO₃ did indeed increase with compression and decrease under tension. In fact, at 850 mV vs RHE, we found that the ε = -1.2% film enhanced the ORR current an order of magnitude compared to unstrained (bulk) LaNiO₃ while, conversely, the ε = +2.7% film had an order of magnitude lower current than the bulk . These contrasting results due to strain-induced orbital polarization clearly delineate the potential for tailoring catalytic activity through epitaxial oxide control.
The work was supported by the U.S. Department of Energy, Basic Energy Sciences, Materials Sciences and Engineering Division and by the LDRD Program of ORNL. Use of the Advanced Photon Source was supported by the U. S. Department of Energy, Office of Science, under Contract No.DE-AC02-06CH11357.

Oxygen vacancies are often present in complex oxides as point defects and their effect on the electronic properties is typically uniform and isotropic. Exploiting collective behavior of oxygen vacancies in order to generate novel structures and functional properties remains a challenging goal. In this presentation we show that epitaxial strontium chromite films can be transformed, reversibly and at low temperature, from cubic, metallic perovskite SrCrO_3-x (SCO, x<0.1) to rhombohedral, semiconducting SrCrO_2.8. As the concentration of oxygen vacancies in perovskite SrCrO_3-x increases, the vacancies start to interact and aggregate into {111}-oriented SrO₂ planes interleaved between layers of tetrahedrally-coordinated Cr. In turn, these planes aggregate into ordered arrays so as the oxygen-deficient SrO₂ planes are separated by ~1 nm.
Using ab initio simulations, we reveal the atomic scale mechanisms leading to this ordering including aggregation of the isolated oxygen vacancies into planar quasi-2D structures and diffusion of these structures along the crystalline axes of the perovskite lattice. We also analyze the character of charge density distribution and atomic rearrangements in the vicinity of the oxygen-deficient SrO₂ planes and discuss their effect on the packing of these planes. Furthermore, our simulations reveal that the structural motifs of the oxygen-deficient SrO₂ planes allow for the Grotthuss-type mechanism of the O^2- ion diffusion and predict that the potential energy barrier for this diffusion path is significantly lower than that in the cubic SrCrO_3-x. This property is of considerable importance in solid oxide fuel cells where fast O^2- diffusion reduces the required operating temperature.

Transition metal and rare earth oxides host a diverse array of physical properties from ferroelectricity and catalytic activity to superconductivity and emergent magnetic phenomena. In many cases knowledge of, and control over, the type and number of defects, and their distribution within the solid, is critical to the observed behavior. In this talk I will highlight examples where solid state powder synthesis, optical floating zone crystal growth, and structural and physical properties characterization (across many lengthscales) are combined to elucidate the role of defects and non-stoichiometry in complex oxides. One specific example is that of the pyrochlore Pr₂Zr₂O₇. This relative of cubic zirconia is a candidate quantum spin ice compound whose low temperature specific heat and magnetic properties are extremely sensitive to preparation method. We find that the sensitivity arises from local changes in stoichiometry due differential volatilization, oxidation, and spinodal decomposition coupled to low-energy vibrational dynamics of the Pr₄O framework. By precisely controlling the floating zone growth conditions, it is possible to control for these effects and produce cm³-sized crystals of intrinsic Pr₂Zr₂O₇. As these methods are applicable to a wide range of other oxides, the future for intrinsic single crystals other complex oxides is bright.

