Bulletin of the American Physical Society
APS March Meeting 2020
Volume 65, Number 1
Monday–Friday, March 2–6, 2020; Denver, Colorado
Session X59: Weyl semimetal, theory IFocus
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Sponsoring Units: DMP Room: Mile High Ballroom 3C |
Friday, March 6, 2020 11:15AM - 11:27AM |
X59.00001: Novel Magnetic Weyl Semimetallic State in Layered Material: Fe2Sn Bishnu Karki, Bishnu Prasad Belbase, Madhav Prasad Ghimire Weyl Semimetals (WSMs) are the crystalline substances that are believed to be the sources for massless but charged particle called Weyl fermions. They are found to possess exotic properties such as high mobilities, magneto transport phenomenon, large magneto resistance and quantum hall phenomenon. These properties are expected to be highly applicable for high speed electronics and spintronics, quantum computing, hall effect devices and even in the catalysis. Here we focus on studying the electronic, magnetic and transport properties of hexagonal Fe2Sn. From our density functional theory calculations, the magnetic ground state is found to be ferromagnetic with a total magnetic moment of 4.56μB/formula unit. The electronic state shows metallic behavior with the band crossing close to the Fermi level. Fe2Sn is predicted to be magnetic WSM based on the identification of 16 Weyl points within an 0.01 eV around the Fermi level with chirality . |
Friday, March 6, 2020 11:27AM - 11:39AM |
X59.00002: Engineering Weyl phases and nonlinear Hall effects in Td-MoTe2 Sobhit Singh, Jinwoong Kim, Karin M Rabe, David Vanderbilt MoTe2 has recently attracted much attention due to the observation of pressure-induced superconductivity, exotic topological phase transitions, and nonlinear quantum effects. However, there is debate on the intriguing structural phase transitions among various observed phases of MoTe2, and their connection to the underlying topological electronic properties. In this work, by means of density-functional theory (DFT+U) calculations, we describe the structural phase transition between the polar Td and nonpolar 1T' phases of MoTe2 in reference to a hypothetical high-symmetry structure T0 that exhibits higher-order topological features. We obtain a total of 12 Weyl points, which can be created/annihilated, dynamically manipulated, and switched by tuning a zone-center polar phonon mode in the Td phase. We also report on the existence of a tunable nonlinear Hall effect in Td-MoTe2, and predict an emergent nonlinear surface current under the application of an external electric field. The potential technological applications of the tunable Weyl phase and nonlinear Hall effect will be discussed. |
Friday, March 6, 2020 11:39AM - 11:51AM |
X59.00003: Multiple types of van Hove singularities induced by band inversion Jiawei Ruan, Huaiqiang Wang, Steven Louie, Hai-Jun Zhang Saddle-point Van Hove singularities in the electron density of states can cause system instabilities that lead to exotic phases of matter. Here, we show that such singularities are very common near the Fermi level of topological semimetals and insulators. Using a k.p model analysis, we find that multiple types of saddle points with different divergence behaviors can be induced by band inversion. In particular, we uncover a new type of saddle-like dispersion (called “saddle line”) that significantly enhances the divergence in the density of states. We present examples of ab initio electron band structures of topological materials including monolayer 1T’-WS2, monolayer decorated stanene, and Dirac semimetal Na3Bi. Our results suggest a new universal property of topological materials that can lead to further opportunities. |
Friday, March 6, 2020 11:51AM - 12:03PM |
X59.00004: Cd3As2: Dirac semimetal to Weyl semimetal Santu Baidya, David Vanderbilt Dirac and Weyl semimetals have attracted much attention in the past few years. The Dirac compound Cd3As2 has seen renewed interest due to reports of proximity-induced surface superconductivity [1] and quantum Hall effect in thin films [2]. Using first-principles based theoretical methods, we show that by breaking time-reversal symmetry upon the application of an external Zeeman magnetic field, the four-fold degenerate Dirac node in Cd3As2 splits into four non-degenerate Weyl nodes, of which two are simple Weyl nodes of chirality ±1 and other have chiral charges of ±2. We calculate the evolution of the Fermi surfaces and their contributions to the anomalous Hall conductivity as a function of magnetic field and Fermi level position. Our work forms a basis for understanding how the resulting hole and electron pockets may play a role in generating complex states such as unconventional forms of superconductivity. |
