Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session G43: Weyl Semimetals: Theory |
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Sponsoring Units: DCMP Chair: James Analytis, University of California, Berkeley Room: Mile High Ballroom 4B |
Tuesday, March 4, 2014 11:15AM - 11:27AM |
G43.00001: Coulomb scattering of Weyl fermions through a potential barrier Mahtab Khan, Michael Leuenberger We investigate the effects of the Coulomb interaction on the two-dimensional relativistic quantum-mechanical scattering of two Weyl fermions, injected on the opposite sides of a potential barrier. We consider the Coulomb interaction in the standard two-body problem and evaluate the corresponding scattering amplitude. We apply our formalism to describe the scattering of Weyl fermions in two-dimensional materials exhibiting Dirac cones, such as graphene and the surface of 3d topological insulators. We obtain a complex shape for the scattering amplitude due to the angle-dependent Klein tunneling through a potential barrier. We show that the Coulomb interaction leads to shifts and broadenings of the transmission peaks. . [Preview Abstract] |
Tuesday, March 4, 2014 11:27AM - 11:39AM |
G43.00002: Crystalline Topological Insulators and Semimetals with $C_{nv}$ Symmetry A. Alexandradinata, Chen Fang, Matthew J. Gilbert, B. Andrei Bernevig We explore a class of 3D materials with $C_{nv}$ symmetry. For $n =3,4$ and $6$, we find the first-known 3D topological insulators with robust surface modes, but $without$ spin-orbit coupling, and $not$ $needing$ time-reversal symmetry; the relevant symmetries are purely crystalline. To describe these $C_{nv}$ systems, we introduce the notion of a mirror chirality: an integer invariant which characterizes half-mirror-planes in the 3D Brillouin zone. In the evolution between two gapped phases with distinct mirror chiralities, we find that the intermediate gapless phase is a Weyl semimetal. Applications are discussed in the context of photonic crystals. [Preview Abstract] |
Tuesday, March 4, 2014 11:39AM - 11:51AM |
G43.00003: Quantum transport in Weyl semimetals Yuya Ominato, Mikito Koshino Quantum transport in 3D Weyl (massless Dirac) electron system with long-range Gaussian impurities is studied theoretically using a self-consistent Born approximation (SCBA). We find that the conductivity significantly changes the behavior at a certain scattering strength which separates the weak and strong disorder regimes. In the weak disorder regime, the SCBA conductivity mostly agrees with the Boltzmann conductivity, while the agreement fails near the Weyl point where the SCBA conductivity drops to zero linearly to the Fermi energy. In the strong disorder regime, the conductivity is smooth and finite near the Weyl point, and the minimum conductivity becomes larger in increasing the disorder potential, contrary to the usual metallic behavior. We also study the charged impurities, and argue the qualitative difference from the Gaussian case. The theory applies to three dimensional gapless band structures, including Weyl semimetals. [Preview Abstract] |
Tuesday, March 4, 2014 11:51AM - 12:03PM |
G43.00004: Local Electronic Structure around a Single Impurity in an Anderson Lattice Model for Topological Kondo Insulators Cheng-Ching Joseph Wang, Jean-Pierre Julien, Jian-Xin Zhu Shortly after the discovery of topological band insulators, the topological Kondo insulators (TKIs) have also been theoretically predicted. The latter has ignited renewed interest in the properties of Kondo insulators. By starting with a minimal-orbital Anderson lattice model, we explore the local electronic structure in a Kondo insulator. We show for the first time that the two strong topological regimes sandwiching the weak topological regime give rise to a dual location of Dirac cone on the surface of TKI. We further find that, when a single impurity is placed on the surface, low-energy resonance states are induced in the weak scattering limit for the strong TKI and the resonance level moves monotonically across the hybridization gap with the strength of impurity scattering potential; while low energy states can only be induced in the unitary scattering limit for the weak TKI, where the resonance level moves universally toward the center of the hybridization gap. These impurity induced low-energy quasiparticles will lead to characteristic signatures in scanning tunneling microscopy/spectroscopy, which has recently found success in probing exotic properties in heavy fermion systems. [Preview Abstract] |
Tuesday, March 4, 2014 12:03PM - 12:15PM |
G43.00005: Chiral magnetic effect of Weyl fermions and its applications to cubic noncentrosymmetric metals Sumanta Tewari, Pallab Goswami When the right and the left handed Weyl points are separated in energy, they give rise to a non-dissipative charge current along the direction of a uniform applied magnetic field, even in the absence of an external electric field. This effect is known as the chiral magnetic effect and is a hallmark of the underlying chiral anomaly of the Weyl fermions. According to the linearized continuum theory of Weyl fermions, the induced current is proportional to the magnetic field strength and the energy separation with a universal coefficient $e^2/h^2$. By considering a generic tight binding model for the cubic non-centrosymmetric metals, we show that such a system naturally supports a set of Weyl points, which are separated in energies. We also show the existence of the chiral magnetic effect for generic band parameters, and recover the universal result of the continuum Weyl fermions for a restricted parameter regime. Our work proves that the cubic non-centrosymmetric metals can serve as suitable platforms for realizing Weyl fermions and the exotic chiral elctrodynamic phenomena, which have promising technological applications. [Preview Abstract] |
