Symposium EM5 : Materials and Mechanisms of Correlated Electronic Phenomena in Oxide Heterostructures

Combining the power and possibilities of heterostructure engineering with the collective and emergent properties of quantum materials, quantum-matter heterostructures open a new arena of solid-state physics. Unprecedented electronic states can be engineered in these structures, giving rise to unforeseeable opportunities for scientific discovery and potential applications [1]. I will present our work in the nascent field of quantum-matter heterostructures, and comment on perspectives and challenges of the field.

[1] H. Boschker and J. Mannhart, Quantum Matter Heterostructures, Ann. Rev. Cond. Matt. (2016)

Due to electronic correlations and / or competing ordering phenomena, transition metal oxides (TMO) show functionalities which make them promising candidates for sensors or devices in future information technologies. Such devices will consist of heterostructures of different TMO's, where the interfaces play the crucial role. Due to the sensitivity of TMO's to external parameters such as strain, chemical and electronic doping, magnetic and electric fields etc., their properties are easily altered at the interface.
We studied the effect of temperature and magnetic field on the interfacial magnetization of La_1/3 Sr _2/3FeO3 (LSFO) and La_2/3Sr_1/3MnO₃ (LSMO) heterostructures.
LSFO and LSMO bulk systems have been well investigated in recent years, as they have very interesting properties. LSFO is a model system for a Verwey transition. For the used stoichiometry of LSFO, iron has the nominal valence of +3.67 composed of Fe³⁺ and Fe⁴⁺. At T_CO = 200 K charge ordering sets in, where a charge disproportionation of Fe⁴⁺ into Fe³⁺ and Fe⁵⁺ occurs. This leads to a step-wise increase in resistivity, the so called Verwey transition. This “metal-insulator” transition is accompanied by a magnetic transition into an antiferromagnetic state. On the other hand LSMO is an oxide with a rich phase diagram and in the used stoichiometry LSMO is a ferromagnet with a T_C = 335 K. As both systems have the same crystal structure with a perovskite pseudo-cubic unit cell and a lattice parameter of 3.87 Å at room temperature, one can expect that the differences in the electronic and spin structure at the interface are more crucial than strain effects. Also the lattice mismatch to the STO substrate is comparably small with 0.8 % tensile strain. The resistivity of LSFO increases by about 8 orders of magnitude between T_V = T_CO = T_N = 200 K and 5 K, due to charge ordering and charge disproportionation. The electronic doping influences the interface behavior by potential changes of the charge carrier density at the interface and one expects a tuning of the hole density in LSMO.
The LSMO and LSFO layers (each 200 Å thickness) of high quality have been grown with an high pressure oxygen sputtering system and a state-of-the-art oxide molecular beam epitaxy (MBE) machine with an atomic oxygen plasma source to achieve fully oxidized samples. In-situ LEED of the grown layer reveals a well-ordered surface. Additionally, x-ray reflectometry and diffractometry measurements prove the good quality of our samples. Information about the nuclear and magnetic scattering length density of the entire film has been measured at the polarized beam reflectometer (PBR) at NIST Center for Neutron Research at different temperatures after field-cooling (0.5 T) and zero-field-cooling. These measurements probed the evolution of the magnetic structure at the interface, as the number of charge carriers will be varied drastically with the temperature.

A rather unique feature of the two-dimensional electron gas (2-DEG) formed at the interface between the two insulators LaAlO₃ and SrTiO₃ is to host both gate-tunable superconductivity and strong spin-orbit coupling. In the present work, we use the disorder generated by Cr substitution of Al atoms as a tool to explore in further details the nature of superconductivity and spin-orbit coupling in these interfaces. A reduction of the superconducting T_c is observed with Cr doping consistent with an increase of electron-electron interaction in presence of disorder. The evolution of spin-orbit coupling with gate voltage and Cr doping is extracted from magnetotransport experiments. The corresponding spin diffusion length is found to be essentially independent of the disorder in agreement with a D'Yakonov-Perel mechanism where the spin-orbit relaxation time varies inversely with the elastic scattering time. In addition, we show that the strength of the spin-orbit coupling increases linearly with the carrier density of the 2-DEG and therefore the interfacial electric field E_z in agreement with a Rashba type of spin-orbit interaction.

The Ruddlesden-Popper phases Sr_n+1Ir_nO_3n+1 are discussed to exhibit a semimetal to insulator transition for the series n=∞,2,1 with an infinite number, 2, and 1 layers of SrIrO₃, where the n=1 member is a so-called spin-orbit driven Mott insulator [1-4]. Here the spin-orbit coupling is large enough to split overlapping, mixed bands into a manifold of narrow bands with well-defined effective total angular momentum J_eff. With lowering the dimensionality of the IrO₂ plane structure these bands become successively more susceptible to Mott localization since the coupling strength gets enhanced even though the local onsite Coulomb repulsion U is relatively small in 5d as compared to 3d transition metals oxides.

A systematic investigation of the Sr_n+1Ir_nO_3n+1 series, however, is difficult since the bulk compounds are unstable in ambient pressure for n ≥ 3 and thus extremely challenging to grow as single crystals. An alternative way to investigate the effect of dimensionality on the electronic properties is the fabrication of ultrathin SrIrO₃ films with varying thickness.

We have grown high-quality SrIrO₃ thin films on SrTiO₃ by pulsed laser deposition and investigated their electronic structure by soft x-ray angle resolved photoemission and transport measurements. Indeed, a semimetal-insulator transition is found as function of film thickness as evidenced by the opening of a gap at the chemical potential and an increase of sheet resistivity by several orders of magnitude. We discuss the nature of the observed transition based on density-functional calculations including U (DFT+U) and taking thickness-dependent octahedra rotation and tilting into account.

[1] S. J. Moon et al., Phys. Rev. Lett. 101, 226402 (2008)
[2] B. J. Kim et al., Phys. Rev. Lett. 101, 076402 (2008)
[3] B. J. Kim et al., Science 323, 1329 (2009)
[4] J. Matsuno et al., Phys. Rev. Lett. 114, 247209 (2015)

Strong interplay between order parameters such as charge, spin, orbital, and lattice in transition metal oxides has proven to produce novel physical phenomena, while interfacial coupling between dissimilar materials is an effective technique to tune such parameters. We have observed that such coupling in iridate-manganite superlattices results in novel magnetic and electronic properties. We have fabricated high quality [(SrMnO₃)_m/(SrIrO₃)_n]_z superlattices on SrTiO₃ (001) substrates by pulsed laser epitaxy and found charge transfer driven magnetism at these 3d-5d interfaces in short period superlattices (m ≦ 3). Element specific magnetic probes indicate that both materials are magnetically active, while the magnitude of the magnetization present in SrMnO₃ is roughly an order of magnitude larger than that present in SrIrO₃. In addition the direction of the magnetization of SrIrO₃ can be tuned by varying its layer thickness. Electronic transport measurements indicate the longitudinal conductivities of both components are highly sensitive to charge transfer and layer thickness. The most intriguing result arises from the transverse conductivity where we observe the anomalous Hall effect that coincides with the onset of magnetism. Thus, this system is the prototypical system displaying strong coupling between 3d and 5d complex oxides. We will present the structural, magnetic, and electronic properties of these superlattice samples and discuss the implication of their novel properties.

This work was supported by the U.S. Department of Energy, Office of Science, Basic Energy Sciences, Materials Sciences and Engineering Division.

Topology both in real and momentum space has become an interesting playground in solid state physics for exploring novel functionality that may be used for future dissipationless electronics. We present topological properties of two dimensional electron system confined at heterointerfaces of oxides. The talk will cover high mobility electron systems that display quantum Hall effect in ZnO and SrTiO₃ and topological Hall effect in EuTiO₃.

Due to the absence of macroscopic magnetization, it is generally difficult to control magnetic domains of antiferromagnetic materials. However, in the case of pyrochlore iridates (R₂Ir₂O₇, R: rare-earth ion), two distinct magnetic domains are identified by the characteristic spin structure called all-in-all-out, where all the four spins at the vertices of a tetrahedral site point either inward or outward. We have shown that one of the all-in-all-out domains can be selectively stabilized depending on the polarity of the cooling magnetic field across the antiferromagnetic transition temperature [1]. Here we visualize the local all-in-all-out domains of a Tb₂Ir₂O₇ thin film by scanning SQUID. The present result supports our recent result of the formation of artificial magnetic domain walls at the Tb₂Ir₂O₇/Eu₂Ir₂O₇ heterointerface [2].
In the case of bulk Tb₂Ir₂O₇, spins at Ir⁴⁺ exhibit the all-in-all-out antiferromagnetic transition at 120 K concomitant with a metal-insulator transition, while Tb³⁺ spins start to order below 40 K. Our scanning SQUID measurement after zero-field cooling clearly showed magnetic domains below 40 K, corresponding to Tb³⁺ spin ordering, while the average of magnetization is nearly zero. This result is consistent with macroscopic magnetization measured by SQUID magnetrometer, which indicates negligible values at 0 T. The observation of local magnetic moment indicates that the all-in-all-out spin order of Tb³⁺ tetrahedra are slightly distorted, causing net magnetization.
We also performed scanning SQUID measurement after field cooling under a magnetic field of ~ 0.1 T. First we applied magnetic field at 50 K, which is above the transition temperature of the Tb³⁺ spin ordering. We observed almost complete recovery of the domain pattern at 5 K compared with that of zero-field cooling. On the other hand, when we increased the temperature to 130 K and applied magnetic field, the domain pattern is completely altered. The overall magnetization also biased by the direction of the cooling magnetic field. These results indicate that the magnetic domain of Tb³⁺ cannot be changed by external magnetic field of ~ 0.1 T but is solely determined via the exchange field from Ir⁴⁺. This characteristic behavior of all-in-all-out spin structure expands freedom to control the domain walls in R₂Ir₂O₇ heterostructures.
[1] T. C. Fujita et al., Sci. Rep. 5, 9711 (2015).
[2] T. C. Fujita et al., Phys. Rev. B 93, 064419 (2016).

The recent discovery of the quantum anomalous Hall effect (QAHE) in magnetically doped topological insulators cooled below in the milikelvin regime represents breakthrough in the field of spintronics¹. Theoretically, the QAHE should occur in graphene proximity coupled to a ferromagnetic insulator² but with the promise of much higher operating temperatures for practical applications. Hints of proximity-induced magnetism in graphene coupled to yttrium iron garnet (YIG) films have been reported³ although the QAHE remains unobserved; the lack of a fully developed plateau in graphene/YIG devices can be attributed to poor interfacial coupling and therefore a dramatically reduced magnetic proximity effect.

Here we report the deposition and characterisation of epitaxial thin-films of YIG on lattice-matched gadolinium gallium garnet substrates by pulsed laser deposition. YIG films are characterized by X-ray diffraction, atomic force microscopy vibrating sample magnetometry and ferromagnetic resonance in order to check is quality. Pristine exfoliated graphene flakes transferred mechanically onto the YIG are reported alongside results that correlate the effects of YIG morphology on the electronic and crystal properties of graphene by electrical (low temperature magnetoresistance measurements in Hall-bar-like configuration) and optical (Raman) means.

(1) C.Z. Chang et al., Science 340, 167 (2013) and C.Z. Chang et al., Nature Materials 14, 473 (2015).
(2) C. L. Kane and E. J. Mele, Phys. Rev. Lett. 95, 226801 (2005).
(3) Z. Wang et al., Phys. Rev. Lett. 114, 016603 (2015).

A strong spin-orbit coupling (SOC) inherent to 5d Ir oxides recently emerged as a new paradigm for oxide electronics. For example, we have investigated novel physics of spin-orbital Mott insulators [1] and topological insulators [2] by tuning the electronic phases through superlattice fabrication. We also demonstrated a large spin Hall effect of IrO₂, one of the simplest 5d oxides, indicating that Ir oxides are promising class of spintronic materials [3].
Here we focus on yet another topic on spintronics – magnetic skyrmion [4]. Recently, topological Hall effect (THE), originating from scalar spin chirality, is found to be a promising tool for probing the Dzyaloshinskii-Moriya (DM) interaction and consequent magnetic skyrmions. This interaction arises from broken inversion symmetry and hence can be artificially introduced at interface; this concept is lately verified in the metal multilayers. However, there are few attempts to investigate such DM interaction at interface through electron transport. Here we clarified how the transport properties couple with interface DM interaction by fabricating the epitaxial oxide interface. We observed THE in epitaxial bilayers consisting of ferromagnetic SrRuO₃ and paramagnetic SrIrO₃ over a wide region of both temperature and magnetic field. The magnitude of THE rapidly decreases with the thickness of SrRuO₃, suggesting that the interface DM interaction plays a significant role. Such interaction is expected to realize 10-nm-sized Néel-type magnetic skyrmion. The present results established that the high-quality oxide interface with strong SOC enables us to tune the effective DM interaction; this can be a step towards the future topological electronics.

[1] J. Matsuno, K. Ihara, S. Yamamura, H. Wadati, K. Ishii, V. V. Shankar, H.-Y. Kee, and H. Takagi, “Engineering spin-orbital magnetic insulator by tailoring superlattices”, Physical Review Letters 114, 247209 (2015). [Editors' Suggestion]
[2] D. Hirai, J. Matsuno, and H. Takagi, “Fabrication of (111)-oriented Ca_0.5Sr_0.5IrO₃/SrTiO₃ superlattices — A designed playground for honeycomb physics”, APL Materials 3, 041508 (2015).
[3] K. Fujiwara, Y. Fukuma, J. Matsuno, H. Idzuchi, Y. Niimi, Y. Otani, and H. Takagi, “5d iridium oxide as a material for spin-current detection”, Nature Communications 4, 2893 (2013).
[4] J. Matsuno, N. Ogawa, K. Yasuda, F. Kagawa, W. Koshibae, N. Nagaosa, Y. Tokura, M. Kawasaki, “Interface-driven topological Hall effect in SrRuO₃-SrIrO₃ bilayer”, Science Advances, in press.

Many rich effects have been predicted and/or discovered in oxide heterostructures relying on the influence of interfaces, confinement, and long-range electrostatics. For example, the ionic discontinuity of the interface between a polar material like LaAlO3 and non-polar one like SrTiO3 leads to an electron gas in the SrTiO3, and this same mechanism applies if LaAlO3 is replaced by LaTiO3. However, while LaAlO3 is a band insulator, LaTiO3 is a Mott insulator. What difference does this make? We argue that in the latter case there is additional physics associated with spin excitations in the Mott insulator. In particular, the interplay of a high density two-dimensional electron gas and localized Mott electrons may lead to kinetic magnetism unique to the Mott/band insulator interface. This is illustrated by a minimal bilayer Hubbard model at U = ∞ with a potential difference between the two layers. We combine analytic results with DMRG simulations to show that magnetism, and especially ferromagnetism, is greatly enhanced relative by the proximity of the two subsystems.

One of the many unusual properties of several two-dimensional (2D) oxide interface systems (e.g., LaAlO₃/SrTiO₃) is the presence of a metal–insulator transition (MIT). This feature contradicts the famous prediction of Abrahams, Anderson, Licciardello, and Ramakrishnan[1] that all two-dimensional systems must be insulating. Since the MIT is a quantum phase transition (i.e., one that occurs at T=0K) the transport properties should be independent of the chemical and structural details of the system. Indeed, recent work has demonstrated that a generic phase diagram for the 2D MIT can be constructed for two very different systems: 1) highly disordered RuO₂ nanoskins with carrier concentrations that are one-to-six orders of magnitude higher and with mobilities that are one-to-six orders of magnitude lower than those reported previously for 2D oxides[2] and 2) plasma-functionalized graphene.[3] This phase diagram consists of three regions: metallic, weakly localized insulator with conductivity, σ~logT, and strongly localized insulator. We will present details of the transport properties of the disordered RuO₂ nanoskins and plasma-functionalized graphene near their respective MITs. We will then present transport results for several gated oxide interface systems near their MITs and compare them with those for the RuO₂ nanoskins and functionalized graphene.


[1] “Scaling theory of localization: Absence of quantum diffusion in two dimensions.,” Abrahams, E., Anderson, P. W., Licciardello, D. C. & Ramakrishnan, T. V., Phys. Rev. Lett. 42, 673–676 (1979).

[2] Osofsky, M. S. et al. Disordered RuO₂ exhibits two dimensional, low-mobility transport and a metal–insulator transition. Sci. Rep. 6, 21836; doi: 10.1038/srep21836 (2016).

[3] Osofsky, M. S. et al. Functionalized graphene as a model system for the two- dimensional metal-insulator transition. Sci. Rep. 6, 19939; doi: 10.1038/srep19939 (2016).

Emergent phases in the two-dimensional electron gas (2DEG) formed at the interface between two insulating oxides have attracted a great deal of attention in the past decade. We present ab-initio electronic structure calculations for the interface between a Mott insulator GdTiO₃ (GTO) and a band insulator SrTiO₃ (STO) and compare our results with those for the widely studied LaAlO₃/SrTiO₃ (LAO/STO) interface between two band insulators. Our GTO/STO results are in excellent agreement with experiments, but qualitatively different from LAO/STO. We find an interface carrier density of 0.5e/Ti, independent of GTO thickness in both superlattice and thin film geometries, in contrast to LAO/STO. The superlattice geometry in LAO/STO offers qualitatively the same result as in GTO/STO. On the other hand, for a thin film geometry, the interface carrier density builds up only beyond a threshold thickness of LAO. The positive charge at the vacuum surface that compensates the 2DEG at the interface also exhibits distinct behaviors in the two systems. The compensating positive charge at the exposed surface of GTO charge disproportionates due to correlation effect making the surface insulating as opposed to that in LAO which remains metallic within band theory and presumably becomes insulating due to surface disorder or surface reconstruction.

Ref.: Electronic Structure of Oxide Interfaces: A Comparative Analysis of GdTiO₃/SrTiO₃ and LaAlO₃/SrTiO₃ Interfaces.
Hrishit Banerjee, Sumilan Banerjee, Mohit Randeria, Tanusri Saha-Dasgupta
Sci. Rep. 5, 18647; doi:10.1038/srep18647 (2015)

Ferroelectricity is a property that only insulating materials can exhibit. For this reason, nearly every approach to date has focused on searching for multiferroics in insulating magnetic oxides. Here, we use a different approach. Using density-functional theory we investigate the electronic properties of the polar metallic oxide LiOsO3. We show that the material is close to a Mott transition and that electronic correlations can be tuned to engineer a Mott multiferroic state in the 1/1 superlattice of LiOsO3 and LiNbO3.