Through MBE studies of oxide thin film heteroepitaxy, we have experimentally observed interesting and unexpected surface and interface bonding structures in both simple (MgO, TiO₂) and more complex (Fe₃O₄, Fe₂O₃, BaTiO₃, BaFe₁₂O₁₉) oxide films with SiC and GaN. Investigations into the iron oxide bonding state control of various Fe_xO_y films on SiC and MgO, has led to questions of kinetic control of nucleation and growth that may help elucidate needed process control parameters for effective integration of functional and multi-functional oxide heterostructures integrated on semiconductor substrates. X-ray Photoelectron Spectroscopy, Reflection High-Energy Electron Diffraction, and High-Resolution Tunneling Electron Microscopy of surfaces and interfaces of the following heterostructures will be discussed and compared: MgO/SiC, MgO/GaN, Fe₂O₃/MgO, Fe₂O_3/MgO/SiC, Fe₂O₃/SiC, BaFe₁₂O₁₉/MgO/SiC, BTO/MgO/SiC, and Fe₂O₃/BTO/Fe₂O₃/SiC. Preliminary magnetic and ferroelectric properties of films and structures will be presented. These, in turn, have led to questions of formation and diffusion of anti-phase domain boundaries as a function of interface structure in thin films. In this presentation, I will attempt to stimulate a discussion of what could be common phenomena throughout complex oxide/semiconductor interfaces, in order to identify new understandings might enable us to engineer effective interfaces for multiple material systems.

Oxide films are usually deposited on commercial single crystals. However, we have been developing here a new strategy that uses well-characterized polycrystalline samples where each grain of the substrate can be viewed as a single crystal of a particular orientation.
This approach called Combinatorial Substrate Epitaxy (CSE), in which an infinite number of crystallographic orientations at the surface of the sample are present, can be used to make statistical analyses of the growth modes across epitaxial orientation space.
In this paper, we apply the CSE to various oxide films, including the isostructural Ruddlesden-Popper compounds. For this, we grow a Ca2MnO4 thin film by the pulsed laser deposition technique onto a spark plasma sintered Sr2TiO4 polycrystalline substrate.
The local epitaxial growth of films was investigated by Electron Backscatter Electrons Diffraction and Transmission Electron Microscopy to determine phase formation and preferred epitaxial orientation relationships, further developing our high-throughput synthetic approach.
The thickness-dependence of Ca2MnO4 films will also be investigated.
Finally, other compositions like BiFeO3 and Sr2FeMoO6 will been presented, demonstrating the potential of CSE in the design and growth of a wide range of complex functional oxides.
Partial support of IDS FunMat program (Erasmus Mundus), the LabeX EMC3 (ANR) and MEET (EU) projects are acknowledged.

Pulsed laser deposition (PLD) is the growth method that has had the most impact on the rapid development of complex oxide materials research. Although it has been used in practical film growth for more than 3 decades, the mechanism of PLD remains poorly understood. The non-equilibrium aspects are the most intriguing and most widely recognized characteristics that are unique to PLD. However, there is no clear understanding of what non-equilibrium behavior constitutes and how it affects the film growth process. In this talk I present a breakdown of the mechanism of PLD that is derived from real time surface x-ray diffraction (SXRD) data obtained during homoepitaxial SrTiO₃ film growth. The measurements were performed by collecting the diffracted intensity at the (0 0 ½) anti-Bragg point before, during and after each single laser shot using a CCD camera. The SXRD data are organized into two dimensional intensity maps that contain the complete information about formation and evolution of the surface layer. The specular component of the diffracted intensity describes the distribution of atoms into layers along the surface normal. It was used to develop a time-dependent quantitative description of surface coverage during growth. The diffuse component provides a correlation length that describes the spatial distribution of islands on the growing surface. We developed a growth model that describes the coverage dependence resulting from the interplay of random nucleation and cooperative island growth that occurs by incorporation of growth species at the edges of already existing islands. This cooperative island growth is driven by rapid interlayer transport that is facilitated by the energetic species delivered by the laser plume. The growth curves at the extremes of the nucleation density range have a distinctly different shape and reveal that PLD does not occur by one single growth mechanism. Instead, the changing shape of the growth curves and the island size distributions extracted from diffuse scattering show that the surface structure evolves toward equilibrium that is governed by factors such as the substrate temperature and the dwell time between laser shots, covering the entire span starting from high density random nucleation and ending with cooperative island growth dominant at vanishingly low nucleation densities. The cooperative island growth mode is of universal significance because the minimum interface width is determined by the interaction potential between the growth species and the surface independent of the growth method.
Research supported by the US Department of Energy, Basic Energy Sciences, Materials Sciences and Engineering Division, and performed at the Advanced Photon Source, a DOE-BES user facility.

