Bulletin of the American Physical Society
APS March Meeting 2019
Volume 64, Number 2
Monday–Friday, March 4–8, 2019; Boston, Massachusetts
Session L04: Dirac/Weyl Semimetals -- Materials Prediction IIFocus
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Sponsoring Units: DMP Chair: Dmytro Pesin, University of Virginia Room: BCEC 107C |
Wednesday, March 6, 2019 11:15AM - 11:27AM |
L04.00001: Composite Dirac Semimetal Ziming Zhu, Zhi-Ming Yu, Weikang Wu, Wei Zhang, Fan Zhang, Shengyuan Yang In this work, we investigate the possibility to construct a new topological state, which may be regarded as a combination of a WTI and a Dirac semimetal, hence it may be termed as a composite Dirac semimetal (CDSM). We start with an effective model, which can be derived from a tight-binding model defined on a stacked honeycomb lattice. By analyzing the possible band ordering at the high symmetry points on the rotation axis, we show that a CDSM state can be realized, for which one pair of low-energy bands cross at the Fermi level to form two symmetry-protected Dirac points, whereas another pair of bands have inverted band ordering along the high symmetry path . The hallmark of this state is that on the side surfaces, a pair of Fermi arcs connecting the projected Dirac points coexist with a pair of helical Fermi loops traversing the surface Brillouin zone (BZ). Without breaking any symmetry, the CDSM may undergo a topological phase transition to an insulating state via a band inversion scenario, accompanied by two pairs of helical surface Fermi loops. Finally, by using first-principles calculations, we show that the discussed physics can be realized in a realistic material system. |
Wednesday, March 6, 2019 11:27AM - 11:39AM |
L04.00002: First-Principles Prediction of New Magnetic Weyl Semimetals:
Cobalt-Based Shandites System Yuma Nakamura, Wei Luo, Jinseon Park, Mina Yoon The experimental confirmation of Co3Sn2S2 as the first magnetic Weyl semimetal (MWSM) paved the way to explore new magnetic Weyl semimetals. Cobalt-based shandite consists of T3M2X2 (T = Co; M = Ge, Sn, Pb; X = S, Se) in a rhombohedral arrangement with rotational symmetry, inversion symmetry, and a kagome layer formed by Co. Using a first-principles approach coupled with a global structure search algorithm, we explored energetically stable configurations of Co-based shandites and their alloys (T3M1M2X2, where M1/M2 = Ge, Sn, and Pb). We characterized their electronic properties by establishing a tight-binding Hamiltonian with parameters extracted from maximally localized Wannier functions,and identified new MWSMs. The Weyl points of new MWSMs are formed by p-orbital of elements M located between the kagome layer and d-orbitals of Co on the kagome layer, where their location and energy level changes as a function of M. A wider class of new MWSMs that can be synthesized experimentally is proposed. |
Wednesday, March 6, 2019 11:39AM - 11:51AM |
L04.00003: Pairing and charge order in a strongly-interacting Weyl system: Numerically exact results from auxiliary-field quantum Monte Carlo Peter Rosenberg, Niraj Aryal, Efstratios Manousakis The recent discovery of Weyl semimetals has generated intense interest in their properties. These materials have been predicted to exhibit a number of fascinating behaviors, including topological superconductivity. Here we study a model Weyl system subject to strong interactions using the numerically exact auxiliary-field quantum Monte Carlo technique. We focus on charge and pairing properties, in order to explore the interplay of Weyl physics, charge order, and superconductivity. These high-accuracy results will complement ongoing experimental efforts in Weyl systems, and help guide the search for exotic pairing phenomena in these systems, which can be realized in both real materials and ultra-cold atoms. |
Wednesday, March 6, 2019 11:51AM - 12:03PM |
L04.00004: Mixed axial-torsional anomaly in Weyl semimetals Yago Ferreiros, Yaron Kedem, Emil J Bergholtz, Jens Bardarson We show that Weyl semimetals exhibit a mixed axial-torsional anomaly in the presence of axial torsion, a concept exclusive of these materials with no known natural fundamental interpretation in terms of the geometry of spacetime. This anomaly implies a nonconservation of the axial current---the difference in current of left- and right-handed chiral fermions---when the torsion of the spacetime in which the Weyl fermions move couples with opposite sign to different chiralities. The anomaly is activated by driving transverse sound waves through a Weyl semimetal with a spatially varying tilted dispersion, which can be engineered by applying strain. This leads to sizable alternating current in presence of a magnetic field that provides a clear-cut experimental signature of our predictions. |