With recent advances in MBE growth, increasingly complex correlated oxide heterostructure can be fabricated, leading to new types of materials involving the interplay of different types of physical phenomena. We report on first principles DFT+U modelling of a new type of interface involving cobaltates and titantes: specifically, a (001) superlattice composed of two unit cells of LaCoO_3 followed by two unit cells of LaTiO_3: (LaCoO_3)_2/(LaTiO_3)_2. Interface formation leads to electron doping of the Co eg states due to electron transfer from Ti t_{2g} states with resulting strong polarity and orbital polarization on the Co e_g manifold. Experiments on this system indicate a non-magnetic insulating system. The calculated Co configuration is close to t_{2g}^6 e_g^1, but a simple single-site physical picture with one doped electrons per Co makes the non-magnetic insulating behavior hard to understand. The DFT+U calculations indicate that the insulating behavior, reproduced in the calculation, is due to a non-local behavior of the doped electrons: each pair of Co along (001) form a filled and localized "dimer" state that accepts all the doped electrons; this may be a realization of a "Dimer Mott" state [1]. Hence, the interfacial insulating state is of a completely different nature than their parent compounds and is realized due to an interesting interplay between electron transfer, polarity, orbital polarization and the dimer state.


1) ‘Dimer Mott insulator in an oxide heterostructure’, Ru Chen, SungBin Lee, and Leon Balents, Phys. Rev. B 87, 161119 (2013)

A complete understanding of materials whose transport and thermodynamic properties deviate from Landau Fermi liquid theory remains one of the central problems in physics. Non-Fermi-liquid behavior is usually identified via power-laws in the temperature (T) dependence of properties such as electrical resistance, i.e., R ~ Tⁿ with n ≤ 2. Conversely, a correlated electron system is usually identified as a classic Fermi liquid when n = 2, which is the exponent predicted from the electron-electron scattering contribution to the resistance. A remarkable feature of transport in doped SrTiO₃ is the temperature dependence of the electrical resistivity that is proportional to Tⁿ with n ≤ 2. This suggests electron-electron scattering is the dominant scattering mechanism. It extends to room temperature and above in both three-dimensional, uniformly doped SrTiO₃ and in two-dimensional electron liquids (2DELs) at oxide interfaces. Here we show that Landau Fermi liquid theory does not apply to electron liquids in SrTiO₃, even when n = 2. Using electrostatic gating and chemical doping, we demonstrate that this regime is associated with a scattering rate and an energy scale that are both independent of carrier density. This is in fundamental conflict with Fermi liquid theory, where the relevant energy scale is the Fermi energy. We discuss that the behavior is very similar to systems traditionally identified as non-Fermi liquids (n < 2). This includes the cuprates and other transition metal oxide perovskites, where strikingly similar density-independent scattering rates have been observed. The results indicate that the applicability of Fermi liquid theory should be questioned for a much broader range of correlated materials and point to the need for a unified theory. We will also discuss results from tunneling spectroscopy of the 2DELs. These measurements directly probe the degree to which the low-energy excitations of an interacting electron liquid differ from those of a Fermi gas. We show the emergence of pseudogaps, and, at very low temperatures, of coherence peaks in the tunnel spectra.

Complex oxide interfaces have recently been recognized as a powerful platform to engineer and study novel electronic phases. The discovery of a two-dimensional electron gas (2DEG) that exhibits superconductivity, co-existing with intrinsic ferromagnetism and large gate-tunable spin-orbit coupling in the interface between two wide band-gap insulators, LaAlO₃ and SrTiO₃ (LAO/STO), has stimulated increasing interest in both experimental and theoretical studies of this system [1]. The unique combination of electronic phases in the LAO/STO interface is very promising for oxide-based electronic devices, such as field-effect transistors, or resistive memories. Moreover, the ferromagnetic polarization of the 2DEG may also be used for applications in spintronic devices.
We developed a novel nanoscale patterning method of the 2DEG in the LAO/STO interface using low-energy Ar+ ion beam implantation in combination with optical and electron beam lithography [2,3]. Using this technique, we realized nano-rings and nano-constrictions with a lateral width of 100 nm. We present a systematic characterization of LAO/STO nanostructures, including measurements of the current-voltage characteristics (IVC) as a function of the temperature (down to 20 mK) and of an external magnetic field, perpendicular to the film plane. While the superconducting transition occurred at 300 mK, we observed a non linear behavior in the IVC up to 700 mK, which might be a hint of preformation of superconductivity above the bulk critical temperature. Moreover, we observed a periodic modulation of the critical current in nano-rings as a function of external magnetic flux with period of h/2e, and electrostatic gate-tunable superconductor-insulator transition. The results elucidate the complex nature of the superconducting state with possible inhomogeneous electronic phase separation and co-existence of ferromagnetism and superconductivity on nanoscale. This makes the LAO/STO interface 2DEG a unique playground to create a non-conventional chiral p-wave interfacial superconductor, and paves the way to a possible realization of Majorana physics in LAO/STO heterostructures [4].


[1] A. Ohtomo and H. Y. Hwang, Nature 427, 423 (2004); 441, 120 (2006).
[2] P.P. Aurino et al., Appl. Phys. Lett. 102 201610 (2013).
[3] P.P. Aurino et al., Phys. Rev. B 92 155130 (2015).
[4] L. Fidkowski et al., Phys.Rev. B 87 014436 (2013).

Oxide heterostructures and superlattices have attracted a great deal of attention in recent years in the search for promising emergent physics that is both technologically useful and eminently tunable. Of such oxide interfaces, those involving correlated oxides are particularly appealing, owing to the rich phenomena that develops due to competing orbital, spin and charge degrees of freedom. While a great number of studies have focused on the interface between band and/or correlated insulators, and superconductors and/or ferromagnets, there has been comparatively much less work on correlated metallic hetero-interfaces and superlattices. Here, we focus on the potential of tunable correlated oxides by investigating the spectral function and electronic reconstruction at the interface of the prototypical correlated metal, SrVO₃, and well-known band insulator, SrTiO₃. We use a variety of soft x-ray spectroscopic probes to selectively interrogate the buried SrVO₃ and SrTiO₃ independently, including soft x-ray absorption spectroscopy (XAS), resonant inelastic soft x-ray scattering (RIXS) and soft x-ray linear dichroism (XLD), giving us access to both unoccupied and occupied electronic states. We demonstrate a remarkable level of tunability in the correlated spectral function of SrVO₃ by varying its thickness within the SrVO₃/SrTiO₃ superlattice, which we find to be augmented by interlayer coupling between SrVO₃ layers. At the interface, we report on the electronic redistribution of charge at both SrVO₃ and SrTiO₃ sides, despite the non-polar nature of the interface in these superlattices.

The Boston University program is supported in part by the Department of Energy under Grant No. DE-FG02-98ER45680. The Advanced Light Source, Berkeley, is supported by the US Department of Energy under Contract No. DEAC02-05CH11231. M.G., J.W.L. and S.A.W. gratefully acknowledge financial support from the Army Research Office through MURI grant No. W911-NF-09-1-0398.

Strontium titanate possesses a breadth of properties that is unmatched in the solid state. Many of these properties can be controlled by electron doping, which itself can be achieved by chemical, polarization or gating methods. Using conductive-AFM lithography, it is possible to locally control the metal-insulator transition at the interface between SrTiO₃ and a thin layer of LaAlO₃, enabling the creation of SrTiO₃-based nanostructures that can be used to investigate the properties of SrTiO₃ itself. Here we focus on local control of electron-electron interactions using a “sketched” single-electron transistor. We find that the electron-electron interaction can be tuned between attractive (at low electron density) to repulsive (at high electron density). The attractive regime leads to electron pairing (boson formation) and superconductivity, while the latter leads to fermionic behavior with repulsive interactions. Electron waveguides formed from quasi-1D nanostructures act as ballistic conduits for electrons or electron pairs, representing a new form of quantum transport that may find use in future quantum technologies. The ability to tune electron-electron interactions may also find use in a solid-state quantum simulation platform.

Vanadium oxide (VO_x) has become a focal point of functional oxide research, due to its richness in multivalence, phase diagram and variety of physical properties. Among the multiple phases of VO_x with different oxygen stoichiometries, VO₂ draws extensive attention due to its near room-temperature metal to insulator transition, which renders this material as a promising candidate for new oxide electronic devices. On the other hand, the more oxidized phase V₂O₅ attracts interest due to its open layered structure and applications in Li-ion batteries and catalysis. There has been extensive efforts on investigating the physical and chemical properties of individual VO_x phases, however very few works have touched the connection and switching between multiple VO_x phases. In this work, we show for the first time that epitaxial VO_x thin film grown on (Y,Zr)O₂ (YSZ) substrates can be reversibly switched between the dioxide VO₂ (R phase) and the pentoxide V₂O₅ phases, by applying a low electrical bias up to 2 V. Concurrent with the phase change, we utilized in situ ambient pressure X-ray photoelectron and absorption spectroscopy (AP-XPS/XAS) to reveal the chemical state and electronic structure of VO_x as a function of applied bias. V L_2,3-edge and O K-edge XAS data showed that V₂O₅ to VO₂ phase transition can be switched by a cathodic bias of merely 350 mV at 300 °C in 760 mTorr pO₂. We found a drastic change in the conductivity and oxygen incorporation kinetics in VO_x accompanying the bias-induced phase transition. In situ AP-XPS of V 2p spectra explicitly confirmed the change in the chemical state of VO_x, while revealing that the surface of the VO_x thin film was more oxidized than its bulk. We captured the evolution in electronic structure of VO_x as a function of oxygen stoichiometry which is well controlled by applied biases. The bias-induced phase transition of VO_x thin film was fully reversible. Cyclability of the phase transition was shown by applying a sinusoidal wave of electrical bias and recording the X-ray absorption intensity near the V L-edge or the O K-edge. The results prove that applying electrochemical bias is a simple yet effective way to trigger the phase transition of VO_x by changing its oxygen content. This ability, more importantly, enables the switching between distinct physical and chemical properties possessed by the different phases of VO_x. These results deepen the understanding of the effect of oxygen stoichiometry on the physical properties, especially the electronic structure of VO_x.

An MIM diode is made of a multilayer of metals and an insulator, and is able to switch on/off extremely fast because of the current carried by electron tunneling and no charge in the insulator. A study has been made to employ an MIM diode as a rectifier in a solar rectenna, which receives solar radiation by an antenna and rectifies the high-frequency AC signal to DC by a diode. The rectenna with a GaAs Schottky diode has demonstrated a conversion efficiency higher than 90 % at 2.5 GHz. However, the conventional Schottky diode can respond to the frequency up to 5 THz, not high enough for solar radiation in the frequency range of 150 – 1000 THz.There has been a continuous effort to develop an MIM diode with nickel oxide NiO as a rectifier in a solar rectenna. This thin NiO film is an insulator and must be thin enough for electrons to tunnel through. An MIM with such an extremely thin tunneling layer causes a higher capacitance and poor rectification. In this study, a semiconductor layer replaced an insulator in an MIM diode. In such an MSM diode, the built-in field resulting from ionized impurities may improve the rectification and allow a thicker semiconductor layer for substantial current flow.
In contrast to NiO, non-stoichiometric and nickel-deficient NiO_x has been found to be a p-type semiconductor. We formed p-type NiO_x by UV oxidation of a nickel thin film and fabricated Al/NiO_x/Pt MSM diodes with two different NiO_x thicknesses. Al and Pt have work functions of 4.1 and 5.6 eV, respectively, and are expected to form a Schottky and ohmic contact to NiO_x, respectively. The Ni thin films with thicknesses of 100 and 7 nm were deposited on the Pt electrode and oxidized in an oxygen flow at 350oC for 180 and 35 min, respectively, under UV illumination using a metal-halide lamp. The NiO_x thicknesses were 150 and 10.5 nm after the Ni films were completely oxidized. Note that these thicknesses are much larger than those in the MIM diode showing a substantial current flow. The voltage-independent capacitance suggests that both diodes were fully depleted in the voltage range for the measurement. The MIM-like I-V characteristic showing a higher current with the Al electrode positively biased with respective to the Pt electrode was observed for the thinner NiO_x, while the Schottky-like I-V characteristic showing a higher current with the Al electrode negatively biased was observed for the thicker NiO_x. A higher current density and a higher degree of rectification were obtained for the thinner NiO_x. The rectification was further improved by increasing the oxidation temperature to 400^oC for last 5 min of the 35-min oxidation. A higher temperature lowers the carrier concentration. The characteristics of the MSM diodes are greatly affected by the metal-oxide contacts and the thickness and carrier concentration of the NiO_x, and the transport mechanism will be proposed to explain their I-V characteristics.

The unique identity of two-dimensional systems (2D) stems from quantum confinement effects. When matter is strongly downscaled, entirely new properties can emerge that are not present in the parent bulk crystals. A paradigmatic example is the quantum well formed at the interface between SrTiO₃ (STO) and LaAlO₃ (LAO). Their charge mismatch leads to a built-in electric field inside the LAO film, which eventually induces the formation of a two-dimensional electron gas (2DEG) at the boundary. Yet, in addition to such an electronic reconstruction, here we show that the LAO/STO system also undergoes a “phononic band reconstruction”. We reach such a conclusion by monitoring the atomic-scale structural evolution of the LAO film with thickness. In particular, we exploit annular bright-field (ABF) combined with high angle annular dark field (HAADF) imaging modes in the aberration-corrected transmission electron microscope (STEM). Using this approach we measure, directly in real space, the distortions of the individual A, B, and O sub-lattices of the ABO₃ perovskite structure. These, in turn, provide us with quantitative information over the amplitude of the antiferrodistortive tilts and of the polar distortions of 3, 5 and 7 unit cell thick LAO layers grown on STO. By combining the aforementioned experimental data with a theoretical analysis based on density-functional theory calculations, we find that the interplay between epitaxial strain, quantum confinement and electrostatic buildup strongly modifies, or even suppresses, the antiferrodistortive tilts of the LAO film, unveiling a sharp transition at a thickness of 3 unit cells. Thus, our work demonstrates the coexistence, and possibly strong interplay, of both electronic and phononic reconstructions in LAO/STO, with a similar critical thickness.

The exchange bias effect gives rise to a unidirectional shift of the hysteresis loop when a ferromagnet (FM) is coupled to an adjacent antiferromagnet (AFM). The effect is widely used in spintronic device applications, e.g. to create reference layers with a pinned magnetization direction. However, the microscopic origin of exchange bias remains unclear. Several proposed theories invoke an uncompensated spin structure at the AFM surface and a collinear coupling to the FM spins in order to explain the unidirectional bias[1, 2]. However, the effect has also been observed in perovskites with compensated G-type antiferromagnetism, where staggered Dzyaloshinshkii-Moriya interactions between nearest neighbors could provide an explanation for the exchange bias[3]. In perpendicularly coupled (spin-flop) systems, theoretical studies predict a uniaxial rather than a unidirectional anisotropy, thus no exchange bias [4].

In this study, we investigate (001)-oriented LaFeO₃/La_0.7Sr_0.3MnO₃ (LFO/LSMO) bilayers for a range of LSMO thicknesses. LFO is a G-type antiferromagnet and displays a compensated spin structure at the (001)-oriented AFM/FM interface. We perform magnetometry measurements and observe onset of exchange bias below a critical FM thickness. The AFM and FM domain state is selectively probed using x-ray magnetic linear and circular dichroism, respectively. We find that spin-flop coupling prevails in the system. However, a significant change in the FM and AFM domain structure coincides with the onset of exchange bias. This work may provide new insight into the mechanisms responsible for exchange bias in AFM/FM oxide heterostructures with a compensated spin structure at the interface and spin-flop coupling.

References:
1. M. Kiwi, J. Magn. Magn. Mater. 234, 584-595 (2001).
2. Nogués, J. & Schuller, J. Magn. Magn. Mater. 192, 203-232, (1999)
3. Dong, S., et al., Phys Rev. Lett 103, 127201 (2009)
4. T.C. Schulthess, Phys. Rev. Lett 81, 4516, (1998)

Interface of epitaxial complex oxide heterostructures has been recognized to determine functionalities such as ferroelectric polarization and piezoelectricity in a technological demand to reduce thickness of epitaxial thin films. During the growth of epitaxial thin films, relaxation phenomena produced boundary acting as an interface for lattice parameters as well as functionalities, so-called the critical thickness (t_c) However, individual functionalities arising from strained and relaxed layer below and above t_c have not been resolved partially due to lack of experimental tools. Here we report that contrastable ferroelectric behaviour below and above t_c in BiFeO3 epitaxial film on a SrRuO3/SrTiO3 substrate. We monitored the structural evolution and atomic position near t_c using transmission electron microscopy. time-resolved X-ray microdiffraction studies can provide the piezoelectricity for both strained and relaxed layer, which is also related to the remnant polarization.
A 60 nm-thick tetragonal BiFeO₃ films were deposited onto SrRuO₃ bottom electrode layer on (001)-oriented SrTiO₃ substrate using pulsed laser deposition technique. Pt top electrodes with diameter 60 mm were deposited to form capacitor structure. The piezoelectric response was investigated using time-resolved X-ray microdiffraction combined with applied triangular voltages in the range of up to +3.75 V at 9C beam line of Pohang Accelerator Laboratory. The piezoelectric coefficient of strained region showed an extremely low value (2.4 pm/V) in comparison to relaxed region (32 pm/V) due to the clamping effect posed by substrate. It indicates that the existence of critical thickness for relaxation in epitaxial film can strongly influence piezoelectric properties of ferroelectric thin films.

In a multiferroic tunnel junction (MFTJ) composed of a ferroelectric tunneling barrier and metallic ferromagnetic electrodes, both tunnel magnetoresistance (TMR) and tunnel electroresistance (TER) can be exploited simultaneously. Hence, such artificial MFTJs are promising for high memory density and low power device technology. One interesting possibility is to rely on intermediate magnetization alignments between the electrodes, facilitated by magnetocrystalline effects, to obtain multivalent bit states. Here we investigate the possibility to exploit the trigonal symmetry of (111)-oriented thin films, to induce an energy landscape allowing for degenerate easy-axes of the metallic electrodes. To this end, we rely on (111)-oriented La_0.7Sr_0.3MnO₃ (LSMO) thin films with a 6-fold magnetic easy-axis along the <1-10> and <11-2> in-plane directions [1]. Epitaxial heterostructures of BaTiO₃ (BTO) and LSMO, fabricated by pulsed laser deposition on SrTiO₃(111) substrates, were used as the ferroelectric tunnel barrier and ferromagnetic bottom electrode, respectively. The BTO thickness was varied between 3 nm and 5 nm, while the LSMO thickness was kept at 10 nm. To probe the effect of magnetocrystalline symmetry on the TMR characteristics, we relied on two different ferromagnetic top electrodes; sputter deposited Ni_0.81Fe_0.19 with low magnetic anisotropy, and epitaxial LSMO, both of 10 nm thickness. X-ray diffraction analysis confirmed epitaxial growth of the oxide heterostuctures, while step-and-terrace surfaces were confirmed by atomic force microscopy. Post processing to define the MFTJs was done by electron beam lithography with dry-etching and lift-off techniques. For the all-oxide MFTJs, argon sputter etching was used to define the top electrodes as freestanding LSMO/BTO on the underlying LSMO layer. We show that TER and TMR coexist in both types of our ultrathin MFTJs. In the TMR data, multiple intermediate resistive states are present, which are examined in the framework of stepwise nucleation of magnetic domains along the degenerate easy axes of LSMO(111). The effect of (111)-oriented BTO, including the effect of possible ferroelectric domain structure, on the TER, is also discussed.