Friday, March 6, 2020 12:03PM - 12:15PM |
X59.00005: Dirac and triple point semimetallic phases in Heusler alloys Chiranjit Mondal, Chanchal Barman, Biswarup Pathak, Aftab Alam We predict full Heusler compounds XInPd2 (X = Zr, Hf and Ti) to be potential candidates for type-II Dirac semimetals. The crystal symmetry of these compounds have appropriate chemical environment with a unique interplay of inversion, time reversal and mirror ymmetry. These symmetries help to give six pairs of type-II Dirac nodes on the C4 rotation axis, closely located at/near the Fermi level. Using first principle calculations, symmetry arguments and crystal field splitting analysis, we illustrate the occurrence of such Dirac nodes in these compounds. Bulk Fermi surfaces have been studied to understand the Lorentz symmetry breaking and Lifshitz transition (LT) of Fermi surfaces. Bulk nodes are projected on the (001) and (111) surfaces which form the surface Fermi arcs, that can further be detected by photo-emission and scanning tunneling spectroscopy. By analyzing the evolution of arcs with changing chemical potential, we prove the fragile nature and the absence of topological protection of the Dirac arcs. Our predicted compounds overcome the limitations of the previously reported PtTe2 class of compounds. Further we show breaking of inversion symmetry leads to realize triply degenerate nodal points in Heusler family. |
Friday, March 6, 2020 12:15PM - 12:27PM |
X59.00006: First-Principles Study of Robust Magnetic Weyl Nodal Loops in 5d Cubic Double Perovskites Young-Joon Song, Kwan-Woo Lee Topological phenomena have been extensively studied due to their interesting properties in the condensed matter physics. In particular, topological nodal line phases have special features of dispersionless surface states, so-called drumhead surface states. These flat states lead to a large surface density of states, implying instabilities toward surface magnetism or superconductivity. |
Friday, March 6, 2020 12:27PM - 12:39PM |
X59.00007: A Dirac semimetal with surface hourglass fermions: BaHgSn Tan Zhang, Zhijun Wang, Hongming Weng, Zhong Fang We proposed that BaHgSn is a Dirac semimetal which hosts hourglass-like surface states protected by glid symmetry. Compared with the topological crystalline insulators (TCI) KHgSb, BaHgSn has the similar crystal structure, but it has an additional band inversion induced by the stronger bonding and antibonding state splitting at Γ point, which leads to the bulk Dirac point on AΓ path. Due to this addtional band inversion, BaHgSn has the Mz mirror Chern number C±i=±3, while KHgSb has Z'2=(0;000) and C'±i=±2. BaHgSn has the same hourglass-like states as KHgSb along the $\tilde{X}\tilde{U}$ path on (010) surface, but it has bulk Dirac cone projection along $\tilde{Z}\tilde{\Gamma}$ path and zigzag surface states protected by C±i=±3 along $\tilde{\Gamma}\tilde{X}$ path. When 5% pression is applied along the y axis, the bulk Dirac point is gapped, and BaHgSn becomes a strong topological insulator (TI) with Z2=(1;000). The change of surface states is that bulk Dirac cone projection opens a gap and nontrival zigzag surface states protected by nontirvial Z2 appear along $\tilde{Z}\tilde{\Gamma}$ path. We also calculate the Wilson-loop spectral for (010) surface, the results are consistent with the surface states. |
Friday, March 6, 2020 12:39PM - 12:51PM |
X59.00008: Computational search for Dirac and Weyl nodes in f-electron antiperovskites Anna Pertsova, R.M. Geilhufe, Martin Bremholm, Alexander V Balatsky We present the results of a computational search for three-dimensional (3D) Dirac materials in the scarcely studied group of lanthanide antiperovskites [1]. There is no photoemission data available for these materials. We calculate electronic structures using density functional theory which accounts for spin-orbit coupling and strong correlations. We identify massive bulk Dirac states close to the Fermi level in A3BO, where A=Sm, Eu, Gd, Yb and B=Sn, Pb. The 3D Dirac nodes along the Γ-M direction originate from the band inversion of the cation and anion bands at the Γ point. This band inversion is also an indication of a topological crystalline insulator phase [2]. Most of the predicted materials display a finite magnetic moment due to unpaired electrons in the 4f shell. Specifically, in magnetic Eu3BO the degeneracy of the Dirac nodes is lifted, leading to appearance of Weyl nodes. This offers intriguing possibilities of combining magnetism and Dirac/Weyl physics in pristine materials without doping or proximity effects. |
Friday, March 6, 2020 12:51PM - 1:03PM |
