Bulletin of the American Physical Society
APS March Meeting 2019
Volume 64, Number 2
Monday–Friday, March 4–8, 2019; Boston, Massachusetts
Session B04: Dirac/Weyl Semimetals  Magnetic Topological SemimetalsFocus

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Sponsoring Units: DMP GMAG Chair: Zhiqiang Mao Room: BCEC 107C 
Monday, March 4, 2019 11:15AM  11:27AM 
B04.00001: Magnetic Weyl Semimetal in Quasi Twodimensional Half Metallic Co_{3}Sn_{2}S_{2} Qiunan Xu, Enke Liu, Wujun Shi, Lukas Muechler, Jacob Gayles, Claudia Felser, Yan Sun A Weyl semimetal can exist in a time reversal or inversion symmetry breaking system. Since the Berry curvature is odd under time reversal, the Berry curvature from Weyl points are expected to generate a large anomalous Hall effect in time reversal symmetry breaking Weyl semimetals. In this work, we find a Weyl semimetal phase in half metallic ferromagnet Co_{3}Sn_{2}S_{2} with Weyl points only 60 meV away from the Fermi level, which derive from nodal lines that are gapped by spinorbit coupling. Therefore, the Weylrelated physics should be easy to detected by both ARPES and bulk transport measurements. Due to the Berry curvature deriving from the gapped nodal lines and Weyl points, its anomalous Hall conductivity can reach up to 1200 S/cm. Substituting S by Se, Co_{3}Sn_{2}Se_{2} shows very similar property. Moreover, since Co_{3}Sn_{2}S_{2} is easily grown quasi twodimensional compound, it provides an ideal platform for the study of magnetic Weyl physics and its future application in topological material based spintronic devices. 
Monday, March 4, 2019 11:27AM  11:39AM 
B04.00002: Topological Weyl semimetals in Full Heusler Co_{2}MnX (X = Si, Ge, Sn) Abhishek Sharan, Felipe Crasto de Lima, Shoaib Khalid, Anderson Janotti Topological semimetals exhibit band crossing in the bulk and exotic surface states, called Fermi arcs, which are protected by topology of the associated wavefunctions. These states are characterized by their connectivity and dimensionality in the momentum space. Using firstprinciples calculations we show that ferromagnetic full Heusler compounds Co_{2}MnX (X = Si, Ge, Sn) belong to the category of topological semimetals called Weyl semimetals. Based on electronic structure calculations, we show that these materials show multiple nontrivial band crossings called Hopf links or entangled nodal lines and chainlike nodal lines, in absence of spinorbit coupling, which are protected by mirror symmetry of the lattice. On breaking the timereversal symmetry, we find that these band crossings are protected along multiple zerodimensional Weyl points with different chiralities. We further explore the possibility of identifying nontrivial Fermi arc surface states connecting projection of these Weyl points on surface of opposite chiralities. 
Monday, March 4, 2019 11:39AM  12:15PM 
B04.00003: Discovery of Topological Magnets via Spectroscopy: 2D and 3D Novel WeylDirac materials  Theory and Experiments Invited Speaker: Zahid Hasan In this talk I present our^{*} theoretical and experimental works on 2D and 3D topological magnets in novel Weyl and Dirac materials building up on earlier result but including recent results "A threedimensional magnetic topological phase" Ilya Belopolski et.al., arXiv:1712.09992 (2017); "Topological quantum properties of chiral crystals" Guoqing Chang et.al., Nature Materials (2018); "Topological Hopf and Chain Link Semimetal States and Their Application to Co2MnGa" Physical Review Letters 119, 156401 (2017); "Magnetic Weyl fermion semimetals in the RAlGe family of compounds" Physical Review B (2018) and Jiaxin Yin, Songtian Zhang et.al., "Giant and anisotropic manybody spin–orbit tunability in a strongly correlated kagome magnet" NATURE 562, 91–95 (2018). *Guoqing Chang, Bahadur Singh, SuYang Xu, Guang Bian, ShinMing Huang, ChuangHan Hsu, Ilya Belopolski, Nasser Alidoust, Daniel S Sanchez, Hao Zheng, Hong Lu, Xiao Zhang, Yi Bian, TayRong Chang, HorngTay Jeng, Arun Bansil, Han Hsu, Shuang Jia, Titus Neupert, Hsin Lin, JiaXin Yin, Songtian S. Zhang, Hang Li, Kun Jiang, Bingjing Zhang, Cheng Xiang, Hao Zheng, Tyler A. Cochran, Daniel Multer, Guang Bian, Kai Liu, ZhongYi Lu, Ziqiang Wang, Shuang Jia, Wenhong Wang, Biao Lian, Benjamin J. Wieder, Frank Schindler, Di Wu, Titus Neupert and TayRong Chang. 