The formation of metallic interface between two band insulators of LaAlO₃ (LAO) and SrTiO₃ (STO) have attracted much attention [1], since both the high mobility and the 3d character of induced interface electrons are realized in oxide heterostructures. We have studied the electronic states of LAO/STO(001) and LAO/Nb:STO(001) hetrostructures, where the polar LAO layers were grown on the insulating and conducting substrates by pulsed laser deposition. To probe the electronic states of both the LAO and STO layers near the interface, we have performed angle-resolved hard x-ray photoemission spectroscopy (HAXPES) at BL15XU [2] of SPring-8. The angle-resolved HAXPES of the Ti 2p and Al 2s core-level showed the almost flat band feature in the LAO and STO layers near the interface for both the insulating and conducting STO substrates. From the analysis of the valence band HAXPES spectra, we found that the staggered type (type II) interface is realized in the LAO/STO and its electron affinity is +0.4 eV. We also found the differences in the valence band spectral shapes for LAO/STO and LAO/Nb:STO, indicating that the electronic responses near the interface are sensitive to the insulator-insulator or insulator-semiconductor interface.

References
[1] A. Ohtomo and H. Y. Wang, Nature 427, 423 (2004).
[2] S. Ueda et al., AIP Conf. Proc. 1234, 403 (2010).

Non-ohmic behavior is not expected in metal–insulator granular systems in low-field regime. There is no model to explain this behavior, even though it has been reported in several metal-insulator granular thin films (Fe-Al₂O₃, Co-Al₂O₃, and Ti-SiO₂). In this work, we show additional experimental results of Fe-SiO₂ granular films and propose an explanation for the electrical properties of all above mentioned systems, based on Mott variable range hopping. The experimental results show that the localization length increases and the electrical resistance decreases with the increase of electrical potential or current. The non-ohmic behavior of the resistance and the increase of the localization length with increasing current are explained by the activation of new pathways for electrons in granular thin films that contain variable grain sizes and/or have different distances between grains.

Spin waves (SWs) propagating in magnetic insulator thin films have attracted considerable interest because of their short wavelength and lack of Joule heating loss, making them an excellent candidate as an information carrier in magnetic logic devices. In particular yttrium iron garnet (Y₃Fe₅O₁₂, YIG) is suitable for SW waveguides because of its low magnetic damping parameter and high resistivity. Additionally, YIG can sustain forward volume SWs (FVSWs) because the required out-of-plane saturation magnetic field is low. Such FVSWs are suitable for logic devices because of their high in-plane uniformity. SWs can propagate in any direction in the film plane simplifying the design of the logic circuit. However, fabrication of high quality thin YIG films and preparation of a characterization system for FVSWs has been challenging. Thus, so far the characteristics of propagation of FVSWs in thin YIG films is not well known. In this study, the extinction coefficient of thin YIG for FVSWs was defined and characterized. To fabricate a logic gate, not only the damping parameter but also the extinction coefficient is one of the essential properties because this factor can be changed by the in-plane uniformity of the SW interference and the surface conditions.

A 174 nm thick YIG film was prepared by pulsed laser deposition epitaxially on a gadolinium gallium garnet substrate. The working pressure was 2.6 Pa oxygen, the pulse duration was 10 Hz, the pulse energy was 360 mJ/pulse, and the substrate temperature was 850°C. X-ray diffraction confirmed the single crystallinity of the YIG. In addition, transmission electron microscopy showed the well-ordered crystal structure but with a small amount of disorder at the top of the film. The magnetic properties were measured with vibrating sample magnetometer and electron spin resonance characterization. The magnetic coercivity was 0.2 Oe, and saturation magnetization was 1700 G, close to bulk.

Five pairs of coplanar waveguides (CPW) were fabricated on the prepared YIG films by lithography techniques in order to excite the FVSW. The distance between CPWs was varied from 20 to 160 μm. The thickness of the CPW was 200 nm. A nonmagnetic probe was used to connect the rf signal to the CPW. The magnetic field was applied perpendicular to the film by an electromagnet.

The transmission S₂₁ between five pairs of CPWs were measured and plotted as a function of the distance between two CPWs. The plots were fitted with an exponential curve, and its exponent was defined as the extinction coefficient κ of SW. The κ at a frequency of 7 GHz (the wavelength was hundreds nm) was 4.501x10^-8 m^-1. From this the propagation length was estimated: L_1/e was 22.2 μm. Similarly, the extinction coefficients at other frequencies were obtained. These values enable us to design a logic device with sufficient output energy, and various designs of logic circuit realizing an address decoder, AND, and OR functions will be shown.

The electronic and magnetic properties of strongly correlated oxides are known to depend sensitively on the complex interplay among charge, spin, and lattice. In this study, we present a systematic study of the effect of interfacial charge transfer on the ferroelectric field effect modulation of a charge-transfer type Mott insulator, Sm_0.5Nd_0.5NiO₃ (SNNO). We have fabricated high quality epitaxial oxides heterostructures composed of a ferroelectric Pb(Zr,Ti)O₃ (PZT) gate and a correlated oxide channel. Two types of channel systems have been investigated, the single layer channel of SNNO and the bilayer channel of SNNO buffered with an ultrathin La_0.67Sr_0.33MnO₃(LSMO) film. We find that the magnitude of the ferroelectric field effect is closely related to both the intrinsic carrier density and carrier mobility of the channel material. For the same channel thickness, we have observed a two orders of magnitude enhancement in the room temperature resistance modulation ((R_H-R_L)/R_L) in PZT/SNNO(1.5 nm)/LSMO(2 nm) samples compared with that of PZT/SNNO(3.5 nm) device. The x-ray photoemission spectroscopy (XPS) and x-ray absorption spectroscopy (XAS) data reveal that ~0.1 hole/u.c. is transferred from SNNO film to LSMO film in SNNO/LSMO heterostructure. In addition, the magnetic and transport property of SNNO/LSMO is also modulated due to the charge transfer compared to that of bare LSMO films. The channel thickness dependence of resistance modulation indicate that the enhancement of resistance modulation can be due to the special overlap between the Thomas-Fermi screening length from the PZT/SNNO interface and the charge transfer length from the SNNO/LSMO interface. Our study reveals the critical role of charge in determining the interfacial coupling between various complex oxide thin films, and has important implications in developing ferroelectric-controlled Mott memory devices.

Current flow at the LAO/STO interface is modulated over the STO substrate’s structural domains, and we recently showed that this modulation can affect transport properties of STO based devices by diverting the flow of substantial amount of the current. In our poster we report our studies of the effect of local mechanical stress on the conductivity of the same devices, and relate it to STO domain polarity at low temperatures.

The origins of colossal magnetoresistance in the doped manganites has fascinated researchers over the past two decades, and has remained difficult to resolve given tendency for off-stoichiometry in complex oxides, large numbers of defects, and experimental limitations. For example, recent experiments suggest that colossal magnetoresistance can also be observed in undoped LaMnO₃, when subjected to high pressure [1]. Here, we show new results on epitaxial, pulsed laser deposition grown LaMnO₃ thin films on etched (001) SrTiO₃ substrates, using in-situ conductive atomic force microscopy (AFM) and fast I-V curve acquisition. Mechanical pressure, exerted by the tip upon contact and scanning of the sample, results in a transition from a metallic state on the surface to an insulating state with order of magnitude lower conduction. We find that both bias and pressure are capable of driving the transition. After ‘poling’ square regions with the AFM tip, thousands of IV curves are then captured over the poled and un-poled areas, allowing comparison of the metallic and insulating states. These results suggest that pressure and bias can locally alter chemical composition at the surface, moving the phase diagram and thus facilitating the M-I transition. In conjunction with previous measurements on both single crystal and doped thin film manganites [2], the results reported herein show that pressure induced modification is likely a universal phenomenon in both thin-film and layered manganites.


This research was sponsored by the Division of Materials Sciences and Engineering, BES, DOE (SVK, PM, SS, RKV). This research was conducted at the Center for Nanophase Materials Sciences, which also provided support (APB) and which is a DOE office of Science User Facility.

References
[1] Baldini et al., PNAS 112, 10869 (2015).
[2] Kelly et al., Nanotechnology 25 475302 (2014).

The physics of domain wall conduction in ferroelectrics has been under intense investigation since the first report of its observation in epitaxial BiFeO₃ thin films, and has now been observed in a wide variety of ferroelectrics, suggesting a generalized phenomena at play [1]. However, the fundamental mechanisms that drive the conduction are debated and no consensus has been reached on the reason for conduction in nominally uncharged ferroelectric domain walls. Here, through a combination of both conductive atomic force microscopy (c-AFM) as well as scanning microwave impedance microscopy (SMM), we probe the DC and AC conduction of ferroelectric domain walls in ultra-thin (~10nm) BiFeO₃ thin films. The I-V measurements obtained via c-AFM suggest that the mechanism of conduction is variable range hopping, with hopping parameters modified at the domain walls, presumably due to accumulation of oxygen vacancies (which is further supported by phase-field modeling). Enhanced conduction is also found in switched domains, which relaxes over several hours as the polarization returns to the original state, again suggesting that diffusion of charged defects plays a critical role. To overcome the limitation imposed by the tip junction, we also apply SMM measurements to ferroelectric domain walls and find substantial increase in the AC conductivity after anneals in vacuum. These results point towards the modification of hopping mechanisms at domain walls, providing new insight into electronic phenomena at domain walls and other topological defects in ferroelectrics.


This research was sponsored by the Division of Materials Sciences and Engineering, BES, DOE (SVK, PM, YC, RKV). This research was conducted at the Center for Nanophase Materials Sciences, which is a DOE office of Science User Facility.

References
[1] Vasudevan et al., Adv. Func. Mater. 23, 2592 (2013).

In the vast majority of experimental and theoretical work, ferroelectric switching is analyzed with the implicit assumption of unchanged stoichiometry of chemical elements and the driving force given by the volume integral of polarization-electric field product. However, it is well recognized that even bulk polarization is unstable unless it is screened at the surfaces, by either metallic electrodes, band bending, or surface ionic charges. By the date all explorations have been done by either interpretation of secondary phenomena (e.g. electric potential change or domain structure evolution) or analyzing data of concomitant spectroscopic techniques (e.g. Raman scattering, X-ray scattering). At same time, Secondary Ion Mass Spectrometry (SIMS) provides capabilities for direct spatially resolved mapping of chemical composition as a function of depth, offering a natural pathway to explore chemical changes on ferroelectric surface and bulk induced by locally applied SPM tip bias.
Here we used SIMS to study surface and bulk modifications in chemical composition of ferroelectric bismuth ferrite thin film induced by the local electric field of the Scanning Probe Microscope (SPM) tip. Reversible ferroelectric and irreversible electro-resistive switching have been considered and demonstrated changes in the surface and bulk chemistry. This confirmed significant role of the surface chemistry on the process of ferroelectric polarization reversal. Furthermore, presence of the ions (e.g. Cl^-) that are not anticipated based on chemistry of the system has been revealed. These ions were concentrated on the surface in the pristine sample and could be locally introduced deeply into the film electric field.
Explored phenomena are important from both practical and fundamental points of view. It potentially enables new applications of tip-mediated electrochemical reactions and local material modification and allows systematic study of the chemical processes following ferroelectric switching.

Acknowledgements
This work was conducted at the Center for Nanophase Materials Sciences, which is a Department of Energy (DOE) Office of Science User Facility

The surface charge layer on ferroelectric film plays an important role in the polarization stability in ultrathin film. It could severely influence the measurement of piezoelectric responses and domain morphology in scanning probe characterization techniques. In this work we investigated the ferroelectric phase stability of ultrathin film in equilibrium with a chemical environment which provides ionic charges to compensate its surface charges. We developed a self-consistent phase-field model to incorporate both a ferroelectric ultrathin film and a topped ionic surface charge layer in which the ferroelectricity vanishes, with proper boundary conditions for both ferroelectric polarizations and ionic compensations based on electrochemical equilibria. An (001) oriented single domain Pb(ZrTi)O₃ thin film was chosen as the model system, and the effect of surface ion polarity, oxygen partial pressure, saturation density of surface ions and the film thickness on the polarization and local electric potential distributions have been systematically studied. It was found that negative/positive ions are accumulated at the surface layers when polarization is pointing up/down, and the limiting saturation density is calculated to be ~1.0/6.4×10^-21cm³. Based on these results, we further analyzed how the 180° and 90° domain structures evolve under applied electric field from both planar electrode and scanning probe with and without ionic surface compensation. Our work thus helps understand the new features in ferroelectric thin films brought about by the electrochemical ionic surface compensation.

This study was supported by the U.S. DOE, Office of Basic Energy Sciences (BES), Materials Sciences and Engineering Division (MSED) through FWP Grant No. ERKCZ07 (Y.C., S.V.K.). The phase-field simulation was performed in collaboration with Prof. Long-Qing Chen at Penn State, which is supported by the U.S. Department of Energy, Office of Basic Energy Sciences, Division of Materials Sciences and Engineering under Award No. DE-FG02-07ER46417(Chen).

Pulsed Laser Deposition (PLD) has been established in recent years as a versatile thin film deposition technique for the near stoichiometric synthesis of materials including complex transition metal oxide thin films. These oxides offer a variety of exploitable properties, but despite this rich potential for use in electronics, actual industrial applications are relatively few as they rely on growth of films on substrate materials with sizes suitable for industrial applications, were silicon wafers define the standard of CMOS technology. This demands upscaling of the PLD process to grow high quality films on silicon wafers.

TSST has developed an PLD system with which oxide heterostructure growth on 4” silicon wafers is investigated and optimized, obtaining highly crystalline heterostructures with atomically sharp interfaces. Two important challenges in thin film growth on silicon wafers using PLD have been the focus of research in recent years. First, epitaxial growth of oxides on as-received silicon wafers is intrinsically prohibited by a native siliconoxide layer. Growth of YsZ buffer layers has proven to be an effective method to reduce and remove the native silicondioxide layer to form an epitaxial basis for oxide growth. Second, in PLD the dimension of the plasma plume is smaller than commonly used industrial wafer sizes, which is solved by scanning the plasma plume over the full wafer area. One final hurdle for full CMOS process compatibility remains: significantly lower process temperatures are required compared to small scale PLD growth experiments.

We present the results from this system on the growth of La0.67Sr0.33MnO3 (LSMO) thin films on high quality YsZ//CeO2//SrRuO3 buffer layers on 4'' silicon wafers. Film quality is investigated with X-Ray Diffraction and magnetic characterisation. Rocking curve measurements around the LSMO (002) Bragg reflection show values of ~1degree, which is comparable to the quality of films grown in <1” small scale experiments. A striking growth temperature dependence in the crystallinity of LSMO is shown. With the introduction of the SRO layer LSMO films show single phase crystallinity and magnetic properties for temperatures as low as 250C. When this SRO layer is lacking, fully amorphous film growth is observed at higher temperatures up to 550C.

We suggest that the occurance of this LSMO high quality crystal growth at these remarkable low growth temperatures can be understood by an improved surface diffusion induced by the B-side to A-side termination switch introduced by the SRO buffer layer. This is necessary to promote species intermixing on the surface to obtain preferential phase nucleation and overcome nucleation of various phases. Unlike small scale experiments, the PLD plume is scanned and fully collected on the wafer, where recent experiments have shown the temporal and spatial dependence of the composition of the plasma plume in relation to film crystallinity.[1]
1. R. Groenen et al. APL. Mat.

The development of epitaxial complex oxide heterostructures enables the integration of a wide range of functional materials, offering a promising approach for realizing spintronics devices where oxide properties, such as spontaneous polarizations controllable by strain or applied voltages, can manipulate the electron spin degree of freedom. Doped SrTiO₃, with theoretically predicted long spin lifetimes, has been the subject of recent investigation, with varying interpretations of the three-terminal magnetoresistance observed in ferromagnetic tunnel junctions [1]. One outstanding problem is the difficulty in producing high quality contacts necessary for spin injection into semiconducting SrTiO₃. We solve this problem through band alignment engineering using epitaxial interface dipoles to control the energy alignments between the ferromagnet and semiconducting SrTiO₃. The insertion of an interfacial dipole via LaAlO₃ atomic layers sharply reduces the Schottky barrier height, yielding effectively ohmic behavior at room temperature. With this advance, we can explore spin injection from ferromagnetic electrodes into SrTiO₃ using tunable contacts. We find that the dominant contribution to the magnetoresistance is spin-dependent transport through single defect sites, which can explain the magnetoresistance amplitude, linewidth, and anisotropy, all within a single analytic framework [2]. Furthermore, taking advantage of unique spectroscopic features arising from the strong electron-phonon coupling and superconductivity in SrTiO₃, we can completely characterize the full spectrum of tunneling processes in these junctions [3].

[1] A. G. Swartz et al., Appl. Phys. Lett. 105, 032406 (2014).
[2] H. Inoue et al., Phys. Rev. X 5, 041023 (2015).
[3] A. G. Swartz et al., in preparation.

The two-dimensional superconductor formed at the interface between the complex oxides, lanthanum aluminate (LaAlO₃) and strontium titanate (SrTiO₃) has several intriguing properties that set it apart from conventional superconductors. Most notably, an electric field can be used to tune its critical temperature (Tc), revealing a dome-shaped phase diagram reminiscent of high Tc superconductors. So far, experiments with oxide interfaces have measured quantities which probe only the magnitude of the superconducting order parameter and are not sensitive to its phase. Here, we perform phase-sensitive measurements by realizing the first superconducting quantum interference devices (SQUIDs) at the LaAlO₃/ SrTiO₃ interface. Furthermore, we develop a new paradigm for the creation of superconducting circuit elements, where local gates enable in-situ creation and control of Josephson junctions in nanoscale. These gate-defined SQUIDs are unique in that the entire device is made from a single superconductor with purely electrostatic interfaces between the superconducting reservoir and the weak link. We complement our experiments with numerical simulations and show that the low superfluid density of this interfacial superconductor results in a large, gate-controllable kinetic inductance of the SQUID. Our observation of robust quantum interference opens up a new pathway to understand the nature of superconductivity at oxide interfaces.

