Symposium S.NM12—Synthesis and Control of Dirac or Topological Materials

In many realistic topological materials, more than one kind of fermions contribute to the electronic bands crossing the Fermi level, leading to various novel phenomena that cannot be well understood by single kind of particle picture. Here, using momentum-resolved inelastic electron scattering, we investigate the plasmons in type-II Weyl Semimetal MoTe₂, in which both Weyl fermions and trivial nonrelativistic fermions contribute to the Fermi surface in the T_d phase at low temperature. We observe one plasmon mode in the 1T’ phase at high temperature and two plasmon modes in the topological T_d phase at low temperature. Combining with first-principles calculations, we show that the plasmon modes in the T_d phase, dominated by the interband transitions between the topologically nontrivial bands for Weyl fermions and the trivial bands for nonrelativistic electrons, are attributed to band splitting due to inversion symmetry breaking and the spin-orbit coupling.

The ability to construct nano-scale systems with atomic-level precision by using STM-based single-atom manipulation techniques has led to numerous outstanding examples of Quantum Designer Physics. In particular the combination of single-atom manipulation with spin-sensitive imaging and spectroscopy [1] based on the spin-polarized STM technique [2] offers an exciting approach towards the design of specific properties of 1D spin chains [3,4] as well as 2D arrangements of tailored nanomagnets [5]. Investigating such artificially built spin arrays on superconductor surfaces has recently led to the observation of Majorana zero-energy modes in atomically well-defined magnet-superconductor hybrid systems [6] allowing for a detailed comparison with theoretical predictions. Moreover, prototypes for all-spin atomic-scale spin logic devices could be demonstrated by a bottom-up fabrication technique [7]. Another exciting research direction is the tailoring of the spin dynamics of artificially built atomic arrangements on surfaces, such as few-atom clusters [8,9] or 2D spin arrays [10]. Making use of time- and spin-resolved STM techniques [11] the spin dynamics can be tuned by the choice of the substrate, the chemical identity and number of the adatoms as well as their geometric arrangement. Interestingly, the symmetry of the adatom arrangements can have a decisive influence on the spin dynamics. This can be explained by the effect of anisotropic indirect exchange interactions which can lead to a destabilization of the spin system for atomic arrangements exhibiting a lower symmetry [10].

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[6] H. Kim et al., Science Advances 4, eaar5251 (2018).
[7] A. A. Khajetoorians et al., Science 339, 55 (2013).
[8] A. A. Khajetoorians et al., Science 332, 1062 (2011).
[9] J. Hermenau et al., Nature Commun. 8, 41467 (2017).
[10] J. Hermenau et al., Nature Commun. 10, 2565 (2019).
[11] S. Krause and R. Wiesendanger, in: Atomic- and Nano-Scale Magnetism,
ed.: R. Wiesendanger, Springer Series in Nano Science and Technology,
Springer Nature Switzerland (2018), p. 221.

Cadmium arsenide (Cd₃As₂) has been identified to belong to the family of three-dimensional topological semimetal, featuring topologically stable band crossing (“nodes”) in the bulk and unconventional surface states. Crystal structures reported for Cd₃As₂ differ, however, in a crucial aspect, namely the presence or absence of an inversion center, which determines whether Cd₃As₂ is Dirac or Weyl semimetals. Both types of semimetals exhibit band crossings in the bulk with linear energy dispersion in momentum space that are protected by the presence of certain crystal symmetries. While Dirac semimetals possess both time reversal and inversion symmetry, one of these (or both) is absent in Weyl semimetals. This difference defines their unique electronic structures and has profound effects on their properties. Here, we report on the determination of point group of Cd₃As₂ thin films grown by molecular beam epitaxy (MBE) using convergent beam electron diffraction (CBED). Using CBED patterns recorded from multiple zone axes and Bloch wave simulation, we demonstrate that Cd₃As₂ is a tetragonal structure with 4/mmm point group, possessing an inversion center and four-fold symmetry, indicative of Dirac semimetals. Lastly, we present that high-angle annular dark-field (HAADF) imaging recorded in scanning transmission electron microscopy (STEM) can be used to directly visualize the Cd vacancy ordering which further confirms our conclusion. The findings in this study offer new opportunities to unveil the symmetry-property relationship in a wide range of topological materials.

Coupling between charge, spin, lattice, or orbital degrees of freedom gives rise to remarkable phenomena such as colossal magnetoresistance, metal–insulator transitions and superconductivity. Many aspects of these competing interactions, however, remain elusive. Traditional bulk measurements of phases transitions, which average the material’s response over large length scales, can inferred the presences of spatial inhomogeneity. Understanding their character, however, requires local probes. Direct, real-space measurements of the onset of emerging order are key to understanding the diverse properties of complex electronic materials.

