Bulletin of the American Physical Society
APS March Meeting 2021
Volume 66, Number 1
Monday–Friday, March 15–19, 2021; Virtual; Time Zone: Central Daylight Time, USA
Session P51: Dirac and Weyl Semimetals: Materials and ModelingMaterials Predictions and Transport TheoryFocus Live

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Sponsoring Units: DMP DCMP Chair: Chaoxing Liu, Pennsylvania State University 
Wednesday, March 17, 2021 3:00PM  3:36PM Live 
P51.00001: New insights into topological semimetals Invited Speaker: Jennifer Cano The field of topological semimetals continues to reveal new insights. I will discuss recent developments starting with the classification of nodal fermions in both magnetic and nonmagnetic space groups. I will then introduce higher order Fermi arcs as a bulkedge correspondence for Dirac fermions, and a refinement of the symmetry indicators that predict these hinge states. Finally, I will discuss some material predictions. 
Wednesday, March 17, 2021 3:36PM  3:48PM Live 
P51.00002: Topological properties of buckled 2D hoyneycomb groupV elements Santosh Radha, Walter R L Lambrecht The honeycomb form of 2D group Velements (Sb,As) was recently shown to undergo distinct topological phase transitions as function of buckling.[1] Starting out as a nodal line semimetal in the flat form, Dirac points emerge when slight buckling is introduced, which annihilate in pairs upon further buckling. Here we show that the buckled lowest energy form at the end of this series of topological transitions is a weak topological crystalline insulator (TCI) in the obstructed atomic limit (OAL).[2] Relations to the kagome, Kekul\'e and SuSchriefferHeeger systems are pointed out. Further group theoretical analysis, shows that this system is a (d2) higher order topological insulator. The resulting corner states are shown to be robust as long as the disorder remains restricted to the bulk rather than the edges. The edge states and corner states at zero energy can be gapped by breaking the inversion symmetry, which we show to lead to the possibility of a topological quantized conductance metal insulator transition by application of an electrical field perpendicular to the layers. Finally, we note that the formation of an OAL is a universal property of the annihilation of Dirac fermions of opposite winding. 
Wednesday, March 17, 2021 3:48PM  4:00PM Live 
P51.00003: Ferromagnetic Weyl Fermions in TwoDimensional Layered Electride Gd_{2}C Shuyuan Liu, Chongze Wang, JunHyung Cho Recently, twodimensional layered electrides have emerged as a new class of materials which possess anionic electrons in the interstitial spaces between cationic layers. Here, based on firstprinciples calculations, we discover a timereversalsymmetrybreaking Weyl semimetal phase in a unique twodimensional layered ferromagnetic (FM) electride Gd_{2}C. It is revealed that the crystal field mixes the interstitial electron states and Gd5d orbitals near the Fermi energy to form band inversions. Meanwhile, the FM order induces two spinful Weyl nodal lines (WNLs), which are converted into multiple pairs of Weyl nodes through spinorbit coupling. Further, we not only identify Fermiarc surface states connecting the Weyl nodes but also predict a large intrinsic anomalous Hall conductivity due to the Berry curvature produced by the gapped WNLs. Our findings demonstrate the existence of Weyl fermions in the roomtemperature FM electride Gd_{2}C, therefore offering a new platform to investigate the intriguing interplay between electride materials and magnetic Weyl physics. 
Wednesday, March 17, 2021 4:00PM  4:12PM Live 
P51.00004: Effect of disorder on the transverse magnetoresistance of Weyl semimetals Yaroslav Rodionov, Kliment I Kugel, Boris A Aronzon, Franco Nori We study the effect of random potential created by different types of impurities on the transverse magnetoresistance of Weyl semimetals. It is shown that the magnetic field and temperature dependence of magnetoresistance is strongly affected by the type of impurity potential. Two limiting cases are analyzed in detail: (i) the ultraquantum limit, when the applied magnetic field is so high that only the zeroth and first Landau levels contribute to the magnetotransport, and (ii) the semiclassical situation, for which a large number of Landau levels comes into play. A formal diagrammatic approach allowed us to obtain expressions for the components of the electrical conductivity tensor in both limits. In contrast to the oversimplified case of the δcorrelated disorder, the longrange impurity potential (including that of Coulomb impurities) introduces an additional length scale, which changes the geometry and physics of the problem. It is shown that the magnetoresistance can deviate from the linear behavior as a function of magnetic field for a certain class of impurity potentials. 