The possibility to control magnetism by structural effects in multiferroic structures is of interest for device applications. An interesting route for property control is the choice of substrate orientation, with an accompanying change in symmetry and strain coupling to the deposited thin films. Here we investigate how the interplay between epitaxial strain and octahedral rotations at the interface between SrTiO₃ and La_0.7Sr_0.3MnO₃ affect the magnetism of La_0.7Sr_0.3MnO₃ . In contrast to (001)-oriented SrTiO₃, (111)-oriented SrTiO₃ have three equivalent micro-domain structures in the tetragonal phase, which are degenerate with respect to the strain of the thin film. Furthermore, density functional theory calculations reveal that the effect of the structural phase transition at 105K on octahedral coupling between SrTiO₃ and La_0.7Sr_0.3MnO₃ is different for the (111)-orientation as compared to (001)-orientation. To this end we rely on pulsed laser deposition to fabricate epitaxial La_0.7Sr_0.3MnO₃ thin films on (111)-oriented SrTiO₃ substrates. We focus on effects around the 105K cubic to tetragonal phase transition in SrTiO₃ to simultaneously probe effect of strain and octahedral structure. Measurements of the magnetic properties show a change in both remanent magnetic field and coercive field at 105 K. The effect of the change can be modulated by introducing a BaTiO₃ layer in the structure, and the observed change depend on both stacking order and BaTiO₃ thickness. This points towards the influence of octahedral coupling across an epitaxial interface on the magnetic properties, and the results will be discussed in the framework of strain, octahedral coupling and effective interface symmetry.

The interface between the two band insulators SrTiO₃ and LaAlO₃ unexpectedly has the properties of a two-dimensional electron gas. It is even superconducting with a transition temperature, T_c, that can be tuned using gate bias V_g, which controls the number of electrons added or removed from the interface. The gate bias–temperature (V_g, T) phase diagram is characterized by a dome-shaped region where superconductivity occurs, that is, T_c has a non-monotonic dependence on V_g, similar to many unconventional superconductors. In this talk, the frequency of the quantum resistance-oscillations versus inverse magnetic field is reported for various V_g. This frequency follows the same non-monotonic behaviour as T_c; a similar trend is seen in the low field limit of the Hall coefficient. We theoretically show that electronic correlations result in a non-monotonic population of the mobile band, which can account for the experimental behavior of the normal transport properties and the superconducting dome.

The two-dimensional electron gas (2DEG) hosted in the interface between an epitaxially grown LaAlO₃ (LAO) thin film and a TiO₂-terminated SrTiO₃ (STO) substrate [1] has been massively studied in the last few years. The confinement of mobile electrons to within a few nm from the interface [2], superconductive behavior at low temperatures [3] and electron mobility exceeding 1000 cm2/(V.s) make this system an interesting oxide material to explore the physics of spin injection and transport. However, the existence of a critical thickness for conduction of 4 unit cells of LAO implies a very high access resistance to the 2DEG, which enables the injection but hampers the detection of spin polarized currents.
Recently, we found that depositing a thin overlayer of Co on LAO reduces the critical thickness, enabling 2DEG formation with only 1 unit cell of LAO [4]. Two scenarios arise to explain this phenomenon: (i) a pinning of the Fermi level in the metal, inducing charge transfer in the STO [5]; (ii) the creation of oxygen vacancies at the interface between LAO and the metal, leading to an n-type doping of the SrTiO₃ [6].
For the present study, we have grown LAO/STO heterostructures and capped them with different metallic layers (Ti, Ta, Co, Ni, Fe, Al, Ag, Pt, Pd and Au) in a UHV system combining pulsed laser deposition (to grow the LAO), sputtering (to grow the metal) and X-ray photoelectron spectroscopy (XPS). Magnetotransport measurements were performed at low temperatures (2K) and high magnetic fields (9T) and analyzed with a current-dependent (2+1)-band model in order to extract the carrier density, mobility and number of populated bands of the 2DEG. In addition, we used XPS to explore 4+ to 3+ valence change of interfacial Ti atoms, and possible oxidation of the deposited metal.
The results confirm that for several metals the LAO thickness required for 2DEG formation is reduced but also indicate that it can be increased well above 4 unit cells for other metals.
We will discuss the magnetotransport and spectroscopy data for all samples and discuss them in the light of the two proposed scenarios. Finally, our results suggest that the creation, suppression and manipulation of the 2DEG at the LAO/STO interface, opens a new path for the creation of these two-dimensional electron systems in other exotic oxide-based system.
[1] A. Ohtomo, H. Y. Hwang, Nature 427, 4236 (2004)
[2] M. Basletic et al., Nature Mater. 7, 6215 (2008)
[3] N. Reyren et al., Science 317, 1196 (2007)
[4] E. Lesne et al., Nature Commun. 5, 4291 (2014)
[5] R. Arras et al., Phys. Rev. B 85, 125404 (2012)
[6] L. Yu, A. Zunger, Nature Commun. 5, 5118 (2014)

Generation and utilization of a flow of spin angular momentum of electrons in condensed matter, called spin current, are the key challenge of today’s nano-scale magnetism and spintronics. The discovery of the inverse spin Hall effect (ISHE) [1-3], the conversion of spin current into electric voltage via spin-orbit interaction, has allowed researchers to detect and utilize spin current directly, and, since then, many spin-current driven effects have been discovered by exploiting the ISHE. Here, such newly discovered spin-current effects will be outlined, including light-spin conversion [1,4], plasmon-spin conversion, sound-spin conversion, and heat-spin conversion [5-6], and their common mechanism and future possible application will be discussed. Among them, a typical conversion effect is the spin Seebeck effect (SSE) [5], spin current generation from a temperature gradient. SSE has attracted a great deal of interest since it may realize new type thermo-electric convertors which make full use of the characteristic feature of spins: the non-reciprocal dynamics. This non-reciprocity allows a spin to rectify thermal fluctuation into unidirectional spin current via the spin pumping mechanism, which can be converted into electric power via the ISHE. Spins, working as a natural rectifier in magnets, may thus provide a versatile mechanism of energy conversion in condensed matter. I will show also that the rectification mechanism underlies various spin related phenomena which were found recently. At the end of my talk, spin current generation from mechanical motion of condensed matter will be discussed.

[1] E. Saitoh et al., Applied Physics Letters 88 (2006) 182509.
[2] J. Wunderlich et al., Physical Review Letters 94 (2005) 047204.
[3] T. Kimura et al., Physical Review Letters 98 (2007) 156601.
[4] K. Uchida et al., Nature communications 6 (2015) 5910.
[5] K. Uchida et al., Nature materials 9 (2010) 894.
[6] T. An et al., Nature materials 12 (2013) 549.

Spin-filtering (SF) across an ultra-thin ferromagnetic insulating barrier between two non-magnetic electrodes can generate highly spin-polarized currents due to the preferential tunneling of one of the two spin orientations [1].
The ferromagnetic insulating spinel ferrites (CoFe₂O₄, NiFe₂O₄, MnFe₂O₄) are the candidates for achieving SF at room temperature as their Curie temperatures are well above 300K [2]. However, high efficiency SF at technologically useful temperatures [3,4] has been hampered by low structural quality of the barrier due to the lack of non-magnetic electrodes with the same crystal structure and lattice parameter of these complex oxides.
We report for the first time the successful growth of spinel metallic-superconducting LiTi₂O₄ and ferromagnetic insulating CoFe₂O₄ bilayers on spinel substrates (MgAl₂O₄). This LiTi₂O₄(200nm)/CoFe₂O₄(5nm) system could pave the road to the fabrication of a SF tunnel barrier in a closely lattice-matched all-spinel heterostructure, attempting to largely eliminate defects within the barrier and so greatly enhance the room temperature SF efficiency.


[1] J. S. Moodera, T. S. Santos, and T. Nagahama, J. Phys. Condens. Matter 19, 165202 (2007).
[2] J.-B. Moussy, J. Phys. D. Appl. Phys. 46, 143001 (2013).
[3] A. V. Ramos, M.-J. Guittet, J.-B. Moussy, R. Mattana, C. Deranlot, F. Petroff, and C. Gatel, Appl. Phys. Lett. 91, 122107 (2007).
[4] S. Matzen, J.-B. Moussy, R. Mattana, K. Bouzehouane, C. Deranlot, and F. Petroff, Appl. Phys. Lett. 101, 042409 (2012).

Transparent oxides, especially with p-type conductivity (p-TCO), are highly sought after for fabricating transparent junctions and devices. We show that by substituting La with Sr in LaCrO₃, a Mott insulator, we can effectively dope holes onto the B-site Cr cations and make Sr_xLa_1-xCrO₃ (x<0.25) a high performance p-TCO. Using oxygen-assisted molecular beam epitaxy (MBE), pn junctions consisting of Sr_xLa_1-xCrO₃ (x = 0.125) films and Nb doped SrTiO₃(001) (NbSTO) substrates were fabricated, and their electronic structure and band offset were characterized by in situ X-ray photoelectron spectroscopy (XPS). Core-level XPS and XANES show that Cr3+ is converted to Cr4+ to an extent commensurate with the Sr doping level, as expected from hole doping at Cr sites. XPS band alignment measurements reveal that the valence and band conduction offsets are ~2.0 eV and ~0.9 eV, respectively, thus constituting a pn junction. Matching rectifying behaviors were observed. In addition, variable temperature IV curves reveal transport mechanisms which differ from those of traditional semiconductors. These results are best explained by the characteristics of this particular oxide junction, and the kinds and quantities of defects that are present.

The spin-orbit interaction couples the electrons' motion to their spin. Accordingly, passing a current in a material with strong spin-orbit coupling generates a transverse spin current (spin Hall effect, SHE) and vice-versa (inverse spin Hall effect, ISHE). The emergence of SHE and ISHE as charge-to-spin interconversion mechanisms offers a variety of novel spintronics functionalities and devices, some of which do not require any ferromagnetic material. However, the interconversion efficiency of SHE and ISHE (spin Hall angle) is a bulk property that rarely exceeds ten percent, and does not take advantage of interfacial and low-dimensional effects otherwise ubiquitous in spintronics hetero- and mesostructures. Here, we make use of an interface-driven spin-orbit coupling mechanism - the Rashba effect - in the oxide two-dimensional electron system (2DES) LaAlO₃/SrTiO₃ to achieve spin-to-charge conversion with unprecedented efficiency. Through spin-pumping, we inject a spin current from a NiFe film into the oxide 2DES and detect the resulting charge current, which can be strongly modulated by a gate voltage. We discuss the amplitude of the effect and its gate dependence on the basis of the electronic structure of the 2DES.

In high oxidation state oxides like the trivalent Nickel oxides, tetravalent Co and Fe oxides as well as the parent superconductors BaBiO3 and SrBiO3the cation electron affinity for the formal valence taking Oxygen to be 2- could result in a cation electron affinity that is larger than the Oxygen ionization potential leading to a so called negative charge transfer gap. If the charge transfer energy is strongly negative, then we should really adopt starting electronic configurations such as Ni2+ rather than 3 + or Bi 3+ rather than 4+ with compensation holes in the O 2p valence band for charge neutrality. This leads to very different electronic structures and descriptions of the physical properties and the interpretation of spectroscopies than when starting from the formal oxidation state picture. We demonstrate that with this in mind we can very well explain many of the properties and phases of the Nickelates and the Ba(Sr)Bi O3 perovskite oxides as well as their spectroscopic properties. We discuss the general problem of treating such systems resulting in inverted crystal field [pictures and low spin rather than high spin states even for modest crystal fields. We also use a cluster exact diagonalization calculation to show that the x ray spectroscopy results now agree very well with experiment which has been a problem for decades. In the intermediate range of negative charge transfer gap systems we have to deal with mix valent starting points for which SmB6 is a typical example but also may include 3d transition metal compounds. We describe how these systems often termed as Kondo Lattice problems can be described in terms of mixed valent in momentum space when they are in their coherent low temperature state.

The purposeful introduction of an interface represents one of the most promising routes for tailoring and even inducing novel functionalities at the nanoscale in oxide heterostructures, as demonstrated in recent years.
In this contribution, our systematic study on the electronic and structural effects induced by the insertion of an electrically charged interface (two-dimensional doping) in insulating lanthanum cuprate (La₂CuO₄) will be presented.^[1] By taking advantage of the state-of-the art atomic-layer-by-layer Oxide MBE capabilities, we synthesized for the first time an epitaxial structure in which single LaO layers in La₂CuO₄ are substituted by a two-dimensionally confined dopant layer (SrO), with a predefined periodicity. In this way, a local electronic phase transition into the high-temperature superconducting state (T_c up to ≈ 35 K) was obtained. By combining low-temperature DC measurements, atomically resolved STEM imaging and spectroscopy, Zn tomography, a series of exciting findings was unveiled: (i) the electrical properties can be tuned by simply varying the spacing between the “active” SrO atomic planes; (ii) the final dopant concentration profile is highly asymmetric as a consequence of the specific MBE growth kinetics, being abrupt at the side of the interface facing the substrate (downward), whereas it is smeared over about 2 u.c. in the growth direction (upward); (iii) at the downward interface side, space-charge effects induce the formation of a hole accumulation layer, which is clearly decoupled from the Sr distribution profile and which acts as a compensating charge for the presence of the confined SrO layer.
This work highlights the great potential of space-charge effects in electronic complex oxide structures for high-T_c superconductivity, allowing for the local tuning of the functionalities by simply introducing an interface of a-priori known position and charge. Moreover, it sheds light onto the intimate interaction between the ionic chemistry and the electronic state at the interface.
[1] F. Baiutti, G. Logvenov, G. Gregori, G. Cristiani, Y. Wang, W. Sigle, P. A. van Aken & J. Maier, Nat. Commun. 6, 8586 (2015)

Pulsed laser deposition (PLD) is widely used technique for the fabrication of smooth, stoichiometric and epitaxial thin films of various compound materials. With in-situ monitoring tool such as reflection high energy electron diffraction (RHEED) can provide precise control of the thickness of PLD-grown film. Here, we are reporting the results on RHEED controlled ultra thin films (~6 uc) of LaAl_1-xCr_xO₃/SrTiO₃ (0 ≤ x ≤ 1.0) prepared by PLD. The films are grown on TiO₂ terminated SrTiO₃ (100) single crystal substrate. Purpose of the study is to address the critical role of controlled transition metal (Cr) doping on the quasi-two-dimensional electron gas (q-2DEG) at LAO/STO interface where one end in the series is purely conducting (x = 0) and the other is completely insulating (x = 1). An increase in the sheet resistance proportionate with decrease in the sheet electron density is observed in these heterostructure with increasing Cr at% in the over-layers. X-ray photoemission spectroscopy (XPS) and ultraviolet photoemission spectroscopy (UPS) measurements have been performed to understand the Cr concentration dependent metal-insulator electronic phase transition at the interface. Ti and Cr 2p XPS core level spectra clearly establishes a proportional decrease/increase in Ti³⁺ / Cr²⁺ ion concentration with increasing Cr content in LaAl_1-xCr_xO₃ films. UPS valence band spectra show continued suppression Ti 3d states and simultaneous opening of energy gap with increasing Cr concentration in the films. Our study confirms that polar catastrophe induced electronic reconstruction of Ti ions (Ti⁴⁺/Ti³⁺) drives the interface conductivity in Al rich films. On the other hand, for Cr rich films (x ≥ 0.6), competing electronic reconstruction of Ti ions at interface and Cr ions in the film suppresses the polar catastrophe and results in insulating interface. Our observation strongly favors electronic reconstruction as the origin of quasi-2DEG in these oxide heterostructures, apart from the other possibilities such as interface chemical disorder and O defects.

Ferroelectricity in nanoscale systems has recently attracted tremendous attention both from fundamental science perspective and from technological perspective (such as ferroelectric tunnel junctions). Especially, in the last few years, multiple reports claiming ferroelectricity in ultrathin ferroelectric films based on formation of remnent polarization states, local electromechanical hysteresis loops, and pressure induced switching were made. However, similar phenomena were also reported for traditionally non-ferroelectric materials, including strained and unstrained SrTiO₃, LaAlO₃-SrTiO₃ multilayers, and TiO₂, creating significant level of uncertainty in the field.
Here we pose that in ultrathin ferroelectric films, ferroelectric state is intrinsically inseparable from electrochemical state of the surface, leading to fundamentally new type of mixed ferroelectric state. We first explore theoretically the thermodynamics and thickness evolution of this state. Then, as a model system, we investigate ferroelectricity of a few monolayer (ML)-thick BaTiO₃ films (from 4 to 10 ML) on SrTiO₃-buffered silicon grown by molecular beam epitaxy. The band excitation piezoresponse force microscopy results combined with contact Kelvin probe force microscopy results show the presence of such mixed ferroelectric-electrochemical states. We further discuss how this mixed state can explain multiple prior studies.
This research was sponsored by the Division of Materials Sciences and Engineering, Basic Energy Sciences, Department of Energy (SVK). Research was conducted at the Center for Nanophase Materials Sciences, which also provided support (SMY) and which is a DOE Office of Science User Facility.

Complex oxide heterostructures can show strong correlation effects, novel magnetism, high breakdown voltage, and high 2D electron density (of the order of 10¹⁴ cm⁻²), unattainable in traditional semiconductor heterostructures. High 2D electron densities are of particular interest for studying low-dimensional physics and fabricating novel devices such as plasmonic field-effect transistors. Here, we discuss the electronic properties of a system with a quasi-2D ultrahigh-density electron gas exceeding 3×10¹⁵ cm⁻². This novel 2DEG results from a broken-gap band alignment at the NdTiO₃/SrTiO₃ (NTO/STO) interface which we have experimentally detected and theoretically verified. The 2DEG density is tunable by means of the NdTiO₃ film thickness and controlled exposure to surface adsorbates.

NdTiO₃/SrTiO₃ and capped SrTiO₃/NdTiO₃/SrTiO₃ heterostructures were grown on LSAT(001) using hybrid molecular beam epitaxy. The NdTiO₃ thickness (t) was varied from 2 to 24 u.c.. Uncapped NTO/STO shows no measurable conductivity for t < 4 u.c. However, an insulator-to-metal transition occurs at t ≥ 4 u.c. with two distinct metallic regimes of high and low sheet resistance at 8 u.c. ≤ t ≤ 12 u.c. and t > 12 u.c., respectively. Hall measurements show n-type behavior with a carrier density of ≈0.5 e⁻ per u.c. per interface (≈3×10¹⁴ cm⁻²) and a critical thickness of 10 u.c., followed by an increase leading to saturation at higher t. In contrast, STO-capped heterostructures exhibit metallic behavior for t ≥ 2 u.c. and a much higher carrier density than uncapped NTO/STO for t up to 12.u.c.