Wednesday, March 6, 2019 12:03PM - 12:15PM |
L04.00005: Evolution of the surface states of the Luttinger semimetal under compressive strain and broken inversion symmetry Ewelina Hankiewicz, Julian-Benedikt Mayer, Maxim Kharitonov Luttinger semimetal, the quadratic-node semimetal for $j=3/2$ electrons under full cubic symmetry, is the parent highest-symmetry minimal model for a variety of topological and/or strongly correlated materials, such as HgTe, $\alpha$-Sn, and iridate compounds. Recently, Luttinger semimetal has been demonstrated [1] to exhibit surface states of topological origin that can be attributed to approximate chiral symmetry. In the present work, we theoretically study the effect of the symmetry-lowering perturbations on these surface states within an analytical model. Under compressive strain lowering rotational symmetry, Luttinger semimetal becomes a Dirac semimetal with a pair of double-degenerate linear nodes. Breaking further inversion symmetry, the system turns into a Weyl semimetal, with each Dirac node split into four Weyl nodes. We analyze the corresponding evolution of the surface states,and its relation to the surface states of the full Kane model. |
Wednesday, March 6, 2019 12:15PM - 12:51PM |
L04.00006: The effect of interaction and gauge field in topological semimetals Invited Speaker: Chaoxing Liu Topological semimetals possess topologically protected nodal points or lines with exotic physical properties. Here I focus on the effects of interaction and gauge field on topological Weyl or Dirac semimetals. I will first show that in a nanowire made of Dirac semimetals, interaction enables the emergence of boundary Majorana zero modes that are protected by rotation symmetry under magnetic fields [1]. Therefore, the Dirac semimetal nanowire provides an ideal platform for the realization of 1D interacting topological phase with Majorana physics, which does not require superconductivity and is thus number-conserving. Next I will describe the influence of generalized gauge fields in Weyl semimetals [2,3]. Due to the chirality of Weyl nodes, it was realized that more general form of gauge fields beyond conventional electromagnetic fields can be induced by magnetic fluctuation or strain in Weyl semimetals. As a consequence of gauge fields, zeroth Landau levels can be induced and only propagate in one direction (chiral). The presence of chiral zeroth Landau levels is the origin of chiral anomaly and allows for a robust bulk transport. Such generalization of gauge fields is recently demonstrated in an inhomogeneous Weyl metamaterial experimentally [3]. |
Wednesday, March 6, 2019 12:51PM - 1:03PM |
L04.00007: Level statistics of disordered Weyl semimetalsν Xiang Rong Wang, Chen Wang, Peng Yan
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Wednesday, March 6, 2019 1:03PM - 1:15PM |
L04.00008: Emergent Weyl Nodes and Monopole Charge Density Wave State Yi Li, Eric Bobrow, Canon Sun We study a new class of topological charge density wave states exhibiting monopole harmonic symmetries. When electron and hole Fermi surfaces carry different Chern numbers, the particle-hole pairing exhibits a non-trivial Berry phase inherited from band structure topology independent of concrete density-wave ordering mechanism. The associated density-wave gap functions become nodal, and the net nodal vorticity is determined by the monopole charge of the pairing Berry phase. The gap function nodes become zero-energy Weyl nodes of the bulk spectra of quasi-particle excitations. These states can occur in doped Weyl semimetals with nested electron and hole Fermi surfaces enclosing Weyl nodes of the same chirality in the weak coupling regime. Possible signatures of monopole harmonic charge density waves are proposed for scattering experiments. |
Wednesday, March 6, 2019 1:15PM - 1:27PM |
L04.00009: Non-Abelian Statistics in Momentum Space QuanSheng Wu, Alexey A Soluyanov, Tomas Bzdusek We develop a general theory describing the stability and conversions of a wide range of band-structure nodes. The technique readily applies to a plethora of nodes existing in semimetals and superconductor, while it also reveals various previously unknown nodal features. Especially, we find that band-structure nodes in PT-symmetric systems with negligible spin-orbit coupling carry a non-Abelian charge, thus suggesting non-trivial braiding rules in momentum space. The non-Abelian charge also poses constrains on admissible nodal-line composition in 3D systems, and on the topological transitions between various such compositions. We emphasize that the non-Abelian property arises without the need of electron interactions. |
