Bulletin of the American Physical Society
APS March Meeting 2020
Volume 65, Number 1
Monday–Friday, March 2–6, 2020; Denver, Colorado
Session F59: Computational approaches to magnetic topological materials discoveryFocus
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Sponsoring Units: DMP Room: Mile High Ballroom 3C |
Tuesday, March 3, 2020 8:00AM - 8:12AM |
F59.00001: High-Throughput of Magnetic Topological Materials Using Topological Magnetic Quantum Chemistry Yuanfeng Xu, Zhida Song, Luis Elcoro, Maia Vergniory, Nicolas Regnault, Yulin Chen, Claudia Felser, Andrei Bernevig Using magnetic band theory and topological indices obtained from Topological Magnetic Quantum Chemistry (TMQC), we have performed the first high-throughput of about 500 magnetic topological materials and have discovered over 100 magnetic enforced semimetals and topological insulators. This knowledge is crucial for correct ab-initio calculations of the materials' band structure, which we have performed for each of those compounds including complete phase diagrams at different values of Hubbard potential in LDA+U. Using an in-house code for finding the magnetic band representations at high symmetry points, we then feed this data into the topological machinery of TMQC to determined the topological materials, as well as the obstructed atomic limits. We then pick several candidates for showcasing new topological physics and analyze the topological trends in the materials upon varying interactions. |
Tuesday, March 3, 2020 8:12AM - 8:48AM |
F59.00002: Computational study and discovery of antiferromagnetic topological insulators Invited Speaker: Mikhail Otrokov Magnetic topological insulators (MTIs) are narrow gap semiconductor materials that combine non-trivial band topology and magnetic order. Unlike their nonmagnetic counterparts, MTIs may have some of the surfaces gapped due to breaking the time-reversal symmetry, which enables exotic phenomena having potential applications in spintronics. So far, MTIs were only created by means of doping nonmagnetic TIs with 3d transition metal atoms however, such an approach leads to strongly inhomogeneous magnetic and electronic properties of these materials, restricting the observation of important effects to very low temperatures. Finding intrinsic MTI, i.e. a stoichiometric well-ordered compound, could be an ideal solution to these problems. Using ab initio calculations, we predicted the van der Waals layered compound MnBi2Te4 (MBT) to be the first antiferromagnetic TI (AFMTI) [1,2]. The interlayer AFM ordering makes MBT invariant with respect to the combination of the time-reversal (Θ) and primitive-lattice translation (T1/2) symmetries, S=ΘT1/2, which gives rise to the Z2 classification of AFM insulators. We find Z2=1 for MnBi2Te4, which confirms its topologically nontrivial nature. To date, many experimental groups confirmed the AFMTI state in MBT, with the first observation reported in Ref. [1]. |
Tuesday, March 3, 2020 8:48AM - 9:00AM |
F59.00003: Computational Search for Magnetic and Non-magnetic 2D Topological Materials using Spin-orbit Spillage Kamal Choudhary, Kevin Garrity, Francesca Tavazza Intrinsic two-dimensional materials have a variety of properties that make them attractive for potential topological devices. Using density functional theory-based spin-orbit spillage, Wannier-interpolation, and related techniques, we identify topologically non-trivial intrinsic 2D insulators and semimetals, including both magnetic and non-magnetic materials. Using JARVIS-DFT 2D material dataset we first identify materials with high spin-orbit spillage among 683 materials, resulting in 108 materials with high-spillage values. Then, we use Wannier-interpolation to carry-out Z2, Chern-number, anomalous Hall conductivity, Curie temperature, and edge state calculations. We identify topological insulators and semimetals such as quantum spin-hall insulators (QSHI), quantum anomalous-hall insulators (QAHI), and semimetals. For a subset of predicted QAHI materials, we run GW+SOC and GGA+U calculations. We find that as we introduce many-body effects, only few materials retain non-trivial band-topology, suggesting the importance of high-level DFT methods in predicating 2D topological materials. However, as an initial step, the automated spillage screening and Wannier-approach provides useful predictions for finding new topological materials. |
Tuesday, March 3, 2020 9:00AM - 9:12AM |
F59.00004: Automated Critical Temperature Prediction and High-Throughput Search for Composite Quantum Materials Nathan C. Frey, Matthew Horton, Jason Munro, Vivek Shenoy, Kristin Persson Composite materials with coexisting quantum phases offer exciting opportunities for solid-state device applications and exploring new physics emerging from the interplay between effects including topology, magnetism, and ferroelectricity. However, both the computational design and experimental realization of magnetic topological materials and multiferroics are confounded by the difficulties inherent to predicting and controlling magnetic behavior, which arises from strong electron correlations. We present a workflow to automate the calculation of exchange parameters and critical temperatures with density functional theory and Monte Carlo simulations. We use our recently developed Python Topological Materials package to screen materials for non-trivial band topology. We then apply this approach to suspected magnetic materials in the Materials Project database. By identifying layered materials, we also accelerate the discovery of van der Waals materials with composite topological quantum phases and multiferroic properties. Our method can be used to screen for thermodynamically stable, robust composite quantum materials for solid-state devices and exploring exotic physics like the quantum anomalous Hall effect and axion electrodynamics. |