To address the origin of this behavior, we employ ab initio simulations based on density functional theory. Our simulations show that the metallic regime at low t results from electronic reconstruction in which Ti at interfacial TiO₂ planes acquire an average charge of 3.5+. As t increases, the NTO lower Hubbard band moves to higher energy until it becomes degenerate with the bottom of the STO conduction band. As a result, electron transfer occurs from NTO to STO, and the NTO thickness determines the number of electrons available for transfer, while the STO layer thickness determines the width of the potential well that traps these electrons.

The much higher conductivity threshold in the uncapped system points to surface chemical modification resulting from air exposure. To investigate this further, we calculated formation energies of interstitial oxygen defects in NTO and determined their geometrical configurations. These simulations suggest that NTO can readily adsorb oxygen and, accordingly, convert Ti³⁺ to Ti⁴⁺. This result indicates that surface reactivity can have a strong influence on interfacial conductivity and can be exploited to control the insulator-to-metal transition in this system.

P. Xu, T. C. Droubay, J. S. Jeong, K. A. Mkhoyan, P. V. Sushko, S. A. Chambers, B. Jalan, Adv. Mater. Interfaces (2015), 1500432.

Manipulating electronic/magnetic properties of materials by electric fields is the key to many applications and provides important insights into the electronic structure of the materials involved. I will start with a brief overview of the current status of electric field gating of oxide heterostructures.
Using top-gating we could show the modulation of the normal state and superconducting properties of LaAlO₃/SrTiO₃ 2-DEG systems [1]. Building up on that work, we have now conducted more detailed investigations on the simultaneous top- and back-gating. In particular, the studies include the transition from single to multiband transport character, modification of the Shubnikov-de Haas oscillations under electric field gating, and field-induced alterations of the confinement potential and the 2-DEG band structure [2].
Finally, I will discuss electric field effect studies on LaMnO₃/SrTiO₃ interfaces. A central question is whether in those systems, that show a critical thickness for transition to a ferromagnetic state [3], the magnetism can be tuned by an applied electric field.

[1]: P. Eerkes et al., APL 103, 201603 (2013).
[2]: S. Smink et al., L. Tang et al., publications in preparation
[3]: X. Renshaw Wang et al., Science 349, 716 (2015).

The two-dimensional electron liquid that forms at the interface between the two insulating oxides LaAlO₃ (LAO) and (001) SrTiO₃ (STO) has stimulated enormous research interest, owing to its extraordinary tunable electronic properties, such as high carrier mobility, large spin-orbit coupling, and superconductivity. One of the hallmarks of the system is the transition from an insulating to a conducting state that occurs when the LAO film exceeds 3 unit cells. Such a critical thickness is also observed at LAO/STO interfaces grown along the (110) and (111) orientations.
In the (001) case, the metallic state and the presence of a critical thickness have for origin the polar discontinuity that is found at the interface. The electric field inside the LAO layer can lead to either a Zener breakdown or the creation of oxygen vacancies at the LAO surface.
In this study, we report on the observation of metallic conduction in LAO/STO interfaces capped with STO crystalline films. Heterostructures with LAO layers as thin as 1 unit cell are found to be metallic and superconducting. We discuss these results considering a model based on the polar discontinuity and leading to surface oxygen vacancies.

Epitaxial growth of atomically-sharp interfaces serves as one of the main building blocks of
nanofabrication. Such interfaces are crucial for the operation of various devices including transis-
tors, photo-voltaic cells, and memory components. In order to avoid charge traps that may hamper
the operation of such devices, it is critical for the layers to be atomically-sharp. Fabrication of
atomically sharp interfaces normally requires ultra-high vacuum techniques and high substrate tem-
peratures. We present here a new self-limiting wet chemical process for deposition of epitaxial layers
from alkoxide precursors. This method is fast, cheap, and yields perfect interfaces as we validate by
various analysis techniques. It allows the design of heterostructures with half-unit cell resolution.
We demonstrate our method by designing a hole-type oxide interface SrTiO3/BaO/LaAlO3. We
show that transport through this interface exhibits properties of mixed electron-hole contributions
with hole mobility exceeding that of electrons.

The electron mobility is one of the key parameters in investigating modern mesoscopic physics and covers a wide range of quantum phenomena. Understanding how the charge carrier mobility can be increased in complex oxides is one of the most significant challenges this area is facing if oxide electronic should reach the market. The mobility of complex oxide is still orders of magnitude lower than that of the conventional semiconductors and with the current fabrication method we still cannot fully control the charge at the interface. In this paper I will present our contribution to achieve such high mobility complex-oxide samples, e.g the heterointerface between SrTiO₃ and a spinel γ-Al₂O₃ epitaxial film [1] and the realization of modulation doping in disordered-LaAlO₃/SrTiO₃ interfaces (d-LAO/STO) with a single unit cell spacer layer of LaSrMnO₃ (LSM) inserted at the interface [2], and explain the strategies behind that led us to obtain very high mobilities in these interfaces.

1. Y. Chen and N Pryds et al., “A high-mobility two-dimensional electron gas at the spinel/perovskite interface of γ-Al2O3/SrTiO3”. Nature Comm. 4, 1371 (2013)
2. Y. Chen and N Pryds et al., “Extreme mobility enhancement of oxide 2DEGs via charge transfer induced modulation doping”, Nature Materials 14, 801–806 (2015)

Electron energy loss spectroscopy (EELS) in a new generation of aberration-corrected electron microscopes provides direct images of the local physical and electronic structure inside a material at the atomic scale. The sensitivity and resolution can extend to imaging single dopant atoms or vacancies in their native environments. The ability to simultaneously measure local structural distortions, along with local bonding changes is invaluable for understanding electron-lattice interactions at internal interfaces. Comparable advances in detector technology are now poised to enable a similar revolution in the measurement of structure and fields in materials, enabling direct measurements of probability current flow and orbital angular momentum transfer.

Complete information about the scattering potential of a sample is in principle encoded in the distribution of scattered electrons from a localized beam propagating through it. A new generation of high-speed imaging detectors brings us closer to this goal and will allow us to explore practical limits and identify the most promising methods of analysis. We have recently developed an electron microscope pixel array detector (EMPAD) that functions as a compact and high-speed, high dynamic range electron diffraction camera (Figure 1a). It has single electron sensitivity with a signal/noise ratio of 140 for a single electron at 200 keV [1,2]. It has a dynamic range of 10⁶ for primary electrons– i.e a pixel can detect from 1 to 1,000,000 electrons, and reads out an image frame in under < 1 ms. These properties allow us to record essentially an image of all the transmitted electrons, from the unscattered beam to out beyond the HOLZ lines, and do so for every probe position in a real-space, atomic resolution image. Not only does this allow quantitative and simultaneous annular dark and bright field signals on an absolute scale, but from the analysis of the spatially-resolved diffraction patterns we can extract thickness, strain and tilt, octahedral rotations, polarity and even electric and magnetic fields.

We demonstrate electrical mapping of tetragonal domains and electric-field-induced twin walls in SrTiO₃ as a function of temperature and gate bias utilizing the conducting LaAlO₃/SrTiO₃ interface and low-temperature scanning electron microscopy [1]. Conducting twin walls appear below 105 K, and new twin patterns are observed after thermal cycling through the transition or on electric field gating. The nature of the twin walls is confirmed by calculating their intersection angles for different substrate orientations. Numerous walls formed when a large side- or back-gate voltage is applied are identified as field-induced ferroelectric twin walls in the paraelectric tetragonal matrix. The walls persist after switching off the electric field and on thermal cycling below 105 K. These observations point to a new type of ferroelectric functionality in SrTiO₃, which could be exploited together with magnetism and superconductivity in a multifunctional context.

[1] H.J. Harsan Ma et al., Phys. Rev. Lett. 116, 257601 (2016)

Epitaxial oxide films can support large percent-level strain levels, well beyond their bulk counterparts. As a result, biaxial strain can provide a means of strain-engineering phenomena in correlated electron systems. For example, a strain-induced orbital selective Mott transition has recently been observed in epitaxial thin films of VO₂.¹ In such studies of epitaxial oxides (typically ~ 10 nm or less), contributions from the surface and interfaces must be explicitly considered. This is especially true when employing surface-sensitive spectromicroscopy techniques, such as low energy electron microscopy (LEEM) and photoemission electron spectroscopy, to determine whether strain-induced effects persist to the topmost atomic layer. Here, we report our latest LEEM studies of biaxial-strained VO₂ on various TiO₂ orientations (001), (100) and (110). We conclude that the VO₂ surface is reconstructed for all three strain-engineered orientations; all films showed a rutile-like (1x1) reconstruction regardless of temperature i.e. no structural transition occurs at the topmost atomic layer. The VO₂/TiO₂(110) surfaces displayed a second (2x2) surface reconstruction when prepared at higher oxygen partial pressures, which was again independent of temperature. A transition from the (2x2) to the (1x1) surface reconstruction of VO₂/TiO₂(110) under certain conditions was observed, and explained in terms of electron-beam oxygen desorption. This transition is irreversible and should not be confused with a structural phase transition that accompanies the metal insulator transition. As a result, the topmost atomic layer(s) of epitaxial VO₂ are found to reconstruct and are not representative of the bulk of the epilayer.

(1) Mukherjee, S.; Quackenbush, N. F.; Paik, H.; Schlueter, C.; Lee, T.-L.; Schlom, D. G.; Piper, L. F. J.; Lee, W.-C. Tuning a Strain-Induced Orbital Selective Mott Transition in Epitaxial VO2. arXiv:1603.00485 2016, 1–7. (accepted for publication in Physical Review B Rapid Communication)

Coherent Bragg Rods Analysis (COBRA) [ref. 1], as one of the phase retrieval methods in X-ray diffraction (XRD), provides a powerful approach to resolve ultra-high resolution 3D atomic structure of a material by reverse Fourier transformation of the structure factor reconstructed from its diffraction intensity. By improving the current COBRA routines, we generalized its applicable scope from only tetragonal symmetry to any symmetry of the film and substrate. The performance of our new routines was tested to be capable of handling large dataset simultaneously (>150 rods) without compromising the accuracy. In this work, we applied our new COBRA routines on a tensile strained series of CaTiO₃ thin films, in which exists competing between ferroelectric distortion and oxygen octahedral rotation (OOR) [ref. 2]. High resolution Scanning Transmission Electron Microscopy (STEM) and Density Functional Theory (DFT) study was also performed on the same systems.


We demonstrate high accuracy (~0.02) 3D atomic structure of CaTiO₃ obtained by COBRA analysis, and it is consistent with STEM and DFT study. The evolution of Ca, Ti and O atoms is clearly visible from the film-substrate interface to film surface. Details analysis shows that substrate OOR will effectively clamp the in-plane OOR in CaTiO₃ films within 6 – 8 unit cells, while the out-of-plane OOR in CaTiO₃ is mostly affected by epitaxial strain imposed by substrate. This interfacial OOR clamping effect gives rise to an unusual out-of-plane polarization component at low temperature, which is predicted to be zero under the same tensile strain by current theories without considering interfacial effect [ref. 2,3]. Most impressively, in the CaTiO₃/LSAT (1.2% tensile) system, the almost zero in-plane OOR in CaTiO₃ clamped by LSAT gives rise to a strong room temperature polarization. This work is the first sub-angstrom 3D atomic structural imaging of functional oxides interface, where a direct competing between OOR and ferroelectric distortion is experimentally observed.



1. Y. Yacoby, et al., J. Phys.: Condens. Matter 12 (2000)
2. C.-J. Eklund, et al., PHYSICAL REVIEW B 79, 220101R (2009)
3. Y. Gu, et al., PHYSICAL REVIEW B 85, 064117 (2012)

Using advanced technique combining pulsed laser deposition growth of LaAlO₃, LaTiO₃ and SrTiO₃ we effectively constructed half-integer unit cell number LaAlO₃-SrTiO₃ heterostructures where all interfaces are of LaO-TiO₂ type, and two dimensional electron gas (2DEG) forms symmetric n-type bilayer. Using ultra high vacuum scanning tunneling microscopy we investigated the properties of surface 2DEG in these heterostructures [1]. Our results indicate that surface 2DEG is strongly, within one unit cell, confined at the interface. Tunneling spectroscopy of surface 2DEG reveals thickness dependent band gap changes attributed to quantum size effect.
References:
1. I. Altfeder, H. Lee, J. Hu, R. D. Naguy, A. Sehirlioglu, A. N. Reed, A. A. Voevodin, C. B. Eom, Phys. Rev. B 93, 115437 (2016)

At the interface between insulating oxides such as SrTiO₃/LaAlO₃ or LaTiO₃/SrTiO₃, a superconducting two-dimensional electron gas (2DEG) has been discovered [1-3], whose carrier density can be tuned by applying a gate voltage. The unique possibility of modulating the superfluid density easily and continuously opens new perspectives to tackle fundamental issues in condensed matter physics, such as the Superconductor to Insulator Quantum Phase Transition (QPT) in a two-dimensional system.

Using two different external parameters, the magnetic field and the electric field, we explored the phase diagram of the 2DEG. Studying the magnetic-field driven QPT, we first evidence that the system can be described as a disordered array of superconducting puddles, and show that the critical behavior belongs to the (2+1)D XY universality class in the clean limit at the local scale, and to the dirty limit at the scale of the array, in agreement with the Harris criteria [4].

The electric-field driven QPT reveals an anomalous critical behavior, that we attribute to density driven superconducting fluctuations, different from the regular amplitude and phase fluctuations in a superconductor. We propose a scenario where the observed inhomogeneities origin from an intrinsic electronic phase separation, driven by the non-rigidity of the bands upon filling and Rashba Spin-Orbit coupling [5,6]. Superconductivity being intimately related to the electronic density, the dynamics in the Cooper pair channel is dominated by nearly critical dynamical density fluctuations[7].

References
[1] N. Reyren et al, Science 317, 1196 (2007)
[2] J. Biscaras et al , Nature Communications 1, 89 (2010)
[3] J. Biscaras et al, Physical Review Letters 109, 246808 (2012)
[4] J. Biscaras et al, Nature Materials 12, 542 (2013)
[5] N. Scopigno et al, Physical Review Letters 116, 026804 (2016)
[6] S. Hurand et al, arXiv 1506.06874v1
[7] S. Caprara et al, J. of Physics, Condensed Matter 27, 425701 (2015)

The demand for new materials with novel or coupled properties has spurred much interest in the development of heterostructures. In particular, layered oxide materials have garnered a lot of attention and several design paradigms have been theoretically developed to introduce new functionalities. These models provide paths to designing new functionality such as hybrid improper ferroelectricity and polar metals. Recent work in the hybrid improper ferroelectric system (Ca,Sr)₃Ti₂O₇, has shown an abundance of charge domain walls. This finding is unexpected due to the formation high energy relative to uncharged walls and is currently not well understood. We present atomic-scale imaging charged and uncharged ferroelectric domain walls in insulating Ca₃Ti₂O₇ and metallic Ca₃Ru₂O₇ by aberration corrected scanning transmission electron microscopy. Analysis of the images reveals several structural features characteristic of the different types of domain walls, which is then compared to the DFT predicted structures. We will also discuss the possible role of 180 degree domain walls within rock salt layers stabilizing charged domain walls. These findings reveal important insights into the structure and formation of domain walls in hybrid improper layered oxides.

SrTiO3 (STO) is often used as a substrate for growing thin superconducting films. STO undergoes a structural transition at 105K, where the lattice breaks into tetragonal domains. We use a scanning Superconducting Quantum Interference Device (SQUID) to map the diamagnetic response of Nb and NbN deposited on STO, at their SC phase. We show that STO domains and domain walls cause large scale modulations of the superfluid density, opening possibility for better understanding of the effect of local structure on superconductivity, and a local control of the strength of superconductivity.

Polarity at SrTiO3 ferroelastic domain walls was recently suggested by reflectometry measurements. I will describe our investigation of the possible influence such polarity could have on electronic states of nearby conducting layers. We tracked the electrical response to local pressure applied to the sample. These measurements revealed stripy features where the response to stress was much greater than their surroundings. We relate these stripes to SrTiO3 domain walls, and suggest that SrTiO3 domain walls are effectively local gates. These local gates can be manipulated using external stress and control the electronic behavior of a nearby interface.

Nano-ionic redox processes are important for resistive switches and non-volatile memory cells, neuromorphics, cognitive computing, and solid state electrolyte devices, i.e. batteries and fuel cells. We study such processes here for a memristive device. In standard oxide memristors, channel formation typically requires an irreversible, not well-controlled electroforming process, giving difficulty to independently control ionic and electronic properties. The device performance is also limited by the incomplete understanding of the underlying mechanisms. Here, we develop a novel memristive model material system based on epitaxial, self-assembled Sm-doped CeO₂ and SrTiO₃ films, allowing the separate tailoring of nanoscale ionic and electronic channels at high density (~10¹² inch^-2). We systematically show these devices allow very precise engineering of the resistance states thus enabling large on-off ratios, with high reproducibility. The understanding from the new structures is highly relevant to other nascent technologies for optimising and tuning ionic and electronic conduction in thin films to give strongly enhanced functional properties.

Recent work has shown the formation of three-dimensional polar vortices in ferroelectric/paraelectric (PbTiO₃)_n/(SrTiO₃)_n superlattices [1]. This novel phase is stabilized by the interplay of lattice, charge, and orbital degrees of freedom. The properties of such superlattices exhibit a strong dependence on tunable parameters, including the electrostatic and elastic boundary conditions. Using different superlattice structures to change these boundary conditions, it is possible to observe new polarization states. Previous work has been done on superlattices with a 3-unit-cell layer thickness of SrTiO₃ and varying thicknesses of PbTiO₃ [2]. In structures with 1 and 2 unit cells of PbTIO₃, improper ferroelectricity emerges from antiferrodistortive/ferroelectric coupling at the interfaces, which leads to oxygen octahedra rotations. This work suggests that there is more to be explored in terms of novel polarization states in asymmetric superlattices.
In this work, we employ a systematic study of the ground state electrical polarization in asymmetric superlattices of PbTiO₃ and SrTiO₃ grown by pulsed laser deposition. Using a 10-unit-cell layer thickness of PbTiO₃ while varying the thickness of SrTiO₃ between 2 and 100 unit cells, phase-field modeling predicts a systematic evolution of the polar state in the superlattice as a function of SrTiO₃ layer thickness. We will present results of a combined experimental-theoretical study of such asymmetric superlattices that form the precursors for the emergence complex topologies of electrical polarization in SrTiO₃ and PbTiO₃. Specifically, for very small thicknesses of SrTiO₃, theory predicts it will become polarized by a "proximity effect" from the surrounding layers of PbTiO₃. We study this by experimentally observing changes in the polarization of asymmetric superlattices as a function of SrTiO₃ thickness. We map the electrical polarization in superlattices using high-resolution scanning transmission electron microscopy in conjunction with mesoscale piezoresponse force microscopy studies of the ferroelectric domain structure and x-ray spectromicroscopy with linearly polarized x-rays to probe the spatial evolution of the polar order.
[1] A.K. Yadav, et al., Nature 530, 198-201 (2016).
[2] E. Bousquet et al., Nature 452, 732-6 (2008).