Today, atomic-resolution imaging is routinely achieved by aberration-corrected scanning transmission electron microscopy (STEM). Coupled with spectroscopy STEM provides direct information about the local structure and chemistry of thin films and heterostructures. In this talk, I will first discuss our results on two classes of superconducting materials, the layered perovskite ruthenates and the recently reported infinite-layer nickelates [1], with particular focus on the microscopic structure, strain and the role of defects in these thin film superconductors. Going beyond room temperature STEM, cryogenic sample cooling promises direct access to low temperature phases. While limitations in hardware currently hamper STEM measurements of ruthenates and nickelates in their superconducting states, high-precision cryogenic STEM near liquid nitrogen temperature has been demonstrated [2, 3]. These new capabilities have paved a path to visualize subtle lattice and electronic orders in emergent low temperature phases as I will show in the second part of the talk. Focus will be on the lattice behavior of charge-ordered manganites, where direct imaging can discriminate between multiple structure models, and mapping over larger length scales shows phase coexistence and nanoscale inhomogenities.

[1] D. Li, K. Lee, B. Y. Wang, M. Osada, S. Crossley, H. R. Lee, Y. Cui, Y. Hikita, H. Y. Hwang, Nature 572, 624 (2019).
[2] B. H. Savitzky et al., Ultramicroscopy 191, 56 (2018).
[3] I. El Baggari, B. H. Savitzky, A. S. Admasu, J. Kim, S.-W. Cheong, R. Hovden, L. F. Kourkoutis, Proc. Natl. Acad. Sci. 115, 1445 (2018).
[4] Research performed in collaboration with Ismail El Baggari, Berit H. Goodge, David J. Baek, Michael J. Zachman and the groups of Harold Y. Hwang (Stanford), Darrell G. Schlom (Cornell), Kyle M. Shen (Cornell) and Beth Nowadnick (New Jersey Institute of Technology). This work was supported by the DoD, AFOSR (FA 9550-16-1-0305), NSF (DMR-1539918, DMR-1429155, DMR-1719875), and the Packard Foundation.

Interfaces and heterostructures with topological semimetals offer new opportunities to control and manipulate their unique electronic states and a wealth of associated phenomena, for example, via electric field effect, strain, or symmetry breaking. In this presentation, we will discuss recent progress in heterostructures of a prototype three-dimensional Dirac semimetal, cadmium arsenide (Cd₃As₂), which are grown by molecular beam epitaxy. We show that high-mobility, epitaxial Cd₃As₂ films can be grown in different crystallographic orientations. The easy cleavage plane of single crystals, the tetragonal (112) plane, is also the most common growth surface. In this orientation, the bulk Dirac nodes project on two points in the surface Brillouin zone. The (001) plane differs from the (112) plane and indeed all others in that the two bulk nodes project onto one point, rather than two, of the surface Brillouin zone. Here, we discuss the nature of quantum oscillations and of the quantum Hall effect of thin Cd₃As₂ films grown in both orientations. The quantum Hall effect of (112) films is characterized by a degeneracy factor of two that is lifted in fields above approximately 10 T. Gated magnetotransport and quantum capacitance revealed ambipolar quantum transport, a zero-energy Landau level, and linear dispersion in the 2D states giving rise to the quantum Hall effect. The important feature is that (112) surfaces host double Fermi arcs, which can give rise to two Fermi pockets near the center of the surface Brillouin zone and thus an even integer Hall effect until their degeneracy is lifted by the magnetic field. A single (112) surface can give rise to an integer quantum Hall effect and explain the observed characteristics. By contrast, the states of (001) films are non-degenerate and integer filling factors are due to crossing Landau levels. We will discuss other remarkable phenomena in the quantum Hall effect of (001) films. We will also discuss pathway towards realizing novel topological systems.

In the presence of strong interactions, the fluid of mobile electrons in a metal can spontaneously break the point group symmetries of its underlying host lattice, forming an electronic analogue of a classical liquid crystal. The experimental discovery of 2D electronic liquid crystals (ELCs) was first made nearly 20 years ago in semiconductor heterostructures and has since been reported in several other systems including the copper- and iron-based high-temperature superconductors. However whether or not a 3D version of an ELC can exist has remained unclear. In this talk, I will present signatures of a 3D ELC in the strongly spin-orbit coupled metallic pyrochore Cd2Re2O7 detected using ultrafast coherent phonon spectroscopy and a recently developed spatially-resolved nonlinear optical polarimetry technique. I will discuss implications for the possible topological superconducting phase that has been reported to emerge in Cd2Re2O7 under pressure.

Tungsten carbide (WC) is widely used in industrial field due to its excellent performance in hardness, roughness and high melting temperature. Bulk modulus measuring the resistant to compression is related to phonon dynamics as well. In recent work, phonon transport properties of topological metals WC and WN are investigated, the thermal conductivity of WC is found to be two orders of magnitude higher than that of WN because the large acoustic-optical gap prohibits the acoustic+acoustic®optical scattering[1]. A first-principles study on phonon related properties of tungsten carbide has been published and the calculated phonon dispersion of WC can be referred, but experimental investigations still need to be conducted. In our work phonon dispersion relation of tungsten carbide in high symmetry directions has been measured by inelastic X-ray scattering. Super resolution is applied to extract phonon lifetime. Ab initio phonon calculation based on Density Function Theory is compared to the experimental results with excellent agreements. Furthermore, inelastic scattering intensity has been simulated at various q points. Our dispersion results are essential for understanding the thermal conductivity and chiral phonons in tungsten carbide.
Polarized photons can excite the chiral phonons at Brillouin-zone corners in honeycomb lattices with threefold rotational broken inversion symmetry in monolayer tungsten disulfide. Tungsten carbide with space group P-6m2 (No.187) is a good candidate for experiments for underlying chiral phonons. We generate circular polarized X-ray to detect the phonon intensity change at zone boundary (K) in tungsten carbide. Present scattering theory could not be applied in our experiment results.

