Bulletin of the American Physical Society
APS March Meeting 2016
Volume 61, Number 2
Monday–Friday, March 14–18, 2016; Baltimore, Maryland
Session F26: Weyl Semimetal  Theory 
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Sponsoring Units: DCMP DMP Chair: Catalin Martin, Ramapo College of New Jersey Room: 325 
Tuesday, March 15, 2016 11:15AM  11:27AM 
F26.00001: \textbf{Raman spectroscopy of exfoliated fewlayered ntype Bi}$_{\mathrm{\mathbf{2}}}$\textbf{Te}$_{\mathrm{\mathbf{3}}}$ Fengjiao Liu, Mehmet Karakaya, Pooja Puneet, Ramakrishna Podila, Sriparna Bhattacharya, Apparao M. Rao A novel chemicalexfoliation sparkplasmasintering (CESPS) process was applied to enhance the thermoelectric figure of merit and compatibility factor of fewlayered ntype Bi$_{\mathrm{2}}$Te$_{\mathrm{3}}$. New vibrational modes were observed in the microRaman spectra of the fewlayered Bi$_{\mathrm{2}}$Te$_{\mathrm{3}}$ samples, which are absent in the bulk. Here we focus on the emergence of the new intermediate and highfrequency Raman modes and their dependence on the layer thickness. A detailed Raman study probing the origin of these exfoliation induced defect modes will be presented. [Preview Abstract] 
(Author Not Attending)

F26.00002: Disorder and metalinsulator transitions in Weyl semimetals Hua Jiang, ChuiZhen Chen, Juntao Song, Qingfeng Sun, Ziqiang Wang, X. C. Xie The Weyl semimetal (WSM) is a newly proposed quantum state of matter. It has Weyl nodes in bulk excitations and Fermi arcs surface states. We study the effects of disorder and localization in WSMs and find three novel phase transitions.(I) Two Weyl nodes near the Brillouin zone boundary can be annihilated pairwise by disorder scattering, resulting in the opening of a topologically nontrivial gap and a transition from a WSM to a threedimensional (3D) quantum anomalous Hall state. (II) When the two Weyl nodes are well separated in momentum space, the emergent bulk extended states can give rise to a direct transition from a WSM to a 3D diffusive anomalous Hall metal. (III) Two Weyl nodes can emerge near the zone center when an insulating gap closes with increasing disorder, enabling a direct transition from a normal band insulator to a WSM. We determine the phase diagram by numerically computing the localization length and the Hall conductivity, and propose that the novel phase transitions can be realized on a photonic lattice. [Preview Abstract] 
Tuesday, March 15, 2016 11:39AM  11:51AM 
F26.00003: Critical exponents at the unconventional disorderdriven transition in a Weyl semimetal Sergey Syzranov, Pavel Ostrovsky, Victor Gurarie, Leo Radzihovsky Disordered noninteracting systems in sufficiently high dimensions have been predicted to display a nonAnderson disorderdriven transition that manifests itself in the critical behaviour of the density of states and other physical observables. Recently the critical properties of this transition have been extensively studied for the specific case of Weyl semimetals by means of numerical and renormalisationgroup approaches. Despite this, the values of the critical exponents at such a transition in a Weyl semimetal are currently under debate. We present an independent calculation of the critical exponents using a twoloop renormalisationgroup approach for Dirac fermions in $2+\varepsilon$ dimensions and resolve controversies currently existing in the literature. [Preview Abstract] 
Tuesday, March 15, 2016 11:51AM  12:03PM 
F26.00004: Elastic Gauge Fields in Weyl Semimetals Alberto Cortijo, Yago Ferreiros, Karl Landsteiner, Maria Angeles Hernandez Vozmediano We show that, as it happens in graphene, elastic deformations couple to the electronic degrees of freedom as pseudo gauge fields in Weyl semimetals. We derive the form of the elastic gauge fields in a tightbinding model hosting Weyl nodes and see that this vector electronphonon coupling is chiral, providing an example of axial gauge fields in three dimensions. As an example of the new response functions that arise associated to these elastic gauge fields, we derive a nonzero phonon Hall viscosity for the neutral system at zero temperature. The axial nature of the fields provides a test of the chiral anomaly in high energy with three axial vector couplings. [Preview Abstract] 
Tuesday, March 15, 2016 12:03PM  12:15PM 
F26.00005: Quantum oscillations in Weyl semimetals Jan Borchmann, Tami PeregBarnea In this work we present recent progress on quantum oscillations of a Weyl semimetal in a slab geometry. Based on semiclassical arguments, it has been conjectured that the Fermi arcs present on the surface of the slab can lead to quantum oscillations with a characteristic dependence on the applied magnetic field as well as the thickness of the slab, which differ from the quantum oscillations in the bulk. To further investigate this we present results from a calculation via a Floquet formalism as well as a lattice quantum treatment of the problem. We present results on the oscillation frequency as well as the phase offset. [Preview Abstract] 
Tuesday, March 15, 2016 12:15PM  12:27PM 
F26.00006: Characterization of the Weyl semimetal via the ``Fermi arc'' of WannierStark ladder Kun Woo Kim, WooRam Lee, Yong Baek Kim, Kwon Park Weyl semimetals have been characterized unequivocally by the Fermiarc spectrum of the surface states in photoemission experiments. While successful, such a method reveals the topological nature of the Weyl phase in the bulk indirectly via the surface spectrum. In this talk, we propose an alternative method to characterize the Weyl phase via the bulk spectrum of the WannierStark ladder (WSL) emerging under an electric field. Specifically, we show that, for weaktomoderate strengths of electric field, the WSL exhibits its own ``Fermi arc'' precisely corresponding to the surface spectrum counterpart, which can be used to characterize the Weyl phase directly in the bulk spectrum. [Preview Abstract] 