Many complex oxides are mixed ionic and electronic conductors (MIECs) with partial equilibrium conductivities depending on temperature, doping and oxygen partial pressure. However, in numerous applications these complex oxides are operated upon a DC voltage load and thus severe deviations from equilibrium properties are often found. The corresponding modification of the properties is caused by voltage driven ion transport in conjunction with interfacial resistances. In solid oxide fuel cells or electrolysis cells (SOFCs, SOECs), for example, MIECs are frequently used as electrodes and the overpotential (polarization) of such an electrode strongly affects its ionic and electronic conductivity as well as its catalytic activity. Ferroelectric/dielectric oxides are generally also employed upon field load and ion motion may strongly affect their insulation resistance. In resistive switching devices the resistance change due to ion motion is even used for information storage.
This contribution emphasizes the link between all these phenomena: DC measurements (partly including ambient XPS studies), impedance spectroscopy and ¹⁸O tracer ion injection under field load are used as methods to monitor the drastic effects of voltage on the MIEC properties. For example, it is shown that the electrochemically active zone of mixed conducting thin film electrodes ((La,Sr)FeO₃, Sr(Ti,Fe)O₃, (Ce,Gd)O₂, etc.) on YSZ strongly varies with applied voltage, both in reducing and oxidizing atmosphere. Moreover, spatially varying oxygen vacancy distributions upon field load, onset of grain boundary conduction and resistance degradation are discussed in polarized lead zirconate titanate (PZT). Finally, several stoichiometry polarization related phenomena in SrTiO₃ thin films are presented, including occurrence of inductive loops in impedance spectra measured under bias.

The characterization of oxide catalysts is often limited by heterogeneity of exposed surfaces and the composite nature of electrodes within fuel cells and electrolyzers.¹ Epitaxial thin films can provide well-defined surfaces of known orientation, the nature of which can be tuned through interaction with the substrate, via strain and variation of the electronic structure.^2,3 We have fabricated (001) epitaxial films of LaCoO₃ on substrates with lattice mismatch ranging from -0.5 to +2.6% in order to investigate the relationship between strain/spin state⁴ and catalytic activity for the oxygen reduction and evolution reactions (ORR, OER) in an alkaline environment. The application of tensile strain increases the catalytic activity, with the effect being more notable for ORR. We further find that film thickness has greater influence on activity, with thicker films (>10 nm) being less active. Electrochemical measurements using the facile redox couple [Fe(CN)₆]^3-/4- to probe conductivity in situ suggest that these trends result in part from the ability to inject charge, arising from changes in the electronic structure of the film. These measurements illustrate new ways to tune catalytic activity employing thin film techniques on complex oxides.
References:
[1] J. Suntivich, H. A. Gasteiger, N. Yabuuchi, and Y. Shao-Horn, Journal of the Electrochemical Society 157, B1263 (2010).
[2] W.T. Hong, M. Gadre, Y.-L. Lee, M.D. Biegalski, H.M. Christen, D. Morgan, and Y. Shao-Horn, Journal of Physical Chemistry Letters 4, 2493 (2013).
[3] K.A, Stoerzinger, M. Risch, J. Suntivich, W.M. Lü, J. Zhou, M. Biegalski, H. Christen, A. Ariando, T. Venkatesan and Y. Shao-Horn, Energy & Environmental Science 6, 1582 (2013).
[4] W.S. Choi, J.-H. Kwon, H. Jeen, J.E. Hamann-Borrero, A. Radi, S. Macke, R. Sutarto, F. He, G.A. Sawatzky, V. Hinkov, M. Kim, and H.N. Lee Nano Letters 12, 4966 (2012).

Understanding cation inter-diffusion across the solid electrolyte-cathode oxide interface is critically important for improving the long timescale durability of solid oxide fuel cells (SOFCs). Demonstrated on an oxide heterointerface between an 8%molY₂O₃-stabilized ZrO₂ (YSZ) and a La_0.65Sr_0.3MnO_3-x (LSM) typically used in SOFC, time-dependent evolution of the interdiffusion profiles of multiple cations is numerically simulated by combining a phase-field diffuse interface description with diffusion equation. The simulated interdiffusion profiles agree with independent experimental data in both time and space domains at different temperatures.