New mechanisms that result in controllable ferromagnetism (FM) in a Weyl semimetal are of utmost importance for the advancement of the science of topological materials. Here, we report the FM behavior induced by non-magnetic doping and the emergence of magnetic Weyl states in SrMn_1-xZn_xSb₂. Building on a synergy between experimental data and first principles calculations we will show that mixing non-magnetic SrZnSb₂ with antiferro-magnetic (AFM) SrMnSb₂ leads to FM SrMnSb₂ and SrZn_0.25Mn_0.75Sb₂ with strong spin-orbit coupling. From the computed band structures and fermi surface, we deduce semimetal, small-gap insulator and Weyl semimetal character for SrZnSb₂, SrMnSb₂ and SrZn_0.25Mn_0.75Sb₂ respectively.
Samples of SrMn_1-xZn_xSb₂ (x = 0.065 and 0.262) were synthetized and characterized. Non-trivial de-Haas van Alphen and Shubnikov – de Haas oscillations were measured, supporting the theoretical finding of Weyl semimetal nature for SrMn_1-xZn_xSb₂. Our results show that non-magnetic doping is an important parameter that could be used to discover new magnetic topological Weyl semimetals.

Topology a mathematical concept became recently a hot topic in condensed matter physics and materials science. One important criteria for the identification of the topological material is in the language of chemistry the inert pair effect of the s-electrons in heavy elements and the symmetry of the crystal structure [1]. Beside of Weyl and Dirac new fermions can be identified compounds via linear and quadratic 3-, 6- and 8- band crossings stabilized by space group symmetries [2]. In magnetic materials the Berry curvature and the classical AHE helps to identify interesting candidates. Magnetic Heusler compounds were already identified as Weyl semimetals such as Co₂YZ [3-6] (Y=Ti, Mn; Z=Ge, Sn, Ga, Al), in Mn₃Z [7-9] (Z=Ge, Sn). Co₃Sn₂S₂ is a 2D Weyl semimetal with edge states [8-10].

[1] Bradlyn et al., Nature 547 298, (2017)
[2] Bradlyn, et al., Science 353, aaf5037A (2016).
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[5] Chang et al., Scientific Reports 6, 38839 (2016)
[6] I. Belopolski, et al., Science 365, 1278 (2019)
[7] Kübler and Felser, EPL 108 (2014) 67001 (2014)
[8] Nayak, et al., Science Advances 2 e1501870 (2016)
[9] Nakatsuji, Kiyohara and Higo, Nature 527 212 (2015)
[10] Liu, et al. Nature Physics 14, 1125 (2018)
[11] D. F. Liu, et al., Science 365, 1282 (2019)
[12] N. Morali, et al., Science 365, 1286 (2019)

With the explosion of recent work exploring the out of equilibrium energy landscape of quantum materials, a need has developed to expand the set of available tools to be able to follow these dynamic pathways in as much details as possible. A key aspect of this mode of exploration is the ability to follow these trajectories to map the energy landscape to understand how the phases evolve and ideally seek out new states. However, this requires the ability to extract as much detail about the how the crystalline, electronic and magnetic properties change on ultrafast timescales. Here I will talk about how time-resolved X-ray probes at synchrotrons and free electron lasers can allow one to separate explore these degrees of freedom. Such direct information about the evolution of lattice, electronic, and magnetic order can then be merged with theoretical studies to build a clear model to describe the ultrafast evolution of the system. To illustrate the possibilities, I will present examples ranging from decoupling order parameters in Nickelates and optical creation of hidden structural phases in Titanate superlattices[1]. This work will highlight what is possible with todays X-ray sources and potential experiments we can consider in the very near future.

The work at Argonne is supported by the U.S. Department of Energy, Office of Science, under Contract No. DE-AC02-06CH11357. The development of the materials and ultrafast experiments is supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Award Number DE-SC-0012375.

[1] V. A. Stoica, N. Laanait, C. Dai, Z. Hong, Y. Yuan, Z. Zhang, S. Lei, M. R. McCarter, A. Yadav, A. R. Damodaran, S. Das, G. A. Stone, J. Karapetrova, D. A. Walko, X. Zhang, L. W. Martin, R. Ramesh, L. Q. Chen, H. Wen, V. Gopalan, and J. W. Freeland, Nat Mater 18, 377 (2019).