Tuesday, March 4, 2014 12:15PM - 12:27PM |
G43.00006: Line node semimetals Vivek Aji, Michael Phillips Weyl semimetals are remarkable states of matter that are have chiral surface states, despite being gapless in the bulk. They are characterized by the touching of two non degenerate bands at an even number of points in the Brillouin zone. A variant of these is a semimetal with line nodes, rather than points, which is realized in a heterostructure made up of alternating layers of of topological and magnetic insulators. In this talk we explore the properties of this system such as the low energy density of states, conductivity and expected oscillatory signatures in magnetic fields. In particular we focus on the parametric dependence on magnetization which offers a knob to tune the properties of the system. [Preview Abstract] |
Tuesday, March 4, 2014 12:27PM - 12:39PM |
G43.00007: Exotic Chiral Anomaly of Double-Weyl Fermions Cheung Chan, Hong Yao Double-Weyl points in a 3D topological semimetal are protected by crystallographic point-group symmetries, as predicted in Hg$_{2}$Cr$_{2}$Se$_{4}$. The dispersion of fermions around double-Weyl points is highly anisotropic in momentum space, namely quadratic along $x$- and $y$-directions but linear in $z$-direction in the low energy effective Hamiltonian, which has intriguing physical consequences. For instances, we show that the double-Weyl fermions give rise to anisotropic chiral anomaly, qualitatively different from the usual chiral anomaly of (linear) Weyl fermions. We also discuss how the anisotropic dispersions of double-Weyl fermions affect their transport behaviors. [Preview Abstract] |
Tuesday, March 4, 2014 12:39PM - 12:51PM |
G43.00008: Collective modes in Weyl semimetals Dmytro Pesin, Ivan Panfilov, Anton Burkov Weyl semimetals are three-dimensional crystalline systems where pairs of bands touch at points in momentum space, termed Weyl nodes, that are characterized by a de?nite topological charge: the chirality. Consequently, they exhibit the Adler-Bell-Jackiw (chiral) anomaly. We consider new plasmon modes that appear in Weyl semimetals in a magnetic field due to the existence of the chiral anomaly in such systems. We show that due to the $\sqrt{B}$ dependence of their frequency on the magnetic field magnitude at low levels of doping, these modes couple effectively to the acoustic vibrations, leading to the existence of new hybrid plasmon-phonon modes. We discuss the implications of the existence of such modes for the sound absorption in Weyl semimetals. [Preview Abstract] |
Tuesday, March 4, 2014 12:51PM - 1:03PM |
G43.00009: Beyond the semi-Dirac, semi-Weyl dispersion: extending the tight binding model Yundi Quan, Warren Pickett Weyl semimetals with ``Dirac points'' have gained widespread notice due to the many unusual features they display, including topological characteristics. The discovery of the 2D semi-Dirac semimetal (more precisely, semi-Dirac, semi-Weyl) in thin VO$_2$ slabs with dispersion that is massless in one direction but massive perpendicular, provides an additional peculiarity introduces its own distinct behavior. We have generalized the two-band tight binding model for this system, obtaining new types of extreme bands and density of states. A specific feature is that the bottom of the upper band terminates at a contour (rather than at a point) such that electron doping leads to a pair of large, slightly separated Fermi lines and the DOS becomes 1D-like. Other unusual features will be described. [Preview Abstract] |
Tuesday, March 4, 2014 1:03PM - 1:15PM |
G43.00010: Magnetic Oscillations in Weyl Semimetals Phillip Ashby, Jules Carbotte Weyl semimetals are a three-dimensional phase containing band touchings at isolated points in the Brillouin zone. A Weyl semimetal can be thought of as a higher dimensional generalization of graphene. We study the thermodynamic and transport properties of a Weyl semimetal subject to an applied magnetic field. We examine the quantum oscillations in the magnetization to look for signatures that distinguish the Weyl semimetal from conventional phases of matter. We find distinctive sawtooth-like oscillations in the magnetization that reflect the relativistic nature of the bulk bands. The effect of impurities on these signatures will also be discussed. [Preview Abstract] |
Tuesday, March 4, 2014 1:15PM - 1:27PM |