Monday, March 4, 2019 12:15PM  12:27PM 
B04.00004: Inelastic Neutron Scattering in Semimetallic YbMnBi_{2} Timothy Reeder, Wesley T Fuhrman, Quinn Gibson, Jose A Rodriguez, Yiming Qiu, Matthew Brandon Stone, Alexander I Kolesnikov, Robert Cava, Collin Broholm Layered pcnitide AMnBi_{2} (A=Eu, Sr, Ca, Yb) compounds host quasi2D square antiferromagnetic layers with alternating Bi square nets responsible for relativistic Dirac bands. Novel transport and peculiar optical conductivity that could be brought on by the coupling of these Dirac bands to the squarelattice antiferromagnetic Mn has garnered further interest in these materials, with the hopes of achieving the magnetically ordered Weyl semimetallic phase. We show that the magnetic lattice in YbMnBi_{2}, fits to a frustated J_{1}J_{2} model with antiferromagnetic J_{1} and J_{2} interactions, and J_{2}/J_{1} less than the critical value, 1/2. While we rule out an ordered ferromagnetic coupling that would give rise to a Weyl Semimetal, we explore the relation between relativistic electronic band structure and anisotropic magnetic degrees of freedom. 
Monday, March 4, 2019 12:27PM  12:39PM 
B04.00005: Complicated magnetic phases interplayed with charge density waves in a new topological semimetal Shiming Lei, Jürgen Nuss, Viola Duppel, Bettina Lotsch, Leslie Schoop Topological semimetals have attracted a lot of attentions as they provide a new platform for studies of quantum matter as well as offer a new paradigm for nextgeneration electronics. Within these discovered topological materials, there is a class of materials that share the same structural motifs—sheets of squarearranged atoms, such as that in ZrSiS, CeSbTe and SrMnBi2. Within these materials, CeSbTe stands out as one unique system that offers the opportunities to study its tunable Weyl and Dirac states due to the existence of fieldinduced magnetic phase transitions. Here in this work, we present the synthesis and characterizations of a new series of squarenetbased magnetic topological semimetals. Based on a comprehensive structural characterization by powder xray diffraction, single crystal xray analysis, and transmission electron microscopy on this series of compounds, we observed a crystal symmetry evolution from an orthorhombic phase to a tetragonal one, in accompany with a tunable charge density wave (CDW) modulation. Besides the structural characterizations, we have also performed a comprehensive study on the magnetic properties. The impact of crystal symmetry evolution and CDW on the linear crossing band is also discussed. 
Monday, March 4, 2019 12:39PM  12:51PM 
B04.00006: Creating Weyl Nodes and Tuning Their Energy by Magnetization Rotation in a Metallic Ferromagnet Madhav Prasad Ghimire, Jorge Facio, JhihShih You, Linda Ye, Joseph Checkelsky, Shiang Fang, Efthimios Kaxiras, Manuel Richter, Jeroen Van den Brink Weyl nodes are robust topological features of the electronic structure that can occur at any momentum and energies. To observe the large anomalous effects Weyl nodes need to be close to or at the Fermilevel. However, most materials Weyl nodes are observed slightly away from the Fermilevel. 