Interfaces between dissimilar oxides are attracting significant interest for their potential role in accelerating charge transport and surface reaction kinetics. If well understood and controlled, they can provide a new way to enable high-performance solid-oxide fuel cells, separation membranes as well as fast switching memristors. For example, recent studies have demonstrated that cobaltite hetero-interfaces exhibit orders of magnitude faster oxygen reduction kinetics compared with either single phase. The interfacial strain fields, anisotropy, and electronic interactions between the two phases are the likely mediators behind such an unprecedented enhancement. The underlying mechanisms must be understood quantitatively, so that we can go beyond isolated and empirically found interface structures to rationally designing dissimilar oxide interfaces with superior properties. Towards this goal, we have investigated the local electronic structure at nanometer resolution in model multilayer superlattices and vertical nanostructures that are made of dissimilar cobaltites. To accomplish this, we used a novel combination of in-situ scanning tunneling spectroscopy and focused ion beam milling. We found that the wide band-gap cobaltite is electronically activated at elevated temperatures through an interfacial coupling with a reducible cobaltite. Such electronic activation is expected to facilitate charge transfer to oxygen, and accelerate the reduction kinetics on the surface. Furthermore, based on our computational and experimental work, we have put forth elastic strain to be an important driver of the kinetics of surface reactions and diffusion in functional oxides, and demonstrated these concepts on fluorite, perovskite and Ruddlesden Popper structures. There remains still a large amount of open questions on how dissimilar oxide interfaces impact oxygen diffusion and oxygen exchange on the surfaces. However, these recent results are encouraging for an improved understanding of oxide hetero-interfaces at elevated temperatures and could enable new interfaces with fast oxygen transport and oxygen reduction kinetics.

Ionic conductivity in the solid state must be significantly enhanced in order to facilitate future advancements in a range of technologies including oxygen permeation membranes, sensors, batteries and fuel cells. In many cases, the traditional approach of chemical and structural optimization have only led to incremental increases in conductivity.
To overcome these limitations, alternative routes to fast ion conduction have been explored, including the engineering of nano- and heterostructures. Recent publications have reported up to 8 orders of magnitude enhancement in oxide ionic conductivity by the latter route. The origin of this effect, and even the accuracy of its measurement, however, remain under debate.
Three principle arguments have been proposed to explain the observed phenomena, including formation of space charge regions altering defect concentrations, relaxation of interfacial misfit by the formation of dislocation networks presenting alternative diffusion pathways and interfacial misfit-induced strain altering the ionic migration barrier. The latter arguably presents the greatest scope for ionic conductivity maximization. In all measurements reported thus far, this strain state parameter has been manipulated by changing the interfacial lattice misfit by replacing either the secondary material in the multi-layered heterostructure or the substrate on which the film is grown. This technique is, however, accompanied by many potential sources of error arising from inter-sample variability, including reducibility, impurity content, film growth quality, film thickness, dislocation density, and the sharpness of interfaces.
In this study, we describe a method for directly measuring the effect of strain on ionic conduction in ceramics, a phenomenon we refer to as ionic piezoresistance, by mechanically varying the strain state using a novel mechanical-electrical testing rig. This allows us to avoid the limitations discussed above and directly establish piezoresistive coefficients for ionic conduction in all crystallographic orientations for the first time. Preliminary results are presented and discussed in light of earlier published models.