Tuesday, March 15, 2016 12:27PM  12:39PM 
F26.00007: Phase diagrams of disordered Weyl semimetals Hassan Shapourian, Taylor L. Hughes Weyl semimetals are gapless quasitopological materials with a set of isolated nodal points forming their Fermi surface. They manifest their quasitopological character in a series of topological electromagnetic responses including the anomalous Hall effect. Here we study the effect of disorder on Weyl semimetals while monitoring both their nodal/semimetallic and topological properties through computations of the localization length and the Hall conductivity. We present detailed phase diagrams of three different lattice tightbinding models and we find that weak disorder preserves the nodal points up to the diffusive limit, but does affect the Hall conductivity. We show that the trend of the Hall conductivity is consistent with an effective picture in which disorder causes the Weyl nodes move within the Brillouin zone along a specific direction that depends deterministically on the properties of the model and the neighboring phases to the Weyl semimetal phase. [Preview Abstract] 
Tuesday, March 15, 2016 12:39PM  12:51PM 
F26.00008: Dynamical axion string, screw dislocation in Weyl semimetals and Axion insulators Yizhi You, Gil Young Cho, Taylor Hughes We study the interplay between the geometry and axion string resulting from a chiral symmetry breaking in 3D. The chiral symmetry is spontaneously broken by charge density wave (CDW) order parameter nesting two Weyl points, which turns it into an axion insulator. The phase fluctuation of the CDW order parameter acts as a dynamical axion field coupled to electromagnetic field via $\theta F \wedge F$ term. When the axion insulator is coupled with the background geometry with torsional defects, i.e. screw dislocations, there is a novel interplay between the dislocation and the dynamical axion string. First, we show that the screw dislocation traps an axial charge. This then implies that if an axion string braids with a parallel screw dislocation, there is Berry phase accumulated during the braiding procedure. In addition, the cubic coupling between the axial current and the torsion bilinear shows the Berry phase accumulated by the threeloop braiding procedure, where we braid one dislocation loop around the other dislocation loop where the both are linked by an axion string loop. We also observe a chiral magnetic effect induced by a screw dislocation in the absence of chemical potential imbalance between Weyl points. [Preview Abstract] 
Tuesday, March 15, 2016 12:51PM  1:03PM 
F26.00009: NonFermi liquid phase and nonGaussian itinerant quantum criticality of Weyl semimetals Pallab Goswami A Weyl semimetal is a gapless topological phase in three dimensions, for which the touching points between two nondegenerate bands act as monopoles and antimonopoles of Abelian Berry curvature, with monopole strength $m$. Such a gapless phase can support $m$ Fermi arcs as the protected, zero energy surface states. We consider the stability of a generalized Weyl semimetal with $m>1$ in the presence of interaction and disorder by employing a renormalization group analysis, which is controlled by the parameter $\epsilon=\left(1\frac{1}{m}\right)$. For any $m>1$, we show how the long range Coulomb interaction gives rise to an infrared stable, nonFermi liquid phase without any sharp quasiparticle pole. In the presence of sufficiently strong short range interactions, the nonFermi liquid can transform into a translational symmetry breaking, axionic insulator. We demonstrate that the associated itinerant quantum critical point possesses nonGaussian scaling properties. We establish the stability of the emergent nonFermi liquid phase and the itinerant quantum critical point against weak disorder. Finally, we discuss the scaling properties of physical quantities, the fate of the Fermi arcs, and the experimental relevance of our results for some candidate materials. [Preview Abstract] 
Tuesday, March 15, 2016 1:03PM  1:15PM 
F26.00010: Detecting monopole charge via quantum interference transport Xin Dai, Haizhou Lu, Hong Yao Topological Weyl and doubleWeyl semimetals host different monopole charges in momentum space. How to detect the signature of the monopole charges in quantum transport remains a challenging topic. Here, we reveal the connection between the parity of monopole charge in topological semimetals and the quantuminterference correction to the conductivity. We demonstrate that the parity of monopole charge determines the sign of quantuminterfere correction, with odd and even parity yielding the weak antilocalization and weak localization effect, respectively. This is attributed to the Berry phase difference between timereversed trajectories circulating the great circle of the Fermi sphere that encloses the monopole charges. From standard Feynman diagram calculations, we further show that the weakfield magnetoconductivity is proportional to $\pm \sqrt B$ for doubleWeyl semimetals and Weyl semimetals, respectively, which could be verified experimentally. [Preview Abstract] 
Tuesday, March 15, 2016 1:15PM  1:27PM 
F26.00011: Chiralitydependent Hall Effect