Monday, March 4, 2019 12:51PM  1:03PM 
B04.00007: Hole Doping and Antiferromagnetic Correlations above the Néel Temperature of Topological Semimetal SrMnSb_{2} Yong Liu, Farhan Islam, Wei Tian, Thomas Heitmann, Benjamin Ueland, Andreas Kreyssig, Alan Ira Goldman, Robert J. McQueeney, David Vaknin Neutron diffraction and magnetic susceptibility studies of orthorhombic single crystal SrMnSb_{2} and hole doped Sr_{0.97}K_{0.03}MnSb_{2} confirm the three dimensional antiferromagnetic (AFM) ordering of the Mn^{2+} moments at T_{N }= 297(3) K for SrMnSb_{2 }and T_{N} = 305(3) K for the hole doped compound. Neutron scattering show strong quasi twodimensional AFM correlations that persist to almost twice T_{N. } In conjuction with susceptibility measurements, this observation seems to rule out a recently suggested ferromagnetic phase above T_{N} in SrMnSb_{2}. We also report detailed analysis of deHaas vanAlphen oscilations observed on the parent and doped compounds. 
Monday, March 4, 2019 1:03PM  1:15PM 
B04.00008: Impact of antiferromagnetic order on Landau level splitting of quasitwodimensional Dirac fermions in EuMnBi_{2} Hidetoshi Masuda, Hideaki Sakai, Masashi Tokunaga, Masayuki Ochi, Hidefumi Takahashi, Kazuto Akiba, Atsushi Miyake, Kazuhiko Kuroki, Yoshinori Tokura, Shintaro Ishiwata Dirac fermions in solids have been of current interest for their unique transport properties. The interplay of Dirac fermion with magnetism in magnetic Dirac materials is recently of particular interest. Among them, EuMnBi_{2} is a rare compound that exhibits quantum transport of Dirac fermions coupled with the fieldtunable magnetic order. However, it remains elusive how and to what extent the Diraclike band dispersion is affected. 
Monday, March 4, 2019 1:15PM  1:27PM 
B04.00009: Observation of Magnetic Bubble Domains in the Quasi2D Kagomé Ferromagnetic Weyl Semimetal Co_{3}Sn_{2}S_{2}^{1} Paul Sass, Weida Wu, Linda Ye, Joseph Checkelsky The Cobased shandite, Co_{3}Sn_{2}S_{2}, is a Weyl^{3} semimetal hosting an itinerant ferromagnetic quasi2D Kagomé lattice of Co ions. This system exhibits strong caxis anisotropy with a curie temperature ~ 174 K and a spontaneous moment of ~ 0.3 μ_{B}/Co.^{2} Lowfield magnetization and AC susceptibility measurements have revealed an anomalous magnetic phase above ~ 126 K and below ~ 40 mT, indicating a possible skyrmion phase. To this end, we carried out variabletemperature magnetic force microscopy studies on Co_{3}Sn_{2}S_{2} single crystals. MFM images under zerofield cooling revealed fingerprintlike magnetic domains, while lowfield cooling through T_{C} resulted in seemingly periodic magnetic bubble domains, which is consistent with the quasi2D nature of the magnetic ordering. Detailed temperature and cooling field dependence of the magnetic domain patterns will be presented. 
Monday, March 4, 2019 1:27PM  1:39PM 
B04.00010: Discovery of MagneticWeyl Semimetallic Phase in a Doped Topological Crystalline Insulator Anh Pham, Panchapakesan Ganesh Materials that can host a magneticWeyl semimetallic phase represent a novel platform for fundamental physics studies and can also be potentially applicable for quantum computing. The search for such magneticWeyl semimetals is ongoing given the recent discovery of Weyl semimetals which show absence of time reversal or inversion symmetry [1, 2]. In our study, we demonstrate a new pathway to engineer magnetic Weyl semimetals by doping topological crystalline insulators (TCI) such as SnTe with a magnetic transition metal element, such as Cr. The magnetic dopant breaks both time reversal and inversion symmetry while maintaining the band inversion between Snp and Tep orbitals, thus resulting in Weyl nodes in the bulk. These Weyl nodes were characterized through chirality calculation, large intrinsic anamolous Hall conductivity (AHC), and presence of Fermi arcs in the surface. Furthermore, the Weyl semimetallic phase is maintained even under dilute dopant concentrations, suggesting experimental feasibility of realizing this system. 