The manganites have emerged as a cornerstone of research in the past two decades, due to the extraordinarily rich spectrum of phenomena they exhibit including half-metallicity [1], giant magnetoresistance [2], spatially inhomogeneous phase transitions [3], and magneto-elastic coupling [4]. In thin films, properties of the manganites can be heavily influenced by strain [5], oxygen stoichiometry [6], surface adatoms and e.g. presence of reconstructions [7]. Yet, few attempts have focused on obtaining atomic resolution of the as-grown surfaces of these complex oxide thin films, due to the inherent difficulty of sample preparation, and even fewer attempts have been made at manipulating surfaces with the Scanning Tunnelling Microscopy (STM) tips to determine the links between physical properties and local atomic ordering. Here, we report atomic manipulation with STM on the surfaces of 25 unit-cell thick La_5/8Ca_3/8MnO₃ (LCMO) grown on TiO₂-terminated SrTiO₃ (STO) substrates. We demonstrate that by applying triangular waveforms of increasing amplitude to STM tips in-situ, with the Z-feedback off, it is possible to form oxygen vacancies, remove atoms from layers below, rearrange atoms in the surrounding lattice, and therefore cause reactions to occur at the atomic level. Interestingly, experiments reveal that little activity is induced at negative biases, and that most of the reactions occur at about ~+2.3V, coinciding with a current spike or hysteresis loop opening in the current traces. These experiments open the pathway to big-data studies of oxide surfaces as combinatorial libraries, where the energy for vacancy formation can be directly correlated with local atomic structure within a single sample, and provides a template for studying physical and electrochemical processes at the single atom scale, allowing greater insight into electronic phenomena in the manganese oxides than has been previously possible.
References
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This research was sponsored by the Division of Materials Sciences and Engineering (RKV, AT, SVK) and by the Scientific User Facilities Division (APB) of BES, DOE. Research was conducted at the CNMS, which is sponsored at ORNL by the Scientific User Facilities Division, BES, DOE.

Complex oxides with perovskite structure have long attracted attention of researchers due to the vast array of functional properties and physical behaviors that they exhibit. Many properties such as ferroelectricity, magnetism, ionic and electronic conductivity are closely coupled to structural parameters. Modern Scanning Transmission Electron Microscopy (STEM) allows high precision mapping of local structure in every unit cell, including strain, polar displacements, degree and type of octahedral tilts. In many ionic perovskites, structure is closely coupled to oxygen content, in turn enabling local quantitative measurements of oxygen content from microscopy data. Electron Energy Loss Spectroscopy (EELS) allows for local probing of chemical composition, as well as electronic and magnetic structure, making the combination of STEM and EELS ideal for discovery of structure-properties correlations at atomic scale. Theoretical studies using Density Functional and Ginsburg-Landau approaches help uncover physical mechnisms underlying such correlations.
Tracking different order parameters in the vicinity of the BiFeO₃ surface, we can resolve discrepancies in the estimates of the ferroelectric “dead layer” thickness obtained by different techniques. In LaFeO₃/SrFeO₃ digital superlattices, we can observe unusual coupling of polar behavior and oxygen vacancy ordering. Finally, we can demonstrate that electron beam can induce oxygen vacancy ordering in LaCoO₃, with the ability to control the vacancy injection. The prospects of incorporating "big data" approaches into STEM data analysis, especially in dynamic conditions, will also be discussed.
Research supported by the U.S. Department of Energy (DOE), Basic Energy Sciences (BES), Division of Materials Sciences and Engineering, and through user projects supported by ORNL&’s Center for Nanophase Materials Sciences (CNMS), which is sponsored by the Scientific User Facilities Division, Office of Basic Energy Sciences, U.S. DOE.