Irradiation with light is able to dynamically push quantum materials away from equilibrium, driving them into states with transiently perturbed magnetic and electronic properties. This field has become a rich playground for fundamental research but may also be relevant for technical applications that require ultrafast switching capabilities.
In this talk, I will focus on our use of intense terahertz and mid-infrared light fields to control quantum solids by resonant excitation of crystal lattice vibrations. This approach has been demonstrated to switch between different phases and reveal hidden states not accessible at equilibrium. Examples are the control of insulator-metal transitions[1]^,[2],[3], magnetism[4]^,[5] and ferroelectricity[6]^,[7], as well as the enhancement of superconducting coherence[8].
Anharmonicities in the crystal lattice dynamics, like rectification[9]^,[10], parametric amplification[11] and harmonic generation[12] of optical phonons, are an important ingredient of the microscopic physics underlying these phenomena. I will present the fundamentals of these nonlinear physics and discuss how they connect to the manipulation of the magnetic and electronic phases.

[1] M. Rini et al., Nature 449, 72 (2007).
[2] A.D. Caviglia et al., Physical Review Letters 108, 136801 (2012).
[3] V. Esposito et al., Physical Review Letters 118, 247601 (2017).
[4] M. Först et al., Nature Materials 14, 883 (2015).
[5] T.F. Nova et al., Nature Physics 13, 132 (2017).
[6] R. Mankowsky et al., Physical Review Letters 118, 197601 (2017).
[7] T.F. Nova et al., Science 364, 1075 (2019).
[8] W. Hu et al., Nature Materials 13, 705 (2014).
[9] M. Först et al., Nature Physics 7, 854 (2011).
[10] R. Mankowsky et al., Nature 516, 71 (2014).
[11] A. Cartella et al., Proceedings of the National Academy of Science 115, 12148 (2018).
[12] A. von Hoegen et al., Nature 555, 79 (2018).

In recent years, topological insulator materials have been attractive to many scientists because of their gapless conductive surface states that are protected against perturbations. Bi₄I₄ and Bi₄Br₄ are a quasi-one-dimensional van der Waals material which may offer a new platform for exploring the physics of topological phases. The β-phase Bi₄I₄ is predicted and experimentally confirmed by photoemission spectroscopy to be a weak topological insulator (WTI) , and undergoes a nontrivial WTI to the trivial insulator α -phase around room temperature. The α-phase of Bi₄Br₄ , on the other hand, is predicted to be intrinsic higher-order topological insulator, with 1D helical hinge states and 3D insulating bulk state. Intrinsic superconductivity in Bi₄I₄ and Bi₄Br₄ under high pressure have also been reported. Here we report the synthesis and transport studies of the Bi₄I₄, Bi₄Br₄, and doped Bi₄I_4-xBr_x materials on the crystals as long as 6 mm. The detailed transport, electronic structures, and the associated control of topological phase transition upon chemical doping will be presented.

Magnetic doping or interfacing magnetic materials with topological insulators such as Bi₂Se₃ has been shown to controllably break time reversal symmetry and induce exotic phenomena such as the quantum anomalous hall effect. In this work we study the effect of a buried paramagnetic metal, Co₇Se₈ within Bi₂Se₃. Trilayer structures were synthesized by molecular beam epitaxy, in which the Co₇Se₈ layer was sandwiched between a top and bottom Bi₂Se₃ layer. Magnetoresistance data reveals a sharp decrease in the weak-antilocalization (WAL) effect with the addition of a single Co₇Se₈ monolayer and is completely suppressed in as few as 4 atomic layers. Conversely, Co-doped Bi₂Se₃ films show only a milder suppression of the WAL effect for equivalent Bi:Co concentrations. As the Co₇Se₈ layer is non-magnetic and obeys time reversal symmetry, it is expected that the topology of the system remains protected. However, the suppression of the WAL effect suggests the spin-momentum locking and topological behavior is destroyed.

Topological semimetals such as Weyl and Dirac systems are three-dimensional phases of matter characterized by topology and symmetry protected gapless electronic excitations. These three- dimensional analogs of graphene have generated a lot of interest recently given that their quasiparticles display properties akin to those of relativistic and chiral fermions in particle physics. Their unconventional electronic structures are predicted to lead to protected surface states and to unconventional responses to applied electric and magnetic fields. In the past few years, we have studied a few of these compounds [1-11] under high magnetic fields, with the goal of i) extracting their electronic structure at the Fermi level in order to ii) compare it with theoretical predictions, and of iii) exposing their transport properties which are expected to be unconventional due to their “topological” character. Here, after a brief introduction, we will focus on the magnetoresistivity and the Hall-effect of the type-I Weyl semimetal TaAs in an attempt to address the strong controversy surrounding its anomalous transport properties in relation to its bulk topological character. For fields and currents along the basal plane, we observe a very pronounced planar Hall effect (PHE) upon field rotation with respect to the crystallographic axes at temperatures as high as T = 100 K [10]. Parametric plots of the PHE signal as a function of the longitudinal magnetoresistivity (LMR), collected at T = 10 K, lead to concentric traces as reported for Na₃Bi and GdBiPt suggesting that both the negative LMR and the PHE observed in TaAs are intrinsically associated to the axial anomaly among its Weyl nodes. For fields nearly along the a axis we also observe hysteresis as one surpasses the quantum limit, where the magnetic torque indicates a change in regime as the field increases, i.e., from paramagnetism and diamagnetism due to Weyl fermions above and below the Weyl node(s), respectively, to a paramagnetic one associated with the field-independent n = 0 Landau level. Hysteresis coupled to the overall behavior of the torque would be consistent with a topological phase transition associated with the suppression of the Weyl dispersion at the quantum limit. This transition leads to the suppression of the negative LMR confirming that it is associated to the Weyl dispersion [10]. Finally, for fields along the c axis the Hall effect reveals two successive changes in slope, or two successive decrements in carrier mobility, one at the quantum limit and a second one at the critical field where a phase transition toward an insulating state was recently reported. This suggests the possibility of two successive phase transitions as a function of the field with the higher-field one involving solely the n = 0 Landau level. For both field orientations we observe Shubnikov–de Haas like oscillations beyond the quantum limit therefore involving quasiparticles at fractional filling factors.