Wednesday, March 17, 2021 4:12PM  4:24PM Live 
P51.00005: Nonreciprocal Thermal Radiation from Magnetic Weyl Semimetals Bo Zhao, Cheng Guo, Christina Garcia, Prineha Narang, Shanhui Fan Objects around us constantly emit and absorb thermal radiation. For reciprocal systems, the emissivity and absorptivity are restricted to be equal by Kirchhoff's law of thermal radiation. This restriction limits the control of thermal radiation and contributes to an intrinsic loss mechanism in photonic energy harvesting systems. Existing approaches to violate Kirchhoff's law typically utilize conventional magnetooptical effects in the presence of an external magnetic field. However, these approaches require either a strong magnetic field (~3T) [1] or narrowband resonances under a moderate magnetic field (~0.3T) [2], because the nonreciprocity in conventional magnetooptical effects is usually weak in the thermal wavelength range. Here we show that the axion electrodynamics in magnetic Weyl semimetals can be used to construct strongly nonreciprocal thermal emitters [3]. Such a thermal emitter can near completely violate Kirchhoff's law over broad angular and frequency ranges without requiring any external magnetic field. 
Wednesday, March 17, 2021 4:24PM  5:00PM On Demand 
P51.00006: The nature of the Zeeman effect and fieldinduced topological semimetals Invited Speaker: Xi Dai In the present talk, I will propose a new way to classify centrosymmetric metals by studying the 
Wednesday, March 17, 2021 5:00PM  5:12PM Live 
P51.00007: FirstPrinciples Design of HalideReduced Electrides with Magnetism and Topological Phases Tonghua Yu, Motoaki Hirayama, Jose A. Flores Livas, Takuya Nomoto, Ryotaro Arita As a novel class of crystals where the excess electrons residing within the cationic framework serve as anions, electrides manifest intriguing magnetic features and topological nature [1], which has aroused great interests among the community of materials science. Here, we propose a design scheme of seeking potential electrides derived from conventional materials. Starting from rareearth element based halides, we exclude the halogen and perform the global structure optimization, so as to obtain excess electrons confined inside interstitial cavities. Spin polarized interstitial states are realized by chemical substitution with magnetic lanthanides. In addition, band topology is particularly explored for the predicted electrides. In this presentation, we primarily report two families of newly designed electrides, A_{2}C_{2} and A_{2}Ge_{ }(A= Y, Gd, etc.), both of which turn out to be topological nodal line semimetals in the absence of spinorbit coupling. Our work opens a new avenue to predict electrides, and reveals the close relationship between electrides, magnetism and topological phases. 
Wednesday, March 17, 2021 5:12PM  5:24PM Live 
P51.00008: NdAlSi, a type II magnetic Weyl semimetal Santu Baidya, David Vanderbilt, Jonathan Gaudet, HungYu Yang, Fazel Tafti, Collin Leslie Broholm The combination of typeII Weyl semimetal and the coexistence of broken timereversal symmetry and inversion symmetry is highly unusual. RAlX materials (R = rare earth, X = Si, Ge) have recently attracted much attention as rare examples of such materials. We focus on NdAlSi due to its interesting magnetic and electronic properties. It shows multiple magnetic phase transitions mediated by RKKYtype interactions with ferromagnetism at 7.2 K and ferrimagnetism (upupdown Nd spin ordering) at 3.3 K. Using firstprinciples densityfunctional theory, we find 20 pairs of Weyl nodes in the paramagnetic phase, 28 pairs of Weyl nodes in the ferromagnetic phase, and 26 pairs of Weyl nodes in the antiferromagnetic phase. We also focus on the Fermi surfaces associated with the Weyl nodes hosting a nesting vector consistent with the experimentally observed magnetic ordering at q ~ [2/3,2/3,0]. Our work exposing the connections between the Weyl node structure and the magnetic ordering should be applicable to this class of materials more broadly, stimulating further interest in their remarkable properties. 