The design of distortions and rotations of the corner-connected BO₆ octahedra across interfaces has emerged as an exciting platform to control electronic or magnetic behavior in ABO₃ perovskite heterostructures. However, interfacial structural modifications often coincide with electronic or orbital reconstructions making it difficult to isolate the effect of octahedral structural coupling on physical properties and to identify the length scale of interfacial octahedral interfacial coupling. Here, we investigate isovalent manganite superlattices, [(La_0.7Sr_0.3MnO₃)_n/(Eu_0.7Sr_0.3MnO₃)_n] (n = 3, 6, 11), as a route to spatial control over electronic bandwidth and ferromagnetism through the creation of octahedral superstructures. The superlattices were grown on (La_0.3Sr_0.7)(Al_0.65Ta_0.35)O₃ (LSAT) by oxide molecular beam epitaxy to minimize strain effect and characterized using scanning transmission electron microscopy (STEM) equipped with electron energy loss spectroscopy (EELS), x-ray and neutron scattering, and temperature dependent magnetization measurements. The EELS results confirm a homogeneous Mn³⁺/Mn⁴⁺ valence state throughout the superlattices, indicating the absence of charge transfer across the interfaces and a uniform electronic state throughout the superlattices. In contrast, the STEM and x-ray diffraction reveal the presence of modulations of the MnO₆ octahedral rotations along the growth direction commensurate with the superlattice period. We show that the Curie temperatures of the constituent materials can be systematically engineered leading to a modulated magnetization in samples where the superlattice period is larger than the interfacial octahedral coupling length scale, while a single magnetic transition is observed in the short period superlattices. The results demonstrate that purely structural effects can be used to spatially confined magnetism in oxide heterostructures and point to the design of rotational gradients or structural delta doping approaches as routes to realize novel electronic or ferroic states in oxide superlattices.
This work is supported by the U. S. Army Research Office under grant number W911NF-12-1-0132. Use of the Advanced Photon Source, an Office of Science User Facility operated for the U.S. Department of Energy (DOE) Office of Science by Argonne National Laboratory, was supported by the U.S. DOE under Contract No. DEAC02-06CH11357. This work has benefited from the use of Polarized Neutron Reflectometry at the Lujan Center at Los Alamos Neutron Science Center, funded by DOE Office of Basic Energy Sciences. Use of the Chemical Imaging Initiative at Pacific Northwest National Laboratory (PNNL), a multi-program national laboratory operated by Battelle for the U.S. DOE under Contract DE-AC05-76RL01830. The work was performed at EMSL, a national scientific user facility sponsored by the DOE's Office of Biological and Environmental Research and located at PNNL.

Oxygen octahedral rotations (OOR) in perovskites couple strongly to their properties, providing another instrument of control for creating materials with desired functionalities. It has recently been demonstrated that OOR can be manipulated in epitaxially grown thin films through careful choice of strain level and crystal symmetry of the substrate.^[1] Spatially resolved characterization of OOR at heterointerfaces is therefore critical not only for understanding the coupling mechanism between OOR and various materials properties, but also for developing better strategies for OOR control.
However, OOR characterization with unit-cell spatial resolution is not trivial. Diffraction techniques using X-rays, neutrons or electrons can fully determine the 3D octahedral rotation information, but they lack spatial resolution.^[2] On the other hand, electron microscopy imaging techniques, including HRTEM^[3], STEM-BF^[4] and STEM-ABF^[5] can be used to directly visualize O columns and determine the rotation angle for one in-phase rotating axis unit cell by unit cell, but with no 3D information. Recently introduced PACBED^[6] endeavors to bridge this gap. Unfortunately it requires extensive simulations, which become especially prohibitive for complex materials with several competing phenomena, since each sample parameter such as thickness, polarization, strain, and tilt, adds an extra dimension to the simulation phase space.
#8203;
In this presentation, we will show that characterizing multiple dimensions of OOR is actually possible by direct oxygen imaging in STEM. Depending on the OOR system, the oxygen atoms will align differently in one of the [110]_pc projection and form specific patterns, which can be captured by our Nion UltraSTEM 200. Therefore in ABF-STEM images, the shapes of the O columns and their symmetry relations to adjacent unit cells have 3D information about the OOR system. We will use two case studies (i.e. CaTiO₃/SrTiO₃ by LQ & MB, and (La,Sr)MnO₃/(Eu,Sr)MnO₃/SrTiO₃ by EJM & SJM) to demonstrate how we use this technique to study the OOR behaviors when two different crystal symmetries meet at the heterointerface. Image simulations with different OOR systems and rotation angles are used to match the experimental results. Using this approach, OOR can in principle be studied in 3D and with unit-cell resolution, without extensive further simulations. It will also enable us to directly visualize out-of-plane OOR in thin films without having to remove the substrate.
[1]Rondinelli, Adv.Mat.(2012)
[2]Johnson-Wilke et al., PRB 88(2013)
[3]Kirkland et al., Ultramicroscopy.107(2007)
[4]Kim et al., Adv Mater 25 (2013)
[5]Okunishi, et al., Micron 43 (2012)
[6]J. Hwang et al., PRB 87 (2013)
*The MSE Divison, US DOE (QH, ARL, AYB); CNMS, sponsored by SUF Division, Office of BES, U.S. DOE (QL and MDB; QH, ARL and AYB through a user proposal) and by the Army Research Office (EJM and SJM), grant W911NF-12-1-0132. JSPS Postdoctoral Fellowship for Research Abroad (RI)