Tuesday, March 3, 2020 9:12AM - 9:24AM |
F59.00005: Computational search for magnetic topological materials in Eu compounds Lin-Lin Wang, Robert-Jan Slager, Hoi Chun Po, Adam Kaminski, Ashvin Vishwanath, Paul C Canfield Topological materials are of great interests for both basic research and future technology. Recent developments of symmetry indicators and similar approaches to diagnose band structure topology have resulted in databases of non-magnetic topological materials via high throughput band structure calculations. However, magnetic topological materials have been much less explored. One of the reasons is that the magnetic ground state structures are not readily available like crystallography databases and also different magnetic configurations can be competing in energy. But this also offers the opportunity to use magnetism to control topological properties of materials. Here we have explored Eu compounds using band structure calculations based on density functional theory and found several Eu compounds can host ferromagnetic Weyl, anti-ferromagnetic Dirac as well as anti-ferromagnetic topological insulating states depending on different magnetic configurations. |
Tuesday, March 3, 2020 9:24AM - 9:36AM |
F59.00006: Magnetic Topological Quantum Chemistry Benjamin Wieder, Luis Elcoro, Zhida Song, Yuanfeng Xu, Nicolas Regnault, Barry Bradlyn, Andrei Bernevig In [Bradlyn et al., Nature (2017)], we introduced Topological Quantum Chemistry (TQC) - a real-space predictive theory of band topology and degeneracy constructed from graph theory and atomic orbitals in the 230 time-reversal- (T-) symmetric space groups (SGs). TQC subsequently fueled the identification of tens of thousands of topological materials, and the discovery of novel topological insulating (TI) phases, including "higher-order" TIs. However, the 230 SGs only represent a fraction of the 1651 Shubnikov SGs, which characterize the symmetries of both magnetic and non-magnetic crystals. In this talk, we extend TQC to the magnetic Shubnikov SGs (MSGs), forming the complete theory of Magnetic Topological Quantum Chemistry (MTQC). Using MTQC, we construct a complete classification of the magnetic atomic limits [i.e., magnetic Elementary Band Representations (MEBRs)]. Through the MEBRs, we also obtain the little-group corepresentations of the MSGs, complete eigenvalue indicators for strong and fragile magnetic TIs and semimetals, and all possible mean-field band connectivities in crystals with lattice-commensurate magnetism. We introduce new tools on the Bilbao Crystallographic Server for accessing this information, and discuss magnetic topological materials identified using MTQC. |
Tuesday, March 3, 2020 9:36AM - 9:48AM |
F59.00007: New classes of band degeneracies in magnetic materials universe Jian Yang We perform a comprehensive study of band degeneracies protected by 3D magnetic space groups in the bands of magnons and of electrons, and obtain all new classes of degeneracies (nodes) that cannot appear in non-magnetic materials. |
Tuesday, March 3, 2020 9:48AM - 10:00AM |
F59.00008: Searching for Double Dirac Fermions Tanya Berry, Rafal Wawrzynczak, Johannes Gooth, Claudia Felser, Tyrel McQueen The field of topological materials is developing rapidly, exhibiting new physical phenomena and inviting potential applications in the emerging fields of quantum computing and sensing. Normally, the maximal degeneracy of states in any electronic system is six. Topological considerations predict that in certain materials the double group cross product of two irreducible representations (irreps.) or the cross product of one irrep. on itself can yield eight states that are degenerate in energy at a high symmetry point in reciprocal space. Materials including CuBi2O4 and PdBi2O4 have been previously predicted to be such 8-fold fermion or double Dirac systems on the basis of their symmetries. However, their insulating behavior hinders photoemission and transport characterizations, which are crucial for unambiguous identification. Here we present a strategy for designing real materials with 8-fold fermions based on chemical principles and provide an update on experimental detection of this novel topological state. |
Tuesday, March 3, 2020 10:00AM - 10:12AM |