This work is supported by the US Department of Energy, Basic Energy Sciences program through Award No. DESC0002613.

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The quantum chiral anomaly enables a nearly non-dissipative current in the presence of chirality imbalance. We propose to utilize the chiral anomaly for the designs of qubits potentially capable of operating at THz frequency and at room temperature with a coherence time to gate time ratio of about 10⁴. The proposed “Chiral Qubit” is a micron-scale ring made of a Weyl or Dirac semimetal, with the |0〉and |1〉states corresponding to the symmetric and antisymmetric superpositions of chiral currents circulating along the ring clockwise and counter-clockwise. A fractional magnetic flux through the ring induces a quantum superposition of the |0〉and |1〉quantum states. The entanglement of qubits can be implemented through the near-field THz frequency electromagnetic fields (EMF). We show that the Hamiltonian of the chiral qubit is similar to that of the superconducting qubit. This means that quantum gates can be implemented in a traditional way, and the algorithms developed for superconducting quantum processors will apply. Light-driven (THz) ultrafast topology switching, demonstrated experimentally in Dirac/Weyl semimetal recently, will be discussed, together with its implications on THz topological quantum gate applications.

The author is grateful for collaborations with Dmitri Kharzeev and Jigang Wang. This work is supported by the US Department of Energy, Office of Basic Energy Science, Materials Sciences and Engineering Division, under contract #DE-SC0012704.

Decades of efforts in improving computing power and experimental instrumentation were driven by our desire to better understand the complex problem of quantum emergence. However, increasing volume and variety of data made available to us today present new challenges. I will discuss how these challenges can be embraced and turned into opportunities by employing machine learning. It is important to note that the scientific questions in the field of electronic quantum matter require fundamentally new approaches to data science for two reasons: (1) quantum mechanical imaging of electronic behavior is probabilistic, (2) inference from data should be subject to fundamental laws governing microscopic interactions. Hence machine learning quantum emergence requires collective wisdom of data science and condensed matter physics. I will review rapidly developing efforts by the community in using machine learning to solve problems and gain new insight. I will then present my group’s results on the machine-learning-based analysis of complex experimental data on quantum matter.

Our understanding of two-dimensional (2D) materials has developed rapidly with the recent development
of theoretical and computational capabilities, as well as the development of experimental synthesis and
characterization tools. Now it has become feasible to perform atomic-level manipulations of 2D materials,
such as introducing atomic defects and dopants, and engineering boundaries by applying strains and
interfaces by growing them on different substrates. The rich parameter space of 2D materials makes them
appealing candidates for new types of flexible electronic devices, such as topological electronics, that are
stable at room temperature and operate at ambient conditions. Theoretical/computational modeling is
desirable to guide experimental efforts in exploring the vast parameter space of 2D material and to identify
desirable parameter space. On the other hand, modeling 2D materials in realistic experimental conditions
is challenging; not only do these materials contain many atoms (exceeding thousands of atoms) but also the
materials’ parameter space is highly complex. Using a prototypical topological 2D system as an example—
e.g., graphene, stanene, and 2D metal-organic frameworks—schemes combining various levels of
theoretical/computational approaches were developed, including a tight-binding approach, high-throughput
density functional theory calculations, data analytics, and large-scale ab initio dynamics using
supercomputer capabilities at the Department of Energy’s Oak Ridge National Laboratory. The developed
schemes are targeted to enhance theoretical predictability for topological materials while significantly
narrowing the gap with the experimental effort toward robust topological materials.

f-electrons possess both strong electronic correlations and strong spin orbit coupling. This combination suggests that f-electron materials are ideal for discovering new topologically non-trivial phases, which arise as a consequence of strong electronic correlations. Broadly speaking, we adopt four approaches to discover new topological states of matter. First, we explore correlation driven narrow gap materials such as SmB₆. Second, in systems such as Eu₅In₂Sb₆ we exploit the fact that rare-earth magnetism is strongly localized, resulting in only minor perturbations to the non-interacting electronic structure. Third, we attempt to exploit (non-symmorphic) crystal symmetries for generating protected nodal crossings. Finally, we examine metallic systems such as U₃Ru₄Al₁₂ where broken time reversal symmetry can yield a finite and tunable Berry phase contribution to transverse transport measurements. This talk will review our strategies, efforts, lessons learned, and unexpected discoveries in searching for such new states of matter.