Electronic and atomic reconstruction at the interfaces between complex oxide thin films can result in intriguing novel functionalities [1]. An important and central question is then the impact of structural changes on ferroic properties of the individual layers, driven by incompatible structural symmetries of the constituent materials. Here we report on the impact of such interfacial reconstructions in a non charge transfer system, i.e. (111)-oriented heterostructures consisting of antiferromagnetic LaFeO₃(LFO) and ferromagnetic La_0.7Sr_0.3MnO₃ (LSMO). At the LFO/LSMO (111) interface a shift from in-phase to out-of-phase octahedral rotations must be accommodated. Scanning transmission electron microscopy and density functional theory (DFT) investigations reveal a lower octahedral symmetry at the interface, which extend 3-5 atomic layers from the interface into the LFO layer. Spin polarized neutron reflectivity and x-ray magnetic circular dichroism measurement are indicative of a non-vanishing net Fe moment 3-5 layers into the otherwise antiferromagnetically ordered LFO. The net Fe moment in LFO is antiferromagnetically coupled to the Mn moment of LSMO. Furthermore the DFT calculations reveal that the change in symmetry along with a change in correlation strengths at the interface is necessary to induce a net Fe magnetic moment. The resulting magnetic state of LFO closest to the interface is ferrimagnetic, affected by the degree of electron localization of the Fe 3d orbitals. Hence our data point towards the importance of structural changes/relaxations near interfaces in combination with electron correlation strength to tailor the effective functional properties of epitaxial interfaces.

[1] H. Y. Hwang, Y. Iwasa, M. Kawasaki, B. Keimer, N. Nagaosa, and Y. Tokura, Nat. Mat. 11, 103 (2012).

The possible coexistence of polarity and metallicity in one material system has recently attracted intensive interest [1-3] since Anderson and Blount's [4] theoretical predictions half a century ago. The polar nature is uncommon since the conducting electrons would be expected to screen out the long-range Coulomb interaction that favors the dipole-dipole cooperative ordering. While domain/domain walls morphology and topology in ferroelectrics are well studied by theory [5] and experiment [6], no such observations are reported in a polar metal system so far. Here by a combination of meso-scale optical second harmonic generation (SHG) imaging and atomic-scale high resolution aberration corrected scanning transmission electron microscopy (STEM), for the first time we achieved a direct observation of the electric domains in a polar metal Ca₃Ru₂O₇ system, unveiling the existence of the 90° “ferroelastic” and 180° “ferroelectric” domain/domain walls. Ca₃Ru₂O₇ is shown to possess intriguing domain structures comprising abundant meandering charged domain twinnings similar to its isostructural ferroelectrics Ca₃Ti₂O₇, as understood under the same framework of hybrid improper “ferroelectricity” mechanism. In addition, for this specific 4d transition metal oxide Ca₃Ru₂O₇, the nanometer-thickness domain walls, as natural existing interfaces, are also found to show unique local conducting properties, which were manifested in our conductive-AFM and Electron Energy Loss Spectroscopy results (EELS). The exotic domain wall behaviors are discussed under the framework of the combined nature of polar order and metallicity in Ca₃Ru₂O₇. Our observations open up the possibility to utilize the nanoscale domain wall in a polar metal for nanoelectronics.

A growing body of work has developed particularly investigating formation of oxygen vacancies in SrTiO₃ (STO) and their properties. One approach involves heteroepitaxy of an oxide with large, negative enthalpy of formation to stabilize an ultra-thin oxygen-deficient STO layer. In order to study the mechanism of formation for this conducting layer, we compare two different spinel oxide overlayers, semiconductor Co₃O₄ and insulator γ-Al₂O₃ . While the data shown here are for STO (001) single crystal substrates, both heterostructures can also be integrated on Si (001) through use of an STO buffer layer.
For deposition of γ-Al₂O₃ , Al metal was evaporated from a cold-lip effusion cell at arate of 1.8 Å/min, with molecular oxygen introduced at a background pressure of 5×10⁻⁶ Torr for the growth. For Co₃O₄, the electron-beam evaporated Co flux was calibrated to a deposition rate of 3-4 Å/minute. Atomic oxygen was introduced by means of an RF plasma source at a power of 250 W with an oxygen background pressure of 1×10⁻⁵ torr. The samples were monitored during growth by in situ RHEED. Substrate temperatures of 700°C and 500°C were used for epitaxy of γ-Al₂O₃ and Co₃O₄, respectively.
In situ XPS measurements of Ti 2p core level provide information on the oxygen vacancy concentration on the STO side of the interface. While the bulk STO substrate and Co₃O₄ films show only two spin-split peaks corresponding to fully oxidized Ti⁴⁺, the spectrum after MBE growth of a thin alumina film reveals a shoulder at lower binding energies corresponding to Ti in a reduced environment. This is consistent with ex-situ electrical measurements. STO substrates and Co₃O₄/STO were both insulating (RS > GΩ), while γ-Al₂O₃/STO heterostructures were highly conductive (R_S ~ 10 kΩ).
The difference can be understood from a thermodynamic point of view. During deposition of alumina on STO, two reactions take place: reduction of STO and oxidation of metal. By comparing two oxides, we illustrate the importance of enthalpy of formation in the oxide overlayer on the oxygen vacancy 2DEG in STO. This tunability lends itself to both the development of sensors and the investigation of novel physical phenomena.

Atomically flat (111) interfaces between insulating perovskite oxides can provide a landscape for new electronic phenomena. For instance, the metallic ion / metal oxide bilayers that comprise the unit cell require both interfacial layers to be polar, generating an intrinsic polar discontinuity. The graphene-like coordination between interfacial metallic ion layer pairs provides alternate band structure leading to topologically protected states. We look at a specific example of an atomically smooth (111) heterointerface, in the model system of (111)-oriented LaAlO₃ / SrTiO₃ (LAO/STO) that preserves many features characteristic of exotic electronic interfacial states, and find that the LAO overlayer relieves the structural reconstruction of the STO (111) surface. The energetics are satisfied by global electronic reconstruction. This is confirmed by the measured transport characteristics of the resulting interfacial conducting layer, the direct determination of the structure and atomic charge distribution by Coherent Bragg Rod Analysis (COBRA), and theoretical calculations of electronic and structural characteristics. Interfacial behaviors of the kind discussed here will likely lead to new interfacial electronic devices.

This work has been done in collaboration with S. Ryu, H. Lee, J. Lee, C. W. Bark, T. Hernandez, T. R. Paudel, Hua. Zhou, D. D. Fong, Y. Zhang, J. Podkaminer, X. Q. Pan, E. Y. Tsymbal, M. S. Rzchowski, and C. B. Eom.

Perovskite bilayers with (111)-orientation combine a honeycomb lattice as a key feature with the
strongly correlated, multiorbital nature of electrons in transition metal oxides. In a systematic DFT+U study we establish trends in the evolution of ground states versus band filling in (111)-oriented (LaXO₃)₂/(LaAlO₃)₄ superlattices, with X spanning the entire 3d transition metal series. The competition between local quasi-cubic and global trigonal symmetry triggers unanticipated broken symmetry phases not accessible for the (001)-growth direction, with mechanisms ranging from Jahn-Teller distortions, to charge-, spin-, and orbital-ordering. Constraining the symmetry between the two triangular sublattices causes X = Mn, Co, and Ti to emerge as Chern insulators driven by spin-orbit coupling. For X = Mn we illustrate how interaction strength and lattice distortions can tune these systems between a Dirac semimetal, a Chern and a trivial Mott insulator [1].Further realizations of the honeycomb lattice and geometry patterns beyond the perovskite structure will be addressed.

Funding by the DFG within SFB/TR80 is gratefully acknowledged.

[1] D. Doennig, S. Baidya, W.E. Pickett and R. Pentcheva, Phys. Rev. B 93, 165145 (2016).

Nickelates form one of the canonical examples of a system driven between a metal and an insulator due to strongly competing interactions. The challenge, however, is to understand how to control these phases and, more importantly, which of the interactions (spin, charge, orbital, and lattice) are the key to forming these different states. To gain insight into this problem, research has recently explored extensively the question of how heterostrutures can be utilized to control the different interactions via strain and confinement[1-2]. Though these new states tend to be driven away from the equilibrium ground-state and as we seek to understand how to efficiently control them with external input (e.g. light), one of the grand challenges is to understand how to map the non-equilibrium phase space. This information is crucial not only to seek conditions where new states emerge, but also as a basis for the design of materials that will help meet the energy needs of the future, since many of those applications require materials in highly non-equilibrium environments. In this talk, I will touch on recent work ranging from strain control to optical excitation to watching how nickelates grow in order to illustrate how forefront X-ray tools are gaining insight into non-equilibrium phases.
Work at Argonne is supported by the U.S. Department of Energy, Office of Science, under Contract No. DE-AC02-06CH11357.

References:
[1] J. Chakhalian et. Al., Rev. Mod. Phys. 86, 1189 (2014).
[2] www.annualreviews.org/doi/abs/10.1146/annurev-matsci-070115-032057

Charge transfer is the most important functionality of electrodes. This holds in particular for electrolyte-electrode heterostructures. Typically, charge transfer is assessed with electroanalytical methods. With increasing complexity, particular chemical complexity of electrode materials and architectures, it becomes more and more difficult to comprehend the charge transfer of electrodes with their immediate environment, be it the electrolyte or be it the source/sink of redox partners. Hence, an "integrated" signal like from electrical measurements permits only limited information on the structural origin of charge transfer. This holds particularly in systems where the electrode meets a liquid. I will show how x-ray and electron spectroscopy have emerged in the recent years so well that it is possible to virtually "deconvolute" the charge transfer between electrode and liquids or gases with element specific and even orbital specific sensitivity. In the beginning it was not so easy to design targeted experiments where "living" systems such as batteries or fuel cells or photoelectrochemical cells could be investigated and monitored "operando", means in operation. But it works now. We can look into electrochemical interfaces with very high spatial resolution and energy and momentum transfer resolution. Therefore I will shows examples from battery research and PEC cell research, specifially with soft- ray spectroscopy, where the valence band characteristic can explain at the quantitative level where the charge transfer actually originates from, such as from O2p orbitals or from 3d orbitals, depending on case-to-case.

We will present on the growth of BaSnO₃ grown via a novel hybrid MBE approach, which uses a chemical precursor (hexamethylditin) for Sn, a solid source for Ba and an rf plasma for oxygen. Epitaxial BaSnO₃ films were grown on SrTiO₃ (001), LaAlO₃ (001) and LSAT (001) substrates. The substrate temperature and oxygen pressure were kept fixed at 900 °C and 5×10^-6 Torr respectively, whereas Sn/Ba beam equivalent pressure (BEP) ratio was varied between 9.7 – 30.6 to optimize cation stoichiometry. The out-of-plane lattice parameter determined using high-resolution X-ray diffraction, and the Rutherford backscattering spectroscopy (RBS) were used to optimize cation stoichiometry. An MBE “growth window” was identified for BaSnO₃, where cation stoichiometry was self-regulating, i.e. for a range of Ba/Sn BEP ratio stoichiometric films are attained. Time-dependent reflection high-energy electron diffraction (RHEED) intensity oscillations confirmed that films grew in a layer-by-layer fashion. A careful study of RHEED patterns as a function of time recorded during growth indicated a critical thickness of ~ 1 nm for strain relaxation for films grown on SrTiO₃ at a growth temperature of 900 °C. Atomic force microscopy confirmed smooth surface morphology for stoichiometric films with a root mean square roughness of 1-2 u.c., whereas non-stoichiometric films showed nano-crystallites on film surface indicating a correlation between surface morphology and film stoichiometry. Scanning transmission electron microscopy combined with electron energy-loss spectroscopy and energy dispersive X-ray spectroscopy confirmed the cube-on-cube epitaxy and composition.
Finally, we will present on the electronic transport properties of La-doped BaSnO₃. Carrier density and room-temperature mobility were used to optimize the buffer and active layer thicknesses. An optimum thickness of 124 nm each was determined. Films were grown with different dopant concentrations between 5×10¹⁸ cm^-3 and 1×10²¹ cm^-3. A maximum room-temperature mobility of 120 cm²V^-1s^-1 was observed at a carrier density of 4×10²⁰ cm^-3, which is amongst the highest reported mobility in La-doped BaSnO₃ films on SrTiO₃ substrate reported so far in literature. A metal-to-insulator transition (MIT) was observed at a resistivity value between 0.03 Ωcm – 0.06 Ωcm consistent with Mott’s prediction. This MIT occurred at a carrier density between 5×10¹⁸ cm^-3 and 1×10¹⁹ cm^-3, which is higher that the expected value, indicating the role of compensating defects. A detailed study of temperature dependent mobility indicated different scattering mechanism at play at different temperature range and different carrier concentration. These will be discussed in the framework of further improving the room-temperature mobility in thin films of BaSnO₃.

X-ray absorption spectroscopy (XAS) has already proven to be a powerful experimental technique to probe the valence state and local structural properties with element specificity. Combined with the x-ray magnetic circular dichroism in magnetic fields it allows in-depth investigations on novel functional, multiconstiuent materials like doped or complex oxides. We have recently shown that this tool-box can be augmented by studying oxides using XAS in electrical fields [1]. Upon applying a voltage across a doped oxide the XAS shifts in photon energy. This shift depends linear on the external voltage and in only present in polar samples. It is therefore the x-ray variant of the linear Stark effect. In Co-doped ZnO it was demonstrated that it enables to measure atomic polarization with element specificity since the observed shift was different for Co compared to Zn [1]. Combined XAS in electric and magnetic field is therefore envisioned to be a powerful tool to study magneto-electric phonomena in multiferroic materials and preliminary results for magneto-electric coupling in Mn-doped SrTiO₃ will be discussed.
[1] V. Ney et al., Phys. Rev. B 93 (2016) 035136

Polar interfaces in complex oxides heterostructures constitute a unique playground for 2D electron systems (2DES), where exotic properties such as superconductivity and magnetism can arise from combinations of bulk insulators. Most of the work so far focused on the SrTiO₃/LaAlO₃ system, where fairly localised Ti 3d states host the metallic layer. An interesting research direction involves oxides where 5d orbitals could play an active role in the 2DES formation, thus enhancing planar delocalisation and transport properties. In this work we first theoretically predict and then experimentally investigate the metallic state arising in the SrTiO₃/LaAlO₃/WO₃ multipolar interface. Our density functional theory calculations point to the formation of a 2DES located in the d_xy bands of WO₃, where carriers possess high mobility and are subject to strong spin-orbit coupling. By means of pulsed laser deposition we grow ultra-thin LaAlO₃/WO₃ heterostructures on TiO₂-terminated SrTiO₃ (001) single crystals. Our measurements uncover a thickness-dependent transition from an insulating to a metallic state with electron mobilities up to 50,000 cm²V⁻¹s⁻¹. Low-temperature magnetotransport reveals a surprisingly high magnetoresistance reaching 1,000% at 10 T and 1.5 K, accompanied by non-linear Hall effect and quantum oscillations of conductance. Our results show how interfaces with multipolar character allow large flexibility in the design of the confinement potential, opening novel possibilities for 2DES in strongly correlated materials.

Two-dimensional electron systems (2DES) in functional oxides can show metal-to-insulator transitions, superconductivity, or magnetism, and are thus an active field of research promising for applications. Additionally, these 2DES offer the possibility to explore new physics emerging from the combined effects of electron correlations and low-dimensional confinement.

Up to now, the creation of 2DES in oxide heterostructures required the growth of 5 layers or more of binary (e.g. Al₂O₃) or ternary (e.g. LaAlO₃) oxides on SrTiO₃ using highly evolved techniques, such as pulsed laser deposition. The properties of these 2DES, hence their reproducibility, depend crucially on the growth parameters, while the necessity of a critical thickness of upper oxide layer for the onset of conductivity limits applications that rely on tunneling or charge/spin injection. Alternatively, we recently discovered that 2DES can be realized at the bare surface of various functional oxides, such as the quantum paraelectric SrTiO₃ [1], the strong spin-orbit coupled KTaO₃ [2], or the photo-catalyst TiO₂ [3], and that their electronic structure can be tailored by the choice of the surface termination [4-5], paving the way for the quest of topological states in correlated oxides. These 2DES are generated by oxygen vacancies forming near the oxide surface –a highly appealing mechanism, because of its simplicity and generality. However, such surface 2DES can exist only in ultra-high vacuum (UHV), to preserve the vacancies from re-oxidation, and are not suited for applications at ambient conditions.

In this talk, I will show that one can generate 2DES in numerous oxides just by thermal deposition of an atomic layer of an elementary reducing agent, such as pure Al [6]. The reducing agent pumps oxygen from the substrate, oxidizes into an insulating capping layer, and forms a pristine, homogeneous 2DEG in the first atomic planes of the underlying oxide. As a novel application, we generate a 2D metallic state at the surface of the room temperature ferroelectric BaTiO₃. Such hitherto unobserved coexistence of ferroelectricity and 2D conductivity in the same material is promising for functional devices using ferroelectric resistive switching. More generally, this new, simple and cheap, fabrication route for 2DES is adaptable to numerous oxides, scalable to industrial production, and ideally suited for the realization of mesoscopic devices. Furthermore, I will discuss our recent observation of ferromagnetic nano-domains at the surface of oxygen-deficient SrTiO₃ [7].

[1] A. F. Santander-Syro et al., Nature 469, 189 (2011).
[1] A. F. Santander-Syro et al., Phys. Rev. B 86, 121107(R) (2012).
[3] T. C. Rödel et al., Phys. Rev. B 92, 041106(R) (2015).
[4] C. Bareille et al., Sci. Rep. 4, 3586 (2014).
[5] T. C. Rödel et al., Phys. Rev. Applied 1, 051002 (2014).
[6] T. C. Rödel et al., Adv. Mater. doi:10.1002/adma.201505021 (2016).
[7] T. Taniuchi et al, Nat. Commun. doi: 10.1038/NCOMMS11781 (2016).

Two-dimensional electron systems (2DES) in SrTiO3 arise in many configurations, for example at the LaAlO3/SrTiO3 interface and in SrTiO3 based electric double layer transistors. Knowledge of the band structure of such two-dimensional electron systems not only aids understanding of their electronic properties but is essential for efforts to engineer novel properties. ARPES measurements on the (001) surface of SrTiO3 allow direct experimental band structure determination of an SrTiO3 based 2DES. This offers insight into the effects of confinement on the Ti 3d conduction band and into the many body interactions in the resulting two-dimensional system.

