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 Two-dimensional Half Metallic Co3Sn2S2 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 Co3Sn2S2 with Weyl points only 60 meV away from the Fermi level, which derive from nodal lines that are gapped by spin-orbit coupling. Therefore, the Weyl-related 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, Co3Sn2Se2 shows very similar property. Moreover, since Co3Sn2S2 is easily grown quasi two-dimensional 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 Co2MnX (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 first-principles calculations we show that ferromagnetic full Heusler compounds Co2MnX (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 chain-like nodal lines, in absence of spin-orbit coupling, which are protected by mirror symmetry of the lattice. On breaking the time-reversal symmetry, we find that these band crossings are protected along multiple zero-dimensional Weyl points with different chiralities. We further explore the possibility of identifying non-trivial 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 Weyl-Dirac 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 three-dimensional 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 R-AlGe family of compounds" Physical Review B (2018) and Jiaxin Yin, Songtian Zhang et.al., "Giant and anisotropic many-body spin–orbit tunability in a strongly correlated kagome magnet" NATURE 562, 91–95 (2018). *Guoqing Chang, Bahadur Singh, Su-Yang Xu, Guang Bian, Shin-Ming Huang, Chuang-Han Hsu, Ilya Belopolski, Nasser Alidoust, Daniel S Sanchez, Hao Zheng, Hong Lu, Xiao Zhang, Yi Bian, Tay-Rong Chang, Horng-Tay Jeng, Arun Bansil, Han Hsu, Shuang Jia, Titus Neupert, Hsin Lin, Jia-Xin Yin, Songtian S. Zhang, Hang Li, Kun Jiang, Bingjing Zhang, Cheng Xiang, Hao Zheng, Tyler A. Cochran, Daniel Multer, Guang Bian, Kai Liu, Zhong-Yi Lu, Ziqiang Wang, Shuang Jia, Wenhong Wang, Biao Lian, Benjamin J. Wieder, Frank Schindler, Di Wu, Titus Neupert and Tay-Rong Chang. |
Monday, March 4, 2019 12:15PM - 12:27PM |
B04.00004: Inelastic Neutron Scattering in Semimetallic YbMnBi2 Timothy Reeder, Wesley T Fuhrman, Quinn Gibson, Jose A Rodriguez, Yiming Qiu, Matthew Brandon Stone, Alexander I Kolesnikov, Robert Cava, Collin Broholm Layered pcnitide AMnBi2 (A=Eu, Sr, Ca, Yb) compounds host quasi-2D 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 square-lattice 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 YbMnBi2, fits to a frustated J1-J2 model with antiferromagnetic J1 and J2 interactions, and J2/J1 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 next-generation electronics. Within these discovered topological materials, there is a class of materials that share the same structural motifs—sheets of square-arranged 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 field-induced magnetic phase transitions. Here in this work, we present the synthesis and characterizations of a new series of square-net-based magnetic topological semimetals. Based on a comprehensive structural characterization by powder x-ray diffraction, single crystal x-ray 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, Jhih-Shih 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 Fermi-level. However, most materials Weyl nodes are observed slightly away from the Fermi-level. |
Monday, March 4, 2019 12:51PM - 1:03PM |
B04.00007: Hole Doping and Antiferromagnetic Correlations above the Néel Temperature of Topological Semimetal SrMnSb2 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 SrMnSb2 and hole doped Sr0.97K0.03MnSb2 confirm the three dimensional antiferromagnetic (AFM) ordering of the Mn2+ moments at TN = 297(3) K for SrMnSb2 and TN = 305(3) K for the hole doped compound. Neutron scattering show strong quasi two-dimensional AFM correlations that persist to almost twice TN. In conjuction with susceptibility measurements, this observation seems to rule out a recently suggested ferromagnetic phase above TN in SrMnSb2. We also report detailed analysis of de-Haas van-Alphen 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 quasi-two-dimensional Dirac fermions in EuMnBi2 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, EuMnBi2 is a rare compound that exhibits quantum transport of Dirac fermions coupled with the field-tunable magnetic order. However, it remains elusive how and to what extent the Dirac-like band dispersion is affected. |
Monday, March 4, 2019 1:15PM - 1:27PM |
B04.00009: Observation of Magnetic Bubble Domains in the Quasi-2D Kagomé Ferromagnetic Weyl Semimetal Co3Sn2S21 Paul Sass, Weida Wu, Linda Ye, Joseph Checkelsky The Co-based shandite, Co3Sn2S2, is a Weyl3 semi-metal hosting an itinerant ferromagnetic quasi-2D Kagomé lattice of Co ions. This system exhibits strong c-axis anisotropy with a curie temperature ~ 174 K and a spontaneous moment of ~ 0.3 μB/Co.2 Low-field 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 variable-temperature magnetic force microscopy studies on Co3Sn2S2 single crystals. MFM images under zero-field cooling revealed fingerprint-like magnetic domains, while low-field cooling through TC resulted in seemingly periodic magnetic bubble domains, which is consistent with the quasi-2D 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 Magnetic-Weyl Semimetallic Phase in a Doped Topological Crystalline Insulator Anh Pham, Panchapakesan Ganesh Materials that can host a magnetic-Weyl semimetallic phase represent a novel platform for fundamental physics studies and can also be potentially applicable for quantum computing. The search for such magnetic-Weyl 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 Sn-p and Te-p 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: g-Tensors 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 semimetal-semiconductor 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 tight-binding 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 tight-binding Hamiltonian. Then, from that Hamiltonian, we derive and calculate g-tensors of the electrons and holes and show that the large spin-orbit couplings of Bi and Sb result in giant effective g-factors 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 g-factors. 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 Co2TiGe Rishi Bhandia, Bing Cheng, Tobias L Brown-Heft, 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 time-reversal symmetry breaking Weyl semimetals remain elusive. In addition to being of great interest to the spintronics community due to its half-metallicity, the Heusler alloy Co2TiGe has attracted interest recently due to theoretical predictions suggesting it hosts time-reversal symmetry breaking Weyl semimetal states. Recently, MBE-grown thin films of Co2TiGe have become available, allowing for time-domain 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 ZrTe5 Yichul Choi, John Villanova, Kyungwha Park When time-reversal symmetry is broken by an external Zeeman field in a Dirac semimetal or small-gap insulator, rich nodal structures can be induced, including Weyl nodes or nodal lines. ZrTe5 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 ZrTe5 due to Zeeman splitting. However, a concrete theoretical study of the nodal structure of ZrTe5 under Zeeman splitting covering the full Brillouin zone is still missing. We construct a Wannier-function based tight-binding model from first principles to investigate the effect of Zeeman splitting in ZrTe5 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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