Predictive capabilities of the widely used band theory paradigm, despite its spectacular successes, have been limited to weakly correlated materials.[1-3] I will discuss how advanced density functionals are enabling new insights into the electronic structure, phase diagrams and magnetism of a wide variety of materials that have been considered to be so strongly correlated as to lie outside the scope of first-principles treatment. A spectacular example is that of the cuprate high-Tc superconductors in which the density functional theory fails to correctly predict the half-filled parent compounds to be insulators. In sharp contrast, however, the recently constructed SCAN functional [4] not only reproduces the insulating character and magnetism of the half-filled cuprates, but also captures the transition to the metallic state with doping without invoking any free parameters such as the Hubbard U. [5,6] I will also discuss with examples drawn from our recent work on the opportunities for a new generation of predictive modeling of correlated topological and non-topological phases of quantum matter more generally. Work supported by the U.S. Department of Energy.

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Efficient electric transport is one of the key topics in modern electronics. Generally, electrons scattering is unavoidable for ordinary current flowing through the materials. Different from this, chiral anomaly current in Dirac semimetals is the charge pumping process with inhibited electrons scattering. Here, we investigate the application of chiral anomaly current for field-effect efficient electronics with integrated sensing and logic functions. The type-II Dirac is theoretically predicted through first-principle calculations in Dirac semimetal PtSe₂ and experimentally investigated by magnetotransport behaviors. Angle-dependent longitudinal negative magnetoresistance (MR) is observed in prepared devices and when electric field is parallel to the magnetic field, there is the maximum magnetic conductance value proportional to B². Besides sensitive to the angles and magnetic field strength, chiral anomaly current can be well field-modulated by ion liquid. Moreover, AND-OR logic operation and three-inputs coincidence detection are demonstrated by using chiral anomaly properties. Our results provide new sights on achieving field-modulated electronics with integrated sensing and logic functions based on the chiral anomaly current with inhibited electrons scattering.

The rapid evolution of topological matter has led to strong interest in electronic and spintronic devices that can leverage the unique and attractive physics of the topological surface states. One path to integrate both topological-based devices with classical electronics (CMOS) would be to grow the materials on silicon. Because topological properties can be affected by strain and because the mismatch in thermal expansion coefficients between silicon and bismuth telluride is approximately a factor of 10, techniques to mitigate strain are of interest. In this report, we investigated the use of a unique strain-compliant barium-fluoride-based buffer layer to mitigate the effects of thermal expansion strain between Bi₂Te₃ and Si. Under our previous research we have shown that BaF₂ can be latticed matched to Si by first growing a compliant BaSi₂ layer. These thin films have been grown in our MBE (molecular beam epitaxy) system of various thicknesses and partial coverage. With partial coverage, the result is (111) BaF₂triangles. We observe that growing Bi₂Te₃ on BaF₂reduces twinning because of the excellent lattice matching. Ionic substrates such as BaF₂ have been shown to n dope graphene and their strong bonds can influence several monolayers, thus the nature of the substrate can be used to control the properties of the topological insulator. Recently, we have grown Bi₂Te₃ on BaF₂/Ba-Si/Si with good results. Growing on a substrate with partial coverage showed good Bi₂Te₃ domain alignment with the BaF₂ domain. The BaF₂ triangles effectively create a nanostructure surface where the topological insulator is exposed to different surface energies. Hall probe results have shown that the Bi₂Te₃ grown on BaF₂/ Si is n type with good mobility. Excellent crystal structure was confirmed by XRD (x-ray diffraction). Here we report our results of growing Bi₂Te₃ on BaF₂/Si and Bi₂Se₃ on SrF₂/Si of various thickness and partial coverage as well as discuss potential applications for DoD.

Weyl semimetals (WSM) are topologically non-trivial materials hosting massless chiral fermions. However, to date nearly all measurements of Weyl semimetals have been conducted on bulk crystals, which have limited ability to tune via external parameters such as electrostatic gating or strain. Here, we report the first epitaxial growth of the Weyl semimetal candidate Gd_xLa_1-xPtSb, using molecular beam epitaxy. We grow these materials in two forms: (1) epitaxial Gd_xLa_1-xPtSb films on Al₂O₃ (0001) substrates, and (2) semi-freestanding membranes grown on graphene-terminated Al₂O₃, using the so-called ``remote epitaxy.''

For x<0.95 the Gd_xLa_1-xPtSb films adopt a hexagonal stuffed wurtzite structure (LiGaGe-type, P6_3 mc), while the x=1 end member GdPtSb has cubic half Heusler structure. Angular dependent magnetotransport measurements of GdPtSb reveal a negative magnetoresistance that is highly dependent on angle, consistent with the chiral anomaly. More detailed measurements as functions of Gd/La doping (magnetic moment and cubic/hexagonal structural transition) and electrostatic gating will also be discussed.

We also demonstrate the epitaxial growth of LaPtSb on graphene-terminated Al₂O₃ (0001). Despite the presence of monolayer graphene, the LaPtSb film adopts epitaxial registry with the underlying Al₂O₃ substrate, as confirmed by X-ray diffraction and transmission electron microscopy measurements. Due to the weak coupling between film and substrate across the graphene barrier, the LaPtSb film can be mechanically exfoliated using tape, to yield a free-standing membrane. Using in-situ photoemission, electron diffraction, and scanning probe microscopy, we will comment on the atomic scale mechanisms for growth. We will also discuss how the magnetic and transport properties Gd_xLa_1-xPtSb membranes can be actively tuned via strain.