X59.00009: Strain induced Kramers-Weyl Fermions Jinxin Hu, Yingming Xie, Chengping Zhang, Xi Dai, Kam Tuen Law Kramers-Weyl fermions are recently found to be widely supported in chiral crystals and exhibit many novel properties. However, in achiral crystals due to the presence of mirror or roto-inversion symmetries, Kramers Weyl fermions are often obscured to emerge due to the presence of nodal lines. In this work, we show Kramers-Weyl fermions are easily generated by removing the degeneracy of these nodal lines through strain effects. Remarkably, based on realistic DFT calculations, in the strained Rashba semiconductor BiTeI, we find a single Kramers-Weyl fermion near Fermi energy. Moreover, a strain induced quantized circular photogalvanic effect (CPGE) is further established. Our work paves a way to study the topology, optoelectronics by the tuning the chirality of crystals. |
Friday, March 6, 2020 1:03PM - 1:39PM |
X59.00010: Insights from magnetic Weyl semimetals: the Berry phase and beyond Invited Speaker: Binghai Yan It is known that Bloch electrons pick up an anomalous velocity because of the Berry curvature in the magnetic material. Recent discovery of magnetic Weyl semimetals (WSMs) provides a novel, ideal platform to examine the Berry curvature-induced transport phenomena, such as the anomalous Hall effect (AHE) and the thermal version of AHE. We focus on the Wideman-Franz law that governs the fundamental correlation between the charge and heat transport. We reveal a novel mechanism [1] to violate the Wideman-Franz law at the finite temperature by the Berry curvature distribution rather than the inelastic scattering effect, which is distinct from the ordinary (longitudinal) transport. Beyond the linear-response phenomena like AHE, WSMs can generate giant nonlinear optical response (such as the DC photocurrent), which is commonly attributed to the Berry curvature too. As a WSM turns magnetic, however, we find a new class of photocurrent [2,3] by re-examining the nonlinear response theory. It is contributed by the diabatic effect, instead of the Berry phase. Because this is a leading-order phenomena, the induced photocurrent is expected to be much larger compared to the non-magnetic case. |
Friday, March 6, 2020 1:39PM - 1:51PM |
X59.00011: Pseudo Dirac Nodal Sphere and its Topological Phase Transitions in a Semimetallic Carbon Network Shi-Zhang Chen, Yuan-Ping Chen, Wenhui Duan Because the global symmetry requirement is hard to achieved, topological nodal sphere is generally a pseudo phase, in which multiple nodal rings form the spherical backbone while the other points on the sphere are approximately degenerate. The physical understanding of nodal sphere, however, remains unclear. Here, we present a tight-binding (TB) model in a tetragonal crystal to simulate the evolution from a nodal sphere to a nodal surface. To provide clear picture, by performing first-principle calculations, a tetragonal carbon network, in which carbon nanotubes (CNTs) interconnected by graphene nanoribbons, is used to identify the pseudo nodal sphere phase. By applying strains, many topological phase transitions and novel semimetallic phases are achieved. The tensile strains lead to a transition from a nodal sphere to a nodal point, and eventually a trivial insulator; while a compressive strain leads to a transition to a nodal tube (uniaxial strains) or a nodal crossbar (biaxial strain), eventually to nodal surfaces, which is in good agreement with our TB modeling. Our results suggest that nodal sphere and nodal surface share the same origin, and their morphologies depend on the inter-CNTs hopping. |
Friday, March 6, 2020 1:51PM - 2:03PM |
X59.00012: Topological metals induced by Zeeman effect song sun, Zhida Song, Hongming Weng, Xi Dai In the present work, we propose a new way to classify centrosymmetric metals by studying the Zeeman effect caused by an external magnetic field described by the momentum dependent g-factor tensor on the Fermi surfaces. Nontrivial U(1) Berry's phase and curvature can be generated once the otherwise degenerate Fermi surfaces are splitted by the Zeeman effect, which will be determined by both the intrinsic band structure and the structure of g-factor tensor on the manifold of the Fermi surfaces. Such Zeeman effect generated Berry's phase and curvature can lead to three important experimental effects, modification of spin-zero effect, Zeeman effect induced Fermi surface Chern number and the in-plane anomalous Hall effect. By first principle calculations, we study all these effects on two typical material, ZrTe5 and TaAs2 and the results are in good agreement with the existing experiments. |
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