I will present recent high-resolution angle-resolved photoemission spectroscopy (HR-ARPES) measurements that reveal the evolution of the strength and nature of electron phonon interactions in the SrTiO3 surface 2DES as the carrier density is tuned [1]. At low density we observe dispersive replica bands separated from the main subbands by 100 meV. This is the hallmark of Fröhlich polarons formed by preferential coupling to a longitudinal optical phonon. As the carrier density of the 2DES increases the polaronic replica bands are suppressed and a low energy kink emerges. This is indicative of a transition to a conventional retarded electron-phonon interaction described by Migdal-Eliashberg theory.

Several authors predicted an unconventional Rashba spin splitting of the SrTiO3 2DES subbands. This has been corroborated by transport measurements of the LaAlO3/SrTiO3 interface that found gate-voltage dependent Rashba spin-splitting as large as 10 meV. I will present spin- and angle-resolved photoemission spectroscopy (SARPES) measurements of the SrTiO3 2DES that support this picture [2]. Negligible polarization of the photocurrent is observed due to the complexity of the 2DES subband structure in combination with experimental resolution that is much greater than the magnitude of the spin splitting. This SARPES signal is inconsistent with a subband structure dominated by either Rashba or exchange splitting comparable to the experimental resolution of 100 meV.

[1] Z. Wang, S. McKeown Walker et. al., Nature Materials doi:10.1038/nmat4623 (2016).
[2] S. McKeown Walker et. al., accepted at Physical Review B (2016).

We report surface chemical cation composition analysis of high quality superconducting LiTi₂O₄ thin films, grown epitaxially on MgAl₂O₄ (111) substrates by pulsed laser deposition. The superconducting transition temperature of the films was 13.8 K.
Surface chemical composition is crucial for the formation of a good metal/insulator interface for integrating LiTi₂O₄ into full-oxide spin-filtering devices in order to minimize the formation of structural defects and increase the spin polarization efficiency. In consideration of this, we report a detailed angle resolved X-ray photoelectron spectroscopy analysis. Results show Li segregation at the surface of LiTi₂O₄ films. We attribute this process due to outdiffusion of Li toward the outermost LiTi₂O₄ layers.

Magnetic thermogalvanic effects such as the anomalous Nernst effect in conducting ferromagnets, and the spin Seebeck effect involving ferromagnetic insulators have been widely studied in the context of spintronics and thermoelectrics. We have developed a technique that allows us to characterize magnetic thermogalvanic effects in patterned structures on the micron scale. We have used this approach to probe magnetic interactions in a variety of materials, leading to the discovery of the spin Seebeck effect in a paramagnetic insulator [1] that is a classical spin liquid at low temperatures and in an antiferromagnetic insulator with uniaxial anisotropy [2]. Our approach also allows us to characterize more complex magnetic structures in patterned devices that may not otherwise be possible to measure using standard magnetometry and scattering techniques. In particular, I will discuss how we characterize non-collinear magnetism in patterned wires of an MBE grown superlattice of La_2/3Sr_1/3MnO₃ (LSMO) and LaNiO₃ (LNO) [3]. The non-collinear magnetic structure of the superlattice and its dependence on magnetic field and temperature were first characterized in a large area sample using polarized neutron reflectometry. Subsequently, using a combination of the anomalous Nernst effect and anisotropic magnetoresistance measurements, we determined how the magnetic structure in a patterned wire of the same superlattice responds to changes in both the direction and magnitude of the applied magnetic field. The results from our transport measurements, while being much quicker to carry out, are in excellent agreement with results from neutron scattering. Our findings also suggest a pathway to a novel memory device that combines features of both antiferromagnetic and ferromagnetic memories.

[1] S. M. Wu, et al., “Antiferromagnetic spin Seebeck effect” Phys. Rev. Lett. 116,097204 (2016)
[2] S. M. Wu, et al., “Paramagnetic spin Seebeck effect”, Phys. Rev. Lett. 114,186602 (2015)
[3] J. Hoffman et al., “Oscillatory Non-collinear Magnetism Induced by Interfacial Charge Transfer in Metallic Oxide Superlattices” arxive 1411.4344 (under review).

Heterostructures made of transition metal oxides are an emerging class of materials which may replace at some point conventional semiconductors for specific applications. We have developed a density functional theory plus dynamical mean field theory (DFT+DMFT) approach for such heterostructures. This allows for treating strong electronic correaltions which are of fundamental importance for many transition metal oxides. Besides the pros and cons of the approach I will discuss some physical results:



First, I will show how to exploit the unique properties of oxide heterostructures for high-efficiency solar cells [1]: The intrinsic electric field of polar heterostructures allows for efficiently separating the created electrons and holes.
Our proposed solar cell has been realized experimentally [2] which also proves the existence of a polar field. An important effect of electronic correlations in this respect is impact ionization [3] which may help to overcome the Shockley- Queisser limit of 38% efficiency. Besides, I will present the theory of spin-orbit coupling in oxide heterostructures [4] which may also lead to topological surface states [5], and discuss prospects to use SrVO3 heterostructures as a Mott transistor [6].



[1] E. Assmann et al., Phys. Rev. Lett. 110, 078701 (2013).

[2] L. Wang et al. Phys. Rev. Applied 3, 064015 (2015); M. Nakamura et al.
Phys.
Rev. Lett. 116, 156801 (2016).

[3] P. Werner, K. Held, and M. Eckstein, Phys. Rev. B 90, 235102 (2014).

[4] Z. Zhong et al. Phys. Rev. B 87, 161102(R) (2013); Mater. Interfaces,
1400445
(2015).

[5] L. Si et al., unpublished (2016).

[6] Z. Zhong et al., Phys. Rev. Lett. 114, 246401 (2015).

Orthorhombic distortions combined with breaking of inversion symmetry have been proposed as new ingredients for the realization of artificial ferroelectric materials in perovskite heterostructures [1]. Coupling these structural distortions to a magnetic order would potentially lead to a new multiferroic compound. Ab-initio calculations have recently suggested vanadate layers as building blocks for the fabrication of artificial multiferroic superlattices [2].

Motivated by these theoretical predictions, we have investigated the structural properties of LaVO₃ thin films grown by pulsed laser deposition. LaVO₃, a d² Mott insulator perovskite, exhibits in its bulk form a low temperature G-type orbital order linked to a Jahn-Teller distortion and a C-type antiferromagnetic state. Growing on different substrates, we have applied tensile and compressive strain to the layers, achieving an orbital reconstruction of the t_2g states [3]. The occurrence or absence of the structural phase transition, detected by low-temperature X-ray diffraction, provide evidence of these novel electronic configurations.

[1] J.M. Rondinelli, C.J. Fennie, Adv. Mater. 14 1961–1968 (2012). N. A. Benedek, A. T. Mulder, and C. J. Fennie, J. Solid State Chem. 195, 11 (2012).
[2] J. Varignon, N. C. Bristowe, E. Bousquet, and P. Ghosez, Coupling and electrical control of structural, orbital and magnetic orders in perovskites, Sci. Rep. 5, 15364 (2015).
[3] G. Sclauzero and C. Ederer, Structural and electronic properties of epitaxially strained LaVO₃ from density functional theory and dynamical mean-field theory, Phys. Rev. B 92, 235112 (2015).

Thermal properties of thin films are sensitive to the presence of interfaces. Control of phonon and electron transport by nanostructured interfaces is valuable to engineer materials for thermoelectric energy conversion. Self-assembled oxide nanocomposite films, in which one phase grows as columns oriented out-of-plane within another phase and both phases are epitaxial with the substrate, provide a model system with well-defined vertical interfaces that allow the contribution of interfaces to thermal properties to be quantified. A spinel/perovskite system consisting of pillars of ferrimagnetic CoFe₂O₄ (CFO) in a matrix of ferroelectric BiFeO₃ (BFO) was synthesized by pulsed laser deposition on (001) SrTiO₃. The pillars have a square or rectangular cross-section and the interface between the BFO and CFO consists of (110) planes. The density of the vertical interfaces was varied from 1582 µm^-1 to 2351 µm^-1 by tuning the volume fractions of CFO (8.5% - 33.6%) via combinatorial deposition. The cross-plane thermal conductivity of the nanocomposite films, probed using time-domain thermoreflectance (TDTR), increases from 3.5 W/mK to 6.6 W/mK with increasing CFO volume fraction.
To simulate heat transport in the nanocomposite films, steady state gray variance-reduced Monte Carlo simulations at room temperature were performed. The domain consisted of a BFO matrix and a single CFO centered pillar, where the square pillar was fixed to have sides of length 12nm and the remaining matrix dimensions were set to maintain the desired volume fraction. The simulated thermal conductivity exhibits a consistent increasing trend with CFO volume fraction.
In contrast, the in-plane thermal diffusivity, probed using transient thermal grating (TTG) spectroscopy, decreases by 12% as the vertical interface density increases from 1582 µm^-1 to 2213 µm^-1 and CFO volume fraction increases from 8.5% to 11.9%. This result indicates an anisotropic phonon scattering mechanism in the vertical nanocomposite films. Based on the systematic study, we demonstrate the influence of self-assembled vertical interfaces in oxide heterostructures on the heat transport.
This work is supported by S3TEC, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Basic Energy Sciences, under Award NO. DE-FG02-09ER46577.

Perovskite Mott insulators with early 3d transition metals occupying the B-site have attracted recent interest for photovoltaic application due to their favorable band gap, large absorption coefficients and potentially carrier multiplication effects due to strong electron-electron correlation present.^1,2,3 LaVO₃ was proposed as a prototypical material with an ideal bandgap ~1.1eV, which has motivated the growth of high quality stoichiometric LaVO₃ to minimize defect concentration that could act as potential nonradiative recombination centers.
In this talk, we will present a systematic growth study of LaVO₃ thin films using hybrid molecular beam epitaxy. A growth window was discovered allowing to grow cation stoichiometric LaVO₃ in a self-regulated manner, as evidenced by intrinsic film lattice parameter and Rutherford backscattering spectroscopy. The impact of stoichiometry on physical properties, namely optical absorption, electrical conductivity as well as vanadium valence state will be discussed in detail. Sharp and pronounced absorption features were found only for stoichiometric films. A double peak with absorption coefficient maxima of 5E-4 cm^-1 around 1.8 and 2.4 eV from electronic transition between the lower V 3d t_2g Hubbard band and the upper V3d t_2g Hubbard band as well as the higher lying unoccupied V3d eg band was found along with a sharp absorption edge at 4.5 eV marking a charge transfer transition from occupied oxygen 2p into unoccupied V3d states. While La rich and stoichiometric films had room temperature resistivities around 5 Wcm, films grown under V-rich conditions showed pronounced resistivity reduction proportional to V excess in the film by over two orders of magnitude, accompanied with an increase in V⁴⁺ valence state determined by X-ray absorption spectroscopy. Photoresponsivity of LaVO₃ devices were measured. Using a bias field of 1 Volt across a 1 mm junction a photoresponsivity larger than 0.1 mA/W was found for a photon excitation energy of 1.6eV, a 4 to 10 fold increase compared to La-rich and V-rich films.

¹ E. Assmann, P. Blaha, R. Laskowski, K. Held, S. Okamoto, and G. Sangiovanni, Phys. Rev. Lett. 110, 078701 (2013).
² E. Manousakis, Phys. Rev. B 82, 125109 (2010).
³ P. Werner, K. Held, and M. Eckstein, Phys. Rev. B 90, 235102 (2014).

Correlated metals SrVO₃ and CaVO₃ have been demonstrated as a new class of transparent conductors (TCs), ideally combining a high electrical conductivity and high optical transparency, with a figure of merit on par with Sn-doped In₂O₃, but at much smaller film thickness.¹ Strong electron correlation present in the two candidate materials pushes the free carrier adsorption edge, represented by the reduced plasma energy, into the infrared down to 1.33 eV despite a high metal-like carrier concentration in excess of low 2×10²² cm^-3. The unique optical properties benefit from the favorable band structure, in which partially-filled vanadium t_2g band is energetically well separated from oxygen 2p and vanadium e_g bands, pushing interband transitions into the ultraviolet spectrum.

In this presentation, we show how to further boost the performance of correlated metals as TCs. In the perovskite structures with corner-connected octahedra, trivalent cation element, lanthanum (La), is used to replace Sr on the A-site to further increase carrier density. Exchanging Sr with La and progressively increasing the composition x in the solid solution La_xSr_1-xVO₃ will reduce the conductivity and eventually drive the system into the Mott insulating phase at x=0.80.² However, it is found that for moderate composition (x < 0.1) the conductivity could be further enhanced without degrading the optical transparency, as corroborated experimentally using x-ray diffraction, electrical transport, spectroscopic ellipsometry and reflectance measurements. The figure of merit of transparent conductors will also be benchmarked against state-of-art materials.

[1] Zhang et al. Nat. Mater. 15 (2016): 132
[2] Miyasaka et al. Phys. Rev. Lett. 85 (2000): 5388

Investigation of high performance thermoelectric materials has resulted in a strong focus on the layered cobaltite, Ca₃Co₄O₉, which has a large ZT and is stable in air. Ca₃Co₄O₉ possesses a misfit layered structure, which, in addition to its positive effect on the material’s electrical and thermal conductivities, results in a complex electronic structure, including ferrimagnetic behavior below 19 K. At high temperatures Ca₃Co₄O₉ is a p-type semiconductor, but it also acts as a metal and then as an insulator with decreasing temperature. Below 100 K the cobaltite begins to form incommensurate spin wave density ordering, which is responsible for the insulator transition. A large segment of the studies on Ca₃Co₄O₉ have focused on adding dopants to improve its thermoelectric performance, primarily by modifying its electrical characteristics. Here, we report the influence of doping Ca₃Co₄O₉ with rare earth elements on the electrical and magnetic properties of this material.

Doping of the ferroelectric materials with the transition metals is one of the routes to design new room-temperature magnetoelectric multiferroics. We utilize this strategy to induce magnetism in a conventional ferroelectric PbTiO₃ (PTO) and Pb(Zr_0.20Ti_0.80)O₃ (PZT). Using first-principles density-functional calculations, we predict that heavy doping of PTO with palladium (Pd) gives rise to a ferromagnetism. Pd is known to exist in both 2⁺ and 4⁺ oxidation states, e.g. in compounds such as in PdO and PdO₂. While isolated isoelectronic substitutions of Pd on Pb and Ti sites do not produce any moment, the simultaneous substitution of Pb and Ti on the neighboring sites with Pd gives rise to a magnetic moment as large as 0.9 B per Pd atom. By analyzing the orbital occupation, we find that the charge redistribution between Pd²⁺ and Pd⁴⁺ sites changes the nominal valence of the ions leading to the magnetism due to unpaired electrons in Pd d-shell. This study corroborates our experimental results showing that a heavily Pd doped Pb(Zr_0.20Ti_0.80)_0.70Pd_0.30O_3-δ (PZT) exhibits a room-temperature ferroelectric and ferromagnetic ordering with a giant magnetoelectric coupling ~0.36 mV/cmOe. This single phase compound has a simple tetragonal crystal structure with space group P4mn as probed by X-ray diffraction (XRD) and Raman studies. The presence of Pd in PZT is confirmed by XPS and XRF studies and assigned with related binding energies of Pd⁺² and Pd⁺⁴ ions as 336.37 eV, 342.9 eV, and 337.53 eV, 343.43 eV, respectively. A sharp first order ferroelectric phase transition was observed at ~569 K (+/-5 K) that is confirmed from dielectric, Raman, and thermal analysis. The requirements of heavy Pd doping and presence of both Pd⁺² and Pd⁺⁴ ionic states in the doped magnetic system are consistent with our theoretical predictions. This finding of the co-existence of the ferromagnetic and ferroelectric orderings with a large magnetoelectric coupling above room temperature is a significant step forward in the development of single phase magnetoelectric material with enhanced functionalities.

Recently, we found a novel route to experimentally control the charge density wave (CDW) order in the perovskite bismuthate BaBiO₃ (BBO), the parent compound of several high-T_c oxide superconductors [1,2]. BBO is known to have an insulating (instead of metallic), ground state exhibiting a CDW that is closely associated with an oxygen breathing distortion. Numerous experimental and theoretical studies have sought to gain insight into the mechanisms that control the physics of these bismuthates; to date, however, only limited progress has been made in hole-doped bulk samples. Here, we show that the electronic and structural properties of BBO are strongly dependent on film thickness, oxygen pressure, and temperature via the combination of spectroscopic techniques and first-principles calculations. In order to investigate such phenomena in more detail, it is highly desirable to make in situ investigations on the optical spectra of BBO films near the CDW gap (i.e. 2.0 eV [3]) under various environmental conditions. In this talk, we will demonstrate how the in situ ellipsometry technique can be used to provide fruitful information related to emergent phenomena in thin film structures.

[1] A. W. Sleight, J. L. Gillson, P. E. Bierstedt, Solid State Commun. 17, 27 (1975).
[2] R. J. Cava et al., Nature 328, 814 (1988).
[3] H. Sato, S. Tajima, H. Takagi, S. Uchida, Nature 338, 241 (1989).

Oxygen vacancies are important functional ion defects in transition metal oxides. A small variation in the oxygen content can lead to the change of the valence state and structure of the transition metal cations, which dramatically modifies the physical properties. [1] Herein, we will give a presentation on our recent works about tailoring physical properties via oxygen vacancies in perovskite oxide films, ionic liquid control of metal-insulator transition in LSMO film [2] and oxygen vacancy migration induced switchable photovoltaic effect in BFO and STO films [3]. By utilizing the electrochemical reaction between LSMO and ionic liquids, we realized a resistance increase of more than four orders of magnitude in LSMO electric double layer transistor (EDLT). Moreover, the distribution of oxygen vacancies is designed under external electric field via the migration of oxygen vacancies in BFO and STO films. The field induced inhomogeneous distribution of oxygen vacancies leads to band bending, producing a switchable photovoltaic effect. We employed scanning transmission electron microscopy (STEM) measurements for imaging the existence of oxygen vacancies. These findings pave a new pathway to manipulate the physical properties based on the ionic degree of freedom for oxide materials.

[1] Kalinin, S. V. & Spaldin, N. A. Functional ion defects in transition metal oxides. Science 341, 858-859 (2013).
[2] Chen Ge, Kuijuan Jin et al. Metal–insulator transition induced by oxygen vacancies from electrochemical reaction in ionic liquid-gated manganite films, Adv. Mater. Interfaces 2, 1500407 (2015).
[3] Chen Ge, Kuijuan Jin et al. Towards switchable photovoltaic effect via tailoring mobile oxygen vacancies in perovskite oxide films, under preparation.