Many physical phenomena in condensed matter arise from strong correlation and spin-orbit coupling (SOC), and their proper control is important for creating novel quantum materials (QMs), such as Mott insulators, quantum spin liquids, topological insulators, Weyl semimetals, etc. to name a few. As the importance of quantum information science emerges, there is an increased attention to QMs to address the need for new materials for addressing challenges in quantum computing and sensing. Among many classes of QMs, complex oxides offer a rich playground to exploit both correlations of Coulombic interactions between electrons often found in 3d transition metal oxides (TMOs) and SOC from heavy metal based 5d TMOs, such as SrIrO₃ and Na₂IrO₃, leading to many intriguing physical properties. Hybrid materials with a proper balancing of the correlation and SOC offer unique opportunities to discover new properties, and heterostructuring or combining dissimilar materials is a great approach to harnessing the complex interplay between quantum wavefunctions and various control parameters, such as proximity, dimensionality, topology, and symmetry. In this talk, I will review recent progresses in topological quantum materials and present our recent work on topological Hall effect from chiral magnetism and its control via interfacial symmetry control. Furthermore, an effort to combine topology and correlation to develop correlated topological materials and an approach to develop novel Berry phase engineered heterostructures will be presented.

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

Momentum Microscopy is a new technique in surface science, in which the momentum distribution of photoelectrons is projected onto an image plane. This image plane can be energy-filtered by a double-hemispherical electrostatic analyzer (NanoESCA) to achieve a monochromatic momentum distribution. Scanning the energy along the valence band is then used for band structure mapping which is a key technique to understand the electronic structure of novel materials.
A new innovative method is to use the monochromatic electron distribution behind the double-hemispherical analyzer to retard it to the needed scattering energy of an Au/Ir (100) single crystal. This allows for spin-polarization dependent reflection, which enables a highly efficient imaging spin-filter technique [1].

We will show recent band-structure measurements of several TMDCs acquired with the NanoESCA and discuss the latest technical improvements of momentum microscopy with respect to 2D material characterization. First results of the imaging spin-filter installed at the ELI-Alps NanoESCA are shown, demonstrating its efficiency in real-space microscopy (e.g. imaging of magnetic domains) and in spin-resolved band-structure mapping.


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Topology a mathematical concept became recently a hot topic in condensed matter physics and materials science. One important criteria for the identification of the topological material is in the language of chemistry the inert pair effect of the s-electrons in heavy elements and the symmetry of the crystal structure [1]. Beside of Weyl and Dirac new fermions can be identified compounds via linear and quadratic 3-, 6- and 8- band crossings stabilized by space group symmetries [2]. In magnetic materials the Berry curvature and the classical AHE helps to identify interesting candidates. Magnetic Heusler compounds were already identified as Weyl semimetals such as Co₂YZ [3-6] (Y=Ti, Mn; Z=Ge, Sn, Ga, Al), in Mn₃Z [7-9] (Z=Ge, Sn). Co₃Sn₂S₂ is a 2D Weyl semimetal with edge states [8-10].

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Irradiation with light is able to dynamically push quantum materials away from equilibrium, driving them into states with transiently perturbed magnetic and electronic properties. This field has become a rich playground for fundamental research but may also be relevant for technical applications that require ultrafast switching capabilities.
In this talk, I will focus on our use of intense terahertz and mid-infrared light fields to control quantum solids by resonant excitation of crystal lattice vibrations. This approach has been demonstrated to switch between different phases and reveal hidden states not accessible at equilibrium. Examples are the control of insulator-metal transitions[1]^,[2],[3], magnetism[4]^,[5] and ferroelectricity[6]^,[7], as well as the enhancement of superconducting coherence[8].
Anharmonicities in the crystal lattice dynamics, like rectification[9]^,[10], parametric amplification[11] and harmonic generation[12] of optical phonons, are an important ingredient of the microscopic physics underlying these phenomena. I will present the fundamentals of these nonlinear physics and discuss how they connect to the manipulation of the magnetic and electronic phases.

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In the presence of strong interactions, the fluid of mobile electrons in a metal can spontaneously break the point group symmetries of its underlying host lattice, forming an electronic analogue of a classical liquid crystal. The experimental discovery of 2D electronic liquid crystals (ELCs) was first made nearly 20 years ago in semiconductor heterostructures and has since been reported in several other systems including the copper- and iron-based high-temperature superconductors. However whether or not a 3D version of an ELC can exist has remained unclear. In this talk, I will present signatures of a 3D ELC in the strongly spin-orbit coupled metallic pyrochore Cd2Re2O7 detected using ultrafast coherent phonon spectroscopy and a recently developed spatially-resolved nonlinear optical polarimetry technique. I will discuss implications for the possible topological superconducting phase that has been reported to emerge in Cd2Re2O7 under pressure.