G43.00011: Weyl semimetal phase in solid-solution zero-gap semiconductors Daichi Kurebayashi, Kentaro Nomura Weyl semimetals are recently found novel magnetic materials with the pseudo-relativistic linear dispersions. Near the band-touching points, the excitations are described by the Dirac-Weyl Hamiltonian. Quasiparticles, Weyl fermions, are assigned by a chirality, and the bulk gap opens only if two Weyl fermions with opposite chirality meet each other. This topological behavior originates in the nonzero Berry curvature enclosing a Weyl point. In TlBi(Se$_{1-x}$S$_x$)$_2$, it was recently found that the bulk gap closes as substituting sulfur by selenium. This vanishing of bulk gap is considered as the topological phase transition, and it is expected that the Weyl semimetal phase can be realized in this regime by breaking time-reversal symmetry. We determine the condition for the Weyl semimetal caused by a magnetic transition in zero-gap semiconductors doped with magnetic impurities. As a model, we use the Wilson Hamiltonian and the s-d exchange Hamiltonian within the mean-field approximation. We calculate the magnetization by solving the Hamiltonian self-consistently and obtain the topological phase diagram. Consequently, we find the Weyl semimetal phase with the finite anomalous Hall conductivity can be realized below the Curie temperature depending on the impurity concentration. [Preview Abstract] |
Tuesday, March 4, 2014 1:27PM - 1:39PM |
G43.00012: Electric field induced spin dynamics and polaritons in Weyl semimetal Jimmy Hutasoit, Chao-Xing Liu In Weyl semimetal, magnetization acts like a ``chiral magnetic'' field that, unlike the conventional magnetic field, couples differently to the left-handed and right-handed Weyl fermions. Integrating out the Weyl fermions, we find a non-local effective theory that describes the interaction between the magnetization and the electromagnetic field. In particular, we find that the system exhibits non-trivial spin dynamics controllable by the external electric field. Furthermore, the coupling between the magnetization and electromagnetic waves give rise to polaritons. [Preview Abstract] |
Tuesday, March 4, 2014 1:39PM - 1:51PM |
G43.00013: Topological States in Ferromagnetic CdO/EuO Quantum Well Haijun Zhang, Jing Wang, Gang Xu, Yong Xu, Shou-Cheng Zhang The quantum anomalous Hall (QAH) effect exhibits a quantized hall conductance without the external magnetic field and the associated landau levels. The topologically protected chiral edge states in a QAH state conducts electric current without dissipation and could be used for interconnects of semiconductor devices. In this talk, based on \textit{ab-initio} calculations, we demonstrate that the ferromagnetic CdO/EuO superlattice is a simple Weyl semimetal with two linear Weyl nodes in the Brillouin zone. The corresponding CdO/EuO quantum well realizes the stichometric quantum anomalous Hall (QAH) state without random magnetic doping, and its working temperature is expected to be close to bulk EuO's Curie temperature (around 70K). In addition, a simple effective model is presented to describe the basic mechanism of spin polarized band inversion in this system. [Preview Abstract] |
Tuesday, March 4, 2014 1:51PM - 2:03PM |
G43.00014: Quantum Oscillations from Fermi-Arcs in Weyl and Dirac Semimetals Andrew Potter, Itamar Kimchi, Ashvin Vishwanath Weyl semi-metals exhibit unusual surface-states whose Fermi-``surface" is not actually a surface, but rather consists of disjoint line segments. Such Fermi-arcs are a fingerprint of the topological aspects of the bulk band-structure. Magnetic field induced quantum oscillations of the density of states have traditionally enabled one to experimentally map out a material's Fermi-surface. On their own, the disjoint nature of surface Fermi-arcs does not permit closed semi-classical orbits in a magnetic field, naively rendering them inaccessible to quantum oscillatory probes. However, a slab of Weyl semi-metal has counter-propagating Fermi-arcs on both the top and bottom surfaces, which together could support closed orbits. Can such orbits which span a a non-local Fermi-surface give rise to quantum oscillations? If so, what happens to these oscillations as the top and bottom surfaces are increasingly isolated in progressively thicker slabs? This talk will address these questions, and apply the results to closely related and recently discovered 3D Dirac semi-metals. [Preview Abstract] |
Tuesday, March 4, 2014 2:03PM - 2:15PM |
G43.00015: Topological protection, disorder, and interactions: Life and death at the surface of a topological superconductor Matthew Foster, Hongyi Xie, Yang-Zhi Chou The key attribute of a 3D topological phase of matter is the prediction of robust, gapless surface states. These are said to be protected from the effects of disorder, in the sense that these states escape Anderson localization. Sufficiently weak interactions also have negligible effect, at least for surface states doped to the Dirac point. Here we consider the \emph{combined} effects of disorder and interactions on the surface states of 3D topological superconductors. Generalizing previous work [Foster and Yuzbashyan, PRL 109, 246801 (2012)], we study the enhancement of interactions due to disorder-mediated wavefunction multifractality, and the suppression of the Altshuler-Aronov correction to the surface quasiparticle spin conductance, due to the topology. We construct global surface state phase diagrams employing numerics, perturbative Finkel'stein non-linear sigma model calculations, and exact conformal field theory results. We establish the restrictive conditions under which surface states can be robust to both disorder and interactions. [Preview Abstract] |
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