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: HighThroughput 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 highthroughput of about 500 magnetic topological materials and have discovered over 100 magnetic enforced semimetals and topological insulators. This knowledge is crucial for correct abinitio 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 inhouse 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 nontrivial band topology and magnetic order. Unlike their nonmagnetic counterparts, MTIs may have some of the surfaces gapped due to breaking the timereversal 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 wellordered compound, could be an ideal solution to these problems. Using ab initio calculations, we predicted the van der Waals layered compound MnBi_{2}Te_{4} (MBT) to be the first antiferromagnetic TI (AFMTI) [1,2]. The interlayer AFM ordering makes MBT invariant with respect to the combination of the timereversal (Θ) and primitivelattice translation (T_{1/2}) symmetries, S=ΘT_{1/2}, which gives rise to the Z_{2} classification of AFM insulators. We find Z_{2}=1 for MnBi_{2}Te_{4}, 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 Nonmagnetic 2D Topological Materials using Spinorbit Spillage Kamal Choudhary, Kevin Garrity, Francesca Tavazza Intrinsic twodimensional materials have a variety of properties that make them attractive for potential topological devices. Using density functional theorybased spinorbit spillage, Wannierinterpolation, and related techniques, we identify topologically nontrivial intrinsic 2D insulators and semimetals, including both magnetic and nonmagnetic materials. Using JARVISDFT 2D material dataset we first identify materials with high spinorbit spillage among 683 materials, resulting in 108 materials with highspillage values. Then, we use Wannierinterpolation to carryout Z2, Chernnumber, anomalous Hall conductivity, Curie temperature, and edge state calculations. We identify topological insulators and semimetals such as quantum spinhall insulators (QSHI), quantum anomaloushall 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 manybody effects, only few materials retain nontrivial bandtopology, suggesting the importance of highlevel DFT methods in predicating 2D topological materials. However, as an initial step, the automated spillage screening and Wannierapproach provides useful predictions for finding new topological materials. 
Tuesday, March 3, 2020 9:00AM  9:12AM 
F59.00004: Automated Critical Temperature Prediction and HighThroughput 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 solidstate 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 nontrivial 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 solidstate 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 LinLin Wang, RobertJan 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 nonmagnetic 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, antiferromagnetic Dirac as well as antiferromagnetic 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 realspace predictive theory of band topology and degeneracy constructed from graph theory and atomic orbitals in the 230 timereversal (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 "higherorder" TIs. However, the 230 SGs only represent a fraction of the 1651 Shubnikov SGs, which characterize the symmetries of both magnetic and nonmagnetic 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 littlegroup corepresentations of the MSGs, complete eigenvalue indicators for strong and fragile magnetic TIs and semimetals, and all possible meanfield band connectivities in crystals with latticecommensurate 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 nonmagnetic 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 CuBi_{2}O_{4} and PdBi_{2}O_{4} have been previously predicted to be such 8fold 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 8fold 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 Z_{2} 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 halfquantized anomalous Hall response. We corroborate our results with various tightbinding models. 
Tuesday, March 3, 2020 10:12AM  10:24AM 
F59.00010: Domainwall states in Chern insulator with inplane 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 ingap domainwall 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 inplane magnetization with azimuthal angle φ_{0}, which breaks the threefold rotation symmetry of original lattice. The topological phase remains the same at different φ_{p}=φ_{0}+2π p/3 (p=02). Along the domain wall, we find onedimensional ingap states whose electronic dispersion depends on the magnetic structure and domain wall width despite the same Chern number at different domains. At a symmetric Yshape junction formed by three regions of different φ_{p}, we find counterintuitive chiral current partition law that is tunable by electrical means. In a network by combining Yshape junction to form a honeycomb, we find that the ingap states form nearly flat Bloch bands with different topological Chern numbers providing a intriguing platform to investigate manybody effects. 
Tuesday, March 3, 2020 10:24AM  10:36AM 
F59.00011: Halfinteger quantization of Hall conductance in semimagnetic 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 twodimensional (2D) Dirac fermions in condensed matter systems, such as graphene and threedimensional (3D) topological insulators, has greatly deepened quantum Hall physics. The anomalous integer quantization of Hall conductance as observed in graphene is understood by the halfinteger topological number of each Dirac cone. However, the Dirac cones always appear in pairs in such 2D lattices, hiding the halfinteger 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 halfinteger quantization phenomena. Here, we will report the observation of half quantized Hall conductance in ‘semimagnetic’ topological insulator films, where one of the surfaces is gapped by magnetic doping whereas the opposite one remains nonmagnetic and gapless. Using timedomain terahertz magnetooptical 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 meanfield 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 MnBi_{2}Te_{4} 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 MnBi_{2}Te_{4}related materials intrinsically host nontrivial band topology, which possess Atype antiferromagnetism characterized by ferromagnetic intralayer and antiferromagnetic interlayer coupling. Extremely rich 2D and 3D topological quantum states emerge in MnBi_{2}Te_{4}, including 3D antiferromagnetic topological insulator with the longsought 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 oddlayer 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 [35]. 
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