Wednesday, March 17, 2021 5:24PM  5:36PM Live 
P51.00009: Twofold degenerate quadruple Weyl nodes in chiral cubic crystals Tiantian Zhang, Ryo Takahashi, Chen Fang, Shuichi Murakami Unlike conventional Weyl nodes, unconventional ones carry a quantized monopole charge C>1, and their existence needs the protection of crystalline symmetries in addition to translation symmetry. There have been many studies on unconventional Weyl nodes, yet we have so far missed one, which is the twofold Weyl node with C=4^{[1]}. In this talk, I will first explain why researchers missed the twofold degenerate quadruple Weyl node (TQW) by introducing the research history of unconventional Weyl nodes. Then, I will explain how to obtain the TQW in crystals. Afterward, a series of LaIrSitype materials that have the TQW in both their spinless electronic band structure and the phonon spectra will be discussed. In the electronic band structure, the TQW will evolve into a fourfold quadruple Weyl node and change both the chirality and the monopole charge after considering spinorbit coupling, which is uncommon in the known Weyl semimetals. At last, I will discuss the relationship between the winding number and pseudospin and offer an intuitive way to understand the complex pseudospin texture of the TQW. If there is enough time, I also would like to make a brief introduction to experimentally detecting the TQW in BaPtGe^{[2]}. 
Wednesday, March 17, 2021 5:36PM  5:48PM Live 
P51.00010: Highthroughput screening for Weyl Semimetals with S4 Symmetry Jiacheng Gao, Yuting Qian, SiMin Nie, Zhijun Wang, Hongming Weng, Zhong Fang Based on irreducible representations (or symmetry eigenvalues) and compatibility relations, a material can be predicted to be a topological/trivial insulator [satisfying compatibility relations] or a topological semimetal [violating compatibility relations]. However, Weyl semimetals usually go beyond this symmetrybased strategy. In other words, Weyl nodes could emerge in a material, no matter if its occupied bands satisfy compatibility relations, or if the symmetry indicators are zero. In this work, we propose a new topological invariant χ for the systems with S4 symmetry [i.e., the improper rotation S4 (≡ IC4z) is a proper fourfold rotation (C4z) followed by inversion (I)], which can be used to diagnose the Weyl semimetal phase. Moreover, χ can be easily computed through the onedimensional Wilsonloop technique. By applying this method to the highthroughput screening in firstprinciples calculations, we predict a lot of Weylsemimetals in both nonmagnetic and magnetic compounds. Various interesting properties (e.g. magnetic frustration effects, superconductivity and spinglass order, etc.) are found in predicted Weyl semimetals, which provide realistic platforms for future experimental study of the interplay between Weyl fermions and other exotic states. 
Wednesday, March 17, 2021 5:48PM  6:00PM On Demand 
P51.00011: Magnetoresistance from Guiding Center Drift of TwoDimensional Electrons in a Smooth Disorder Potential Calvin Pozderac, Brian Skinner Linear magnetoresistance has been observed in a range of lowdensity, twodimensional electron systems. For threedimensional systems, linear magnetoresistance can be explained in terms of semiclassical drift of electrons in a smooth disorder potential. But as of yet there is no analogous theory for twodimensional electron systems. Here we address this issue by studying the magnetoresistance of 2D electrons in a smooth random potential, as created, for example, by charged impurities in the substrate. In the presence of a sufficiently large magnetic field, electron trajectories experience a drift of their guiding centers along equipotential contours, in addition to the rapid cyclotron motion. Scattering from impurities or phonons allows electrons to hop from one equipotential contour to another, and at large enough magnetic field this process dominates their diffusion. We study the resulting electron diffusion constant, which determines the electrical resistance. Using scaling arguments and numerical simulations, we find regimes in which the magnetoresistance scales as B^{0}, B^{10/13}, and B^{10/7}. 
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