Decades of efforts in improving computing power and experimental instrumentation were driven by our desire to better understand the complex problem of quantum emergence. However, increasing volume and variety of data made available to us today present new challenges. I will discuss how these challenges can be embraced and turned into opportunities by employing machine learning. It is important to note that the scientific questions in the field of electronic quantum matter require fundamentally new approaches to data science for two reasons: (1) quantum mechanical imaging of electronic behavior is probabilistic, (2) inference from data should be subject to fundamental laws governing microscopic interactions. Hence machine learning quantum emergence requires collective wisdom of data science and condensed matter physics. I will review rapidly developing efforts by the community in using machine learning to solve problems and gain new insight. I will then present my group’s results on the machine-learning-based analysis of complex experimental data on quantum matter.

Coupling between charge, spin, lattice, or orbital degrees of freedom gives rise to remarkable phenomena such as colossal magnetoresistance, metal–insulator transitions and superconductivity. Many aspects of these competing interactions, however, remain elusive. Traditional bulk measurements of phases transitions, which average the material’s response over large length scales, can inferred the presences of spatial inhomogeneity. Understanding their character, however, requires local probes. Direct, real-space measurements of the onset of emerging order are key to understanding the diverse properties of complex electronic materials.

Today, atomic-resolution imaging is routinely achieved by aberration-corrected scanning transmission electron microscopy (STEM). Coupled with spectroscopy STEM provides direct information about the local structure and chemistry of thin films and heterostructures. In this talk, I will first discuss our results on two classes of superconducting materials, the layered perovskite ruthenates and the recently reported infinite-layer nickelates [1], with particular focus on the microscopic structure, strain and the role of defects in these thin film superconductors. Going beyond room temperature STEM, cryogenic sample cooling promises direct access to low temperature phases. While limitations in hardware currently hamper STEM measurements of ruthenates and nickelates in their superconducting states, high-precision cryogenic STEM near liquid nitrogen temperature has been demonstrated [2, 3]. These new capabilities have paved a path to visualize subtle lattice and electronic orders in emergent low temperature phases as I will show in the second part of the talk. Focus will be on the lattice behavior of charge-ordered manganites, where direct imaging can discriminate between multiple structure models, and mapping over larger length scales shows phase coexistence and nanoscale inhomogenities.

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[4] Research performed in collaboration with Ismail El Baggari, Berit H. Goodge, David J. Baek, Michael J. Zachman and the groups of Harold Y. Hwang (Stanford), Darrell G. Schlom (Cornell), Kyle M. Shen (Cornell) and Beth Nowadnick (New Jersey Institute of Technology). This work was supported by the DoD, AFOSR (FA 9550-16-1-0305), NSF (DMR-1539918, DMR-1429155, DMR-1719875), and the Packard Foundation.

Interfaces and heterostructures with topological semimetals offer new opportunities to control and manipulate their unique electronic states and a wealth of associated phenomena, for example, via electric field effect, strain, or symmetry breaking. In this presentation, we will discuss recent progress in heterostructures of a prototype three-dimensional Dirac semimetal, cadmium arsenide (Cd₃As₂), which are grown by molecular beam epitaxy. We show that high-mobility, epitaxial Cd₃As₂ films can be grown in different crystallographic orientations. The easy cleavage plane of single crystals, the tetragonal (112) plane, is also the most common growth surface. In this orientation, the bulk Dirac nodes project on two points in the surface Brillouin zone. The (001) plane differs from the (112) plane and indeed all others in that the two bulk nodes project onto one point, rather than two, of the surface Brillouin zone. Here, we discuss the nature of quantum oscillations and of the quantum Hall effect of thin Cd₃As₂ films grown in both orientations. The quantum Hall effect of (112) films is characterized by a degeneracy factor of two that is lifted in fields above approximately 10 T. Gated magnetotransport and quantum capacitance revealed ambipolar quantum transport, a zero-energy Landau level, and linear dispersion in the 2D states giving rise to the quantum Hall effect. The important feature is that (112) surfaces host double Fermi arcs, which can give rise to two Fermi pockets near the center of the surface Brillouin zone and thus an even integer Hall effect until their degeneracy is lifted by the magnetic field. A single (112) surface can give rise to an integer quantum Hall effect and explain the observed characteristics. By contrast, the states of (001) films are non-degenerate and integer filling factors are due to crossing Landau levels. We will discuss other remarkable phenomena in the quantum Hall effect of (001) films. We will also discuss pathway towards realizing novel topological systems.

f-electrons possess both strong electronic correlations and strong spin orbit coupling. This combination suggests that f-electron materials are ideal for discovering new topologically non-trivial phases, which arise as a consequence of strong electronic correlations. Broadly speaking, we adopt four approaches to discover new topological states of matter. First, we explore correlation driven narrow gap materials such as SmB₆. Second, in systems such as Eu₅In₂Sb₆ we exploit the fact that rare-earth magnetism is strongly localized, resulting in only minor perturbations to the non-interacting electronic structure. Third, we attempt to exploit (non-symmorphic) crystal symmetries for generating protected nodal crossings. Finally, we examine metallic systems such as U₃Ru₄Al₁₂ where broken time reversal symmetry can yield a finite and tunable Berry phase contribution to transverse transport measurements. This talk will review our strategies, efforts, lessons learned, and unexpected discoveries in searching for such new states of matter.