F59.00009: Axion coupling in the hybrid Wannier representation Nicodemos Varnava, Ivo Souza, David Vanderbilt One of the most important quantized responses of 3D topological insulators (TIs) is due to the axion coupling θ. Strong TIs, axion insulators, and the topological magnetoelectric effect owe their robustness to the quantization of the axion coupling (the axion Z2 index). Time reversal was the first quantizing symmetry to be recognized, but recently a plethora of magnetic symmetries were found to have the same effect. After we enumerate all the quantizing symmetries, we explore how the nature of these symmetries affects the topological properties of the system. One tool that has proven especially useful in that respect is the hybrid Wannier (HW) representation. In this representation the quantizing symmetries can be divided into three distinct classes. By analyzing each of them in turn, we are able to explain whether the connectedness of the HW band structure in a topological phase is robust or fragile. Furthermore, we clarify the correspondence between the connectedness of the HW band structure and the appearance of either topologically protected metallic surfaces or insulating surfaces with half-quantized anomalous Hall response. We corroborate our results with various tight-binding models. |
Tuesday, March 3, 2020 10:12AM - 10:24AM |
F59.00010: Domain-wall states in Chern insulator with in-plane ferromagnetism Yafei Ren, Tao Hou, Zhenhua Qiao Topological phase is global and beyond the Landau paradigm of phases described by local order parameters. In this work, we report the electronic and transport properties of in-gap domain-wall states between domains with the same topology but different local magnetic orders, which lie in plane. We consider a buckled honeycomb lattice model that can host Chern insulating phase from in-plane magnetization with azimuthal angle φ0, which breaks the three-fold rotation symmetry of original lattice. The topological phase remains the same at different φp=φ0+2π p/3 (p=0-2). Along the domain wall, we find one-dimensional in-gap states whose electronic dispersion depends on the magnetic structure and domain wall width despite the same Chern number at different domains. At a symmetric Y-shape junction formed by three regions of different φp, we find counter-intuitive chiral current partition law that is tunable by electrical means. In a network by combining Y-shape junction to form a honeycomb, we find that the in-gap states form nearly flat Bloch bands with different topological Chern numbers providing a intriguing platform to investigate many-body effects. |
Tuesday, March 3, 2020 10:24AM - 10:36AM |
F59.00011: Half-integer quantization of Hall conductance in semi-magnetic topological insulator Masataka Mogi, Yoshihiro Okamura, Minoru Kawamura, Ryutaro Yoshimi, Kenji Yasuda, Atsushi Tsukazaki, Kei Takahashi, Takahiro Morimoto, Naoto Nagaosa, Masashi Kawasaki, Youtarou Takahashi, Yoshinori Tokura The emergence of two-dimensional (2D) Dirac fermions in condensed matter systems, such as graphene and three-dimensional (3D) topological insulators, has greatly deepened quantum Hall physics. The anomalous integer quantization of Hall conductance as observed in graphene is understood by the half-integer topological number of each Dirac cone. However, the Dirac cones always appear in pairs in such 2D lattices, hiding the half-integer number from experimental observations. The 3D topological insulators, on the other hand, can possess a single Dirac cone in each top and bottom surface, serving as an ideal system to explore the half-integer quantization phenomena. Here, we will report the observation of half quantized Hall conductance in ‘semi-magnetic’ topological insulator films, where one of the surfaces is gapped by magnetic doping whereas the opposite one remains non-magnetic and gapless. Using time-domain terahertz magneto-optical spectroscopy as well as electrical transport, we observed half quantized Faraday and Kerr rotations and half quantized Hall conductivity at zero fields. This result provides experimental evidence for the predicted fractional quantized state in 3D topological insulators. |
Tuesday, March 3, 2020 10:36AM - 10:48AM |
F59.00012: Antiferromagnetism and spin density waves in Dirac semimetals Grigory Bednik We perform mean-field study of possible magnetic instabilities in Dirac semimetals. We find that Dirac electrons naturally host antiferromagnetic or spin density wave ground states, though their specific configurations may vary depending on specific model, as well as chemical potential and temperature. We also discuss paramagnetic susceptibility in Dirac semimetals. In the cases, when Dirac electrons do not have orbital momentum, the magnetic properties may be μ and T independent. |
Tuesday, March 3, 2020 10:48AM - 11:00AM |
F59.00013: Intrinsic magnetic topological insulator MnBi2Te4 Jiaheng Li, Duan Wenhui, Yong Xu The interplay of magnetism and topology offers great opportunities to explore variant emerging topological quantum phenomena. Here we will present that van der Waals layered MnBi2Te4-related materials intrinsically host nontrivial band topology, which possess A-type antiferromagnetism characterized by ferromagnetic intralayer and antiferromagnetic interlayer coupling. Extremely rich 2D and 3D topological quantum states emerge in MnBi2Te4, including 3D antiferromagnetic topological insulator with the long-sought topological axion states on the surface, the simple magnetic Weyl semimetal with one pair of Weyl points, as well as intrinsic axion insulators and QAH insulators in even- and odd-layer films, respectively [1]. Furthermore, we will show a series of magnetically controllable topological quantum phase transitions in the material [2]. Relevant experimental progresses will also be introduced [3-5]. |
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