We have investigated the interfacial structure of epitaxial (Ba,Sr)TiO₃ films grown on (111)-oriented SrTiO₃ single-crystal substrates using transmission electron microscopy (TEM) techniques. Compared with the (100) epitaxial perovskite films, we observe dominant dislocation half-loop with Burgers vectors of a<110> comprised of a misfit dislocation along <112>, and a threading dislocation along <110>. The misfit dislocation with Burgers vector of a<110> can dissociate into two ½a<110> partial dislocations and one stacking fault. We found the dislocation reactions occur not only between misfit dislocations, but also between threading dislocations. Via three-dimensional electron tomography, we retrieved the configurations of the threading dislocation reactions. The reactions between threading dislocations lead to a more efficient strain relaxation than do the misfit dislocations alone in the near-interface region of the (111)-oriented (Ba_0.7Sr_0.3)TiO₃ films, which could be responsible for the broadened temperature dependence of permittivity in the (111) BST films.

The light-induced phase transition (PT) in vanadium oxides is a very complex process. External pressure and/or doping could play a crucial role in determining the phase transition process. By employing external pressure or doping, we investigate the variety of possible pathways of V₂O₃ phase transition. At temperatures below the PT point (<150 K) vanadium oxide is an antiferromagnetic insulator, and laser excitation produces an insulator-to-metal PT which is completely different from the light-induced PT at room temperature. Ultrafast processes in vanadium oxides have been investigated from last decades but the information about size-dependent light-induced PT dynamics and recovery processes is still under investigation. The light-induced PT in V₂O₃ with different structural morphologies, at different levels of optical excitation, at different temperatures and pressures is a less explored area. Nevertheless this information is of strong interest for the fundamental understanding of PT dynamics in vanadium and other oxides. Therefore, in this study we investigate the light-induced processes in vanadium oxides by employing light-induced strain and/or doping.

Oxide heterointerfaces have attracted much attention because of their exotic functionalities. Although deeper understanding of oxide surfaces and interfaces would lead to the emergence of electronic and magnetic properties, atomic-scale investigations of the structures and composition of oxide surfaces are still insufficient. We here report a comparison of the extent of Sr segregation on buffered-HF (BHF)-etched and (√13×√13)-R33.7°reconstructed SrTiO₃(001) surfaces.[1] This study reveals that Sr segregation is suppressed on the reconstructed surface. Scanning tunneling microscopy and photoemission studies of both surfaces show distinct differences in terms of atomic arrangements, electronic structures, and chemical states. The emission-angle-dependent Sr 3d core-level spectra indicate that the amount of Sr is low at the surface of the reconstructed surfaces, while the amount increases near the surface of the BHF-etched surface. Our investigations would contribute to solve a variety of open questions in oxide electronics researches including the important role of substrates discussed in a recent study on FeSe on a (√13×√13)-R33.7°reconstructed SrTiO₃(001) substrate.
[1] Ohsawa et al. Appl. Phys. Lett. 108, 161603 (2016).

Tungsten oxide (WO₃) is a favorable material for electrochromic devices, due to the unique properties it possesses. Here we report the fabrication of WO₃ films and nanowire arrays on fluorine doped tin oxide (FTO) substrates for evaluating the electrochromic properties of these samples. The electrodeposition was carried out by using a bath containing sodium tungstate (Na₂WO₄) and hydrogen peroxide (H₂O₂). For optimal electrodeposition the entire parameter space was explored. The WO₃ nanowire arrays were fabricated using anodic alumina oxide (AAO) template. The samples were characterized by optical microscopy, scanning electron microscopy (SEM), X-ray diffractometer (XRD). The electrochromic properties of the WO₃ films and nanowire arrays was compared. This knowledge and outcome of this prototype was transferred in making stretchable electrochromic devices.

Polarization engineering in polar wide band gap semiconductors will form high mobility two-dimensional electron gas (2DEG) in polar heterostructures, leading polar semiconductors such as nitrides or oxides desirable for fabrication of electronic devices capable of operating at high temperature, high frequency, and high power densities. However, the electron transport properties of 2DEG shows a strong time-dependent degradation with the carrier mobility significantly decreased at about several tens of percent as elapsed time changed from several weeks to one year. Such an aging effect on GaN/AlGaN heterostructures have been investigated and ascribed to unstable surface. Surface states are thought to be unavoidable in this material system due to the strong polarization fields and will change with time due to adsorption or oxidation in air ambient. For oxide semiconductors, high carrier mobility and even quantum hall effect have been realized in ZnMgO/ZnO heterostructures fabricated by MBE and MOCVD method. However, less attentions have been paid on its aging effect. Actually, high polarization field and high density of oxygen vacancies will certainly cause a large amount of surface states formed on oxide heterostructures, which should cause a much stronger aging effect on the behavior of 2DEG.
In this presentation, we will concentrate on the impact of surface states on the behavior of 2DEG in ZnMgO/ZnO heterostructures grown by MOCVD method. The varied temperature Hall effect measurements has revealed that the two-dimensional electron gas has been formed in the ZnMgO/ZnO heterostructure for as grown (A) or kept for one year (B). However, the low temperature carrier mobility of B has decreased for several times with a little decrease (about ten percent) obtained on the carrier density, which shows strong differences compared with that observed on nitride system due to much stronger aging effect. Capacitance voltage measurement does verify the change of the carrier density with much broad distribution of the 2DEG observed in B. More interesting, the observed emission related to 2DEG in A has totally disappeared in B, with strong donor bound exciton (D⁰X) and surface exciton appeared instead. This verifies the much high density of surface states formed on oxide heterostructures in B. Peroxide treatment has then been employed to restore the behavior of the 2DEG in B. DIC microscopy and AFM images do show the remove of the adsorption layer after the treatment with clean surface obtained. The treatment has caused D⁰X strongly suppressed with a new emission arose, which may relate to 2DEG but at a little high energy compared with that in B. The carrier density is found to be restored but the low temperature mobility further decreases after pretreatment, which is similar to that observed on nitride system with chemical treatments. Possible causes to the above observations will be presented and discussed.

The two dimensional electron gas (2DEG) at the MgZnO/ZnO heterointerface has attracted attention for its high mobility outside the realm of conventional modulation doping [1] concomitant to strong electron correlations [2]. Rapid progress in quality has been facilitated by careful optimization of the growth technology, leading to significant reduction in background impurities, specifically through the use of ozone molecular beam epitaxy. Alternatively, novel heterostructure design concepts have the potential to limit the effect of various types of disorder, and hence enhance the quality as exemplified in case of AlGaAs/GaAs heterointerfaces [3]. For this, the dependence of 2DEG quality on the various inherent disorders has to be studied carefully.
Along with the background impurities, alloy disorder prevails as a dominant factor limiting the quality due to penetration of the electron wave function in to the MgZnO barrier layer. In order to probe this, we employ a modified interface profile consisting of Mg_0.01Zn_0.99O/ ZnO with a thin interlayer of Mg_xZn_1-xO (x =0.005-0.4). This results in simultaneous control of the wave function penetration and alloy disorder at a constant carrier density as confirmed by solving 1D Schrodinger-Poisson equation self consistently. The effect of this design on heterostructure quality was studied by deducing the transport (τ_tr) and quantum (τ_q) scattering times, as extracted from the low-field mobility and Shubnikov-de Hass oscillations. While the τ_tr shows a strong correlation with x, τ_q remains almost insensitive to x (τ_q~15ps). The large variation in the τ_tr/ τ_q ratio (from 16.2 to 1.5) implies a change in the dominant scattering mechanism from small angle towards large angle with increasing x. The relative insensitivity of τ_q on x indicates the total scattering rate to be independent of the alloy disorder. This implies that another mechanism, likely unintentional background impurities, to be the dominant mechanism limiting the τ_q in the high quality heterostructures. This result therefore emphasizes the need for pursuing ever higher levels of sample purity in order to achieve significant sample quality gains.
References
[1] J. Falson, Y. Kozuka, M. Uchida, J. H. Smet, T. Arima, A. Tsukazaki, M. Kawasaki, Scientific Reports 6, 26598 (2016) .
[2] Y. Kozuka, A. Tsukazaki, and M. Kawasaki, Applied Physics Reviews 1, 011303 (2014).
[3] V. Umansky, M. Heiblum, Y. Levinson, J. Smet, J. Nubler, and M. Dolev, J. Cryst. Growth 311, 1658 (2009).

Epitaxial layers of ternary oxides such as BaTiO₃, SrTiO₃, BiFeO₃, and cuprate superconductors gathered significant scientific and technological interest during the last years. It was demonstrated that important physical properties, such as ferroelectricity, multiferroicity, or colossal magnetoresistance, can be tuned if such layers are deposited on almost (but not perfectly) lattice-matched substrates [1]. Here rare earth scandates play an important role. Bulk growth processes for these substances REScO₃ (RE = Dy, Tb, Gd, Eu, Sm, Nd, Pr) were developed in our institute [2], and substrates are available commercially meanwhile. In this row, PrScO₃ has the largest pseudocubic lattice constant a₀ = 4.021 Å. The quest for larger a₀, and for “fine tuned” a₀ between the values of neighboring RE elements, is difficu< especially CeScO₃ and LaScO₃ are not available as bulk crystals for wafer preparation so far, because their melting points exceed the stability limits of iridium crucibles that are used for the Czochralski process.

This contribution reports on recent phase diagram measurements and bulk crystal growth of (RE,RE’)ScO₃ with low segregation, and that such mixed crystals enable fine-tuning of lattice parameters for substrates [3]. More recent results on pseudobinary systems between REScO₃ and other perovskites such as LaLuO₃ or likely NdLuO₃ show that significantly larger pseudocubic lattice parameters between 4.13 and 4.15 Å are accessible too [4].

[1] D.G. Schlom et al., Elastic strain engineering of ferroic oxides, MRS Bull. 39 (2014) 118–130.
[2] R. Uecker et al., Properties of rare-earth scandate single crystals (Re=Nd-Dy), J. Cryst. Growth. 310 (2008) 2649–2658.
[3] R. Uecker et al., Growth and investigation of Nd_1−xSm_xScO₃ and Sm_1−xGd_xScO₃ solid-solution single crystals, Acta Phys. Pol. A. 124 (2013) 295–300.
[4] R. Uecker et al., Large-lattice-parameter perovskite single-crystal substrates, J. Cryst. Growth, in press, doi:10.1016/j.jcrysgro.2016.03.014

Epitaxially strained BiFeO₃ (BFO) thin films have seen an upsurge of interest over the past decade with the revelation of fascinating characteristics such as giant ferroelectric polarisation, exceptional piezoelectric and magnetoelectric responses to name a few.^[1] The growth of BFO on substrates providing a large in-plane compressive strain which facilitates structural alterations of BFO to that of a tetragonal-like phase (T-phase) and distorted rhombohedral-like phase (R-phase) has opened the door to a plethora of opportunities in terms of controlling nanoscale interfaces and expanding our understanding of solid state physics. In a recent review,^[2] it was advised that there are still a number of exciting possibilities as well as pending questions which need to be addressed before the functionality of mixed-phase BFO thin films can be driven to the next level.
Using both aberration-corrected TEM and STEM we have performed a high resolution study of the lattice parameters which characterise the ‘R’ and ‘T’ phase and mapped the strain evolution between polymorphs. The relative ease through which transitions from one phase to another can be achieved varies depending on the application of stimuli selected. In this study we use a variation of techniques to explore the physical make-up behind the polymorphs of mixed-phase BFO. We highlight the benefits of reversibly reorganising phases via the application of external stimuli with an AFM probe tip. Through precise SEM and FIB techniques we have identified the pre-written AFM areas and milled cross-sectional lamellae across these areas for post-AFM analysis using STEM. With this unique study we detect and compare structural differences between the as-grown polymorphs in a native lamella and polymorphs written through external stimuli via an AFM tip. We show differences in lattice parameters and strain evolution in the newly written polymorphs via nano-beam electron diffraction using a 2nm sized electron probe; this precision has allowed us to explicitly map the strain across a large area of polymorphs. With the addition of EELS focusing on the Fe-L_2,3 and O-K edges we show changes in ELNES features which suggest octahedral distortion variations between polymorphs.
By combining AFM and state-of-the-art STEM techniques in this distinctive way we provide an insight into the link between exciting polymorph switchability seen via AFM on the microscopic scale, and structural polymorph changes seen via STEM on the atomic scale.

[1] R. Ramesh and N. A. Spaldin, Nat. Mater. 6, 21-29 (2007)
[2] V. Nagarajan et al., Appl. Phys. Rev. 3, 011106 (2016)

Hydrogen, the smallest and the lightest atomic elements, is reversibly incorporated into interstitial sites of vanadium dioxide (VO₂), a correlated oxide with 3d¹ electronic configuration, and induces electronic phase modulation. It is widely reported that low hydrogen concentrations stabilize the metallic phase, but the understanding of hydrogen in high doping regime is very limited so far due to the difficulty of heavy hydrogenation.
Here, we demonstrate that as many as two hydrogen atoms can be incorporated into each VO₂ unit cell, and that hydrogen is reversibly absorbed into, and released from, VO₂ without destroying its lattice framework due to the low temperature annealing process. During hydrogenation of VO₂ thin film at 70 °C, sheet resistance of pristine insulating VO₂ decreased by 3 orders of magnitude at the first step, and then it eventually increased by 5 orders of magnitude with significant expansion of out-of-plane lattice parameter (~10.2 %). This hydrogenation process allows us to elucidate electronic phase modulation of vanadium oxyhydride (H_xVO₂), remarkably demonstrating two-step insulator (VO₂) – metal (H_xVO₂) – insulator (HVO₂) phase modulation during inter-integer d-band filling using in-situ electrical measurement. Based on synchrotron X-ray measurement and 1^st principles calculations, the unprecedented insulating HVO₂ with 3d² configuration is attributed to highly doped electrons via hydrogenation process in conjunction with huge lattice expansion. Our finding suggests the possibility of reversible and dynamic control of topotactic phase modulation in VO₂ and opens up the potential application in proton-based Mottronics and novel hydrogen storage.

Oxygen vacancies, V_O, are easily formed in SrTiO₃ (STO) under reducing atmosphere at high temperature, high-energy ion-bombardment, etc. The electron-donor nature of V_O, along with the extremely high electron mobility in this oxide, results in a very large conductivity even for a very small concentration of V_O. For this reason, there is an intense discussion about the precise role of V_O in the formation of a two dimensional electron gas reported at the interface between STO and insulating LaAlO₃ [1,2]. Additionally, these heterostructures are subjected to epitaxial strain, which has been shown to produce an effect on the oxygen vacancy formation energy. Therefore, it is very important to perform a systematic determination of the oxygen formation energy in STO thin films as a function of strain.
In this work, we present a complete thermodynamic study of oxygen vacancy formation in e-doped STO thin films grown by Pulsed Laser Deposition. Hall effect measurements confirmed that each V_O is doubly ionized and contributes with two electrons to the conduction band of the STO. Our results demonstrate that epitaxial strain, both compressive and tensile, causes a decrease in the enthalpy formation of V_O, in good agreement with ab-initio calculations [3].
Moreover, we show that completely re-filling the vacancies by annealing under oxidizing atmosphere, is only possible under precise conditions of temperature and oxygen pressure. This is very important in order to define synthesis and post-annealing protocols to achieve stoichiometric thin-film samples.

References:
[1] N. Reyren, Science, 317, 1196 (2007).
[2] A. Brinkman, Nature, 6, 493 (2007)
[3] S. Y. Choi, Nanoletters, 15, 4129 (2015).

Strongly correlated oxides show electronic properties that are intimately linked to their orbital occupancy. Among them, nickelate perovskites (RNiO₃) stands out as a model systems for structure – correlated physics study, due to its capability of precisely manipulation of steric effect by moving along the 4f rare earth series. There is growing interest in exploiting these properties in emerging electronics and energy technologies as well. In this work, we present a critical comparison of electrical resistivity evolution in SmNiO₃ and EuNiO₃ epitaxial thin films as a function of point defects. Two types of defects are considered: oxygen vacancies created by low oxygen pressure annealing and interstitial defects introduced by protons. Further, we investigate electrical conductivity relaxation dynamics in both nickelates as a function of disorder. This allows us to quantify the oxygen exchange process as a function of sub-lattice defects and how interstitial electron dopants modify the kinetics. We will then discuss magnetotransport at low temperatures in these samples as a function of defect concentration and attempt to correlate magnetism to disorder-induced charge transfer to the nickel orbitals.

A clear understanding of the energetics and strain-control of active ionic defects in oxides is crucial for achieving technical feasibility and efficient performance of future electronic devices relying on these properties [1]. We have utilized atomic layer-by-layer laser molecular beam epitaxy (ALL-laser MBE) and a combination of several bulk-sensitive x-ray spectroscopies (XAS and HAXPES) in conjunction with state-of-the art theoretical calculations to establish a relationship between applied coherent epitaxial strain and oxygen vacancy concentration in ultrathin single-crystalline CaMnO₃ films. We show that the vacancies partially diffuse out of the film when exposed to ambient atmosphere, owing to their high mobility [2], thus requiring an in-situ-grown capping layer to preserve the original strain-induced oxygen-vacancy content. Our findings provide a new way of designing strongly-correlated transition-metal oxides with tunable ionic defect content and suggests a robust characterization platform for detecting and quantifying oxygen vacancy content at buried depths.

[1] S. V. Kalinin and N. A. Spaldin, Functional Ion Defects in Transition Metal Oxides, Science 341, 858 (2013).
[2] Aschauer, U. et al. Strain-controlled oxygen vacancy formation and ordering in CaMnO₃. Phys. Rev. B 88, 054111 (2013).

The remarkable development of complex oxide materials in the last decade has led to tremendous amounts of new phenomena and properties, which can be effectively harnessed for the design of advanced materials for information and energy applications and accelerating materials integration into advanced devices. A compelling manifestation is oxide heterointerfaces exhibiting fascinating emergent behaviours due to subtle combinative contributions of atomic structures and chemistries near interfacial boundaries. Surface/interface X-ray scattering from modern synchrotron sources integrated with phase retrieval direct methods (e.g. COBRA) provides a very powerful toolkit to decipher the subtlety. However, typical X-ray Bragg rods from oxide epitaxial systems are laterally averaged over the coherence length of the X-ray beam such that changes in the meso/nanoscale structure during materials evolution or transition may be impossible to observe or lost.

In this talk, we will discuss recent developments of novel surface X-ray mesoprobes, such as emergent scanning high-energy surface diffraction and surface coherent techniques, to overcome these limitations and enable the rapid capture of large volumes in reciprocal space with high spatial resolution. When applied in situ to epitaxial oxide heterostructures with heterogeneity on the mesoscale, they will allow simultaneous imaging of atomic-level and mesoscale structure in real time and in real conditions for studies of novel interfacial phenomena and functionalities that would be otherwise inaccessible by existing techniques. Future opportunities such as integrating X-ray mesoprobes with thin film growth system, ultrafast pump-probe apparatus, and predictive computational modeling will also be discussed.