Atomically resolved STEM with EELS (scanning transmission electron microscopy and electron energy loss spectroscopy) are used often to understand interfacial systems. Typically, one approximates measured spectra by weighted superpositions of various reference bulk spectra to extract valence of ions and their electronic structure. We have studied the interface of a ferroelectric oxide (BaTiO3) and a colossal magnetoresistive manganite (LaxSr1-xMnO3) where it has already been established that the ferroelectric polarization and resulting screening charge in the manganite creates a dramatic coupling of the atomic geometry, electronic structure, and magnetic state which leads to a strong magnetoelectric coupling. The EELS spectra for this interface show unusual and counterintuitive behavior that are quite non bulk-like and unique to the interface, which we elucidate using combination of experiment and first principles theory. Understanding the link between measured interfacial EELS and the atomic-scale structure and electronic configuration of the interface is of importance in understanding the interfacial behavior of novel materials.

The role of the local charge carrier concentration profiles is paramount for the occurrence of superconductivity in low dimensional systems. Atomic layer-by-layer oxide molecular beam epitaxy (MBE) growth together with state-of-the-art characterization techniques discloses new ways for the synthesis of novel superconducting epitaxial heterostructures. In this contribution, we present the exciting findings of the study of Sr d-doped La₂CuO₄ (LCO). d-doping, which represents a novel approach in the field of layered complex oxides, here relies on the substitution of a full La-O atomic layer by a Sr-O layer. Artificial superlattices having different spacing between two subsequent Sr-O layers were grown on LaSrAlO₄ (LSAO) substrates, and by appropriately tuning the super-lattice structure, high-T_c superconductivity up to about 40 K has been obtained.
The structure and cations redistribution in the Sr-d-doped LCO multilayers on LSAO substrates was investigated using a JEOL ARM 200CF scanning transmission electron microscope (STEM) equipped with a cold field-emission electron source, a DCOR probe corrector, a large solid angle SDD-type JEOL Centurio EDX detector, and a Gatan GIF Quantum ERS spectrometer. LCO exhibits an orthorhombic structure which can be regarded as pseudo-tetragonal, whereas the LSAO substrate has a tetragonal structure.
Cross-sectional STEM images of the interface between LCO and LSAO do not show structural defects, where atomically resolved high-angle annular dark-field (HAADF) and annular bright-field (ABF) images show the orientation relationship between LCO and LSAO. Furthermore, HAADF and ABF images of the Sr-d-doped region, which were simultaneously acquired, illustrate a perfectly defect-free atomic arrangement. Sr redistribution at the interface was studied by atomic resolution HAADF in combination with EDX and EELS. Due to the difference in atomic number (Z_Sr = 38, Z_La = 57), the atomic columns dominated either by La or Sr give rise to different contrast in the HAADF image. In Sr-δ-doped regions the atomic column intensity is significantly lower than in pure LCO. Integrated image intensity profiles perpendicular to the growth direction yield for the Sr-δ-doped region a relatively sharp image intensity drop followed by a slowly increasing intensity pointing at an asymmetric distribution of Sr in growth direction. In addition, Sr-L EDX and Sr-L_2,3 EELS line-scan profiles provide a robust proof that the Sr concentration in LCO is (i) distributed across a few layers and (ii) has an asymmetric profile. These findings, suggesting a rather complex mechanism of charge rearrangement, are discussed [1].
[1] The research leading to these results has received funding from the European Union Seventh Framework Program [FP7/2007-2013] under grant agreement n°312483 (ESTEEM2). Financial support for the ARM200CF project by the Max Planck Society is gratefully acknowledged.