Wednesday, March 6, 2019 1:27PM - 1:39PM |
L04.00010: Classification of composite Weyl fermions Daniel Gosálbez Martínez, Oleg Yazyev Weyl nodes with chiral charge ±1 are classified as types I and type II according to the tilting of the conical band dispersion at the band degeneracy. Their Fermi surface, described by a quadratic form, is different for these two types. We extend this classification to the case of composite Weyl nodes with chiral charge larger than one. When the C4 and C6 rotation symmetries forbid the linear band dispersion on the plane perpendicular to the symmetry axis, new terms with quadratic and cubic momentum dependence must be included. Consequently, the Fermi surface produced by these band degeneracies are described by a 4 or 6 order algebraic surfaces. In this more complex situation, instead of classifying Fermi surfaces, we study numerically the possible Lifshitz transitions of the Fermi surface produced by a composite Weyl node as the chemical potential is varied. We use this methodology to study quadratic Weyl nodes generated by the C4 rotation symmetry. We find that such band degeneracies present four different types of morphologies, two analogous to the type-I and type-II case of the conical Weyl nodes, and two new distinct morphologies within the type-II class. We illustrate the existence of these new types of band degeneracies in real materials such as bcc iron. |
Wednesday, March 6, 2019 1:39PM - 1:51PM |
L04.00011: Computational search for magnetic Weyl semimetals in rare-earth compounds Lin-Lin Wang, Na Hyun Jo, Yun Wu, Brinda Kuthanazhi, Adam Kaminski, Paul Canfield Weyl fermions are of interests for both basic science and future technology. Weyl points in electronic band structure can be generated by breaking inversion or/and time-reversal symmetry to realize linear crossings having monopoles of Berry curvature. Here we have used high throughput band structure calculations based on density functional theory to search for magnetic Weyl semimetals in rare-earth compounds. The existence of Weyl points is identified by calculating Berry curvature. We find that the magnetic Weyl points depend on the specific magnetic ordering structure. |
Wednesday, March 6, 2019 1:51PM - 2:03PM |
L04.00012: Optical Properties of Weyl Semimetals from First Principles Christina Garcia, Jennifer Coulter, Prineha Narang Recent experiments have shown interesting optical behavior in Weyl semimetals, a topologically nontrivial class of materials. Weyl semimetals (WSMs) are characterized by linearly dispersive band touchings in the bulk states (in three dimensions) where electrons can be treated as massless Weyl fermions. This talk will focus on our evaluation of optical properties, hot carrier generation and dynamics in different Weyl semimetals using first principles calculations incorporating electron-phonon interactions. In particular, we explore TaAs, in which record-breaking second-harmonic generation and bulk photovoltaic effects have been recently reported. We will build upon our ab initio framework to include nonlinear optical properties for direct comparison with these exciting observations. |
Wednesday, March 6, 2019 2:03PM - 2:15PM |
L04.00013: Three-dimensional chiral spin liquids in the Kitaev model on a hypernonagon lattice Petr Mishchenko, Yasuyuki Kato, Kevin O'Brien, Troels Bojesen, Tim Eschmann, Maria Hermanns, Simon Trebst, Yukitoshi Motome Chiral spin liquids (CSLs) are exotic quantum states without magnetic order but with broken time-reversal symmetry. Recently, the ground-state and finite-temperature (T) behaviors of CSLs have been studied for the Kitaev models [1] on two and three-dimensional (3D) lattice geometries [2-4]. In this talk, we will present a comprehensive study of the ground state and thermodynamic properties of the Kitaev model on the 3D hypernonagon lattice, which has previsously been explored in the limit of anisotropic interactions [4]. Our numerical simulations employ quantum Monte Carlo technique, which in combination with the Chebyshev polynomial expansion [5], allows us to study systems of up to 2600 spins. We find evidence for a first-order phase transition in the isotropic interactions case similar to the anitostopic limits, but into a different type of CSL ground state. We also present the ground-state phase diagram obtained by variational calculations. |
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