Monday, March 4, 2019 1:39PM  1:51PM 
B04.00011: gTensors and Band Structure Evolution of BiSb Alloys under Magnetic Field Cuneyt Sahin, Dung Vu, WENJUAN ZHANG, Nandini Trivedi, Joseph P C Heremans, Michael Flatté Bi and Sb alloys exhibit novel physical phenomena depending on the antimony concentration, such as semimetalsemiconductor transitions, giant spin Hall conductivities, and topologically protected phases including Weyl semimetal phases. Although the band structures of Bi and Sb are well studied in terms of k.p or tightbinding Hamiltonians, it is still a challenge to predict the behavior of the band edges as a function of the alloy concentration. Simple linear virtual crystal approximations lack the correct symmetries of the electron and hole pockets; on the other hand, alternative VCA approaches are insufficient to describe band crossings at different Sb concentrations. In this work, we introduce a new VCA parametrization describing the symmetries and crossing of the band edges using a 16 band tightbinding Hamiltonian. Then, from that Hamiltonian, we derive and calculate gtensors of the electrons and holes and show that the large spinorbit couplings of Bi and Sb result in giant effective gfactors whose axes of symmetry differ from the crystallographic axes. We also show that the band gap between symmetric and antisymmetric bands at the L point can be closed by a moderate magnetic field due to large gfactors. The closing of this gap produces a Weyl state. 
Monday, March 4, 2019 1:51PM  2:03PM 
B04.00012: THz spectroscopy of the Weyl semimetal candidate Co_{2}TiGe Rishi Bhandia, Bing Cheng, Tobias L BrownHeft, Sean Harrington, Shouvik Chatterjee, Chris Palmstrom, Peter Armitage Weyl semimetals are a class of topological materials that been the subject of intense research in the past few years. While there have been definitive examples of inversion symmetry breaking Weyl semimetals, good examples of timereversal symmetry breaking Weyl semimetals remain elusive. In addition to being of great interest to the spintronics community due to its halfmetallicity, the Heusler alloy Co_{2}TiGe has attracted interest recently due to theoretical predictions suggesting it hosts timereversal symmetry breaking Weyl semimetal states. Recently, MBEgrown thin films of Co_{2}TiGe have become available, allowing for timedomain THz spectroscopy (TDTS) measurements. We present results of the low frequency optical conductivity ofCo2TiGe and discuss our results in the context of prevailing theories. 
Monday, March 4, 2019 2:03PM  2:15PM 
B04.00013: Zeeman Splitting Induced Topological Nodal Structure in ZrTe_{5} Yichul Choi, John Villanova, Kyungwha Park When timereversal symmetry is broken by an external Zeeman field in a Dirac semimetal or smallgap insulator, rich nodal structures can be induced, including Weyl nodes or nodal lines. ZrTe_{5} is known to be located near the phase boundary between strong and weak topological insulator, and recent experiments showed intriguing magnetic transport properties including anomalous Hall effect and the chiral magnetic effect. This indicates that Weyl nodes are developed in ZrTe_{5} due to Zeeman splitting. However, a concrete theoretical study of the nodal structure of ZrTe_{5} under Zeeman splitting covering the full Brillouin zone is still missing. We construct a Wannierfunction based tightbinding model from first principles to investigate the effect of Zeeman splitting in ZrTe_{5} and to fully explore the nodal structure near the Fermi level. We calculate the topological character of induced band crossings and examine the importance of magnetic field direction effect on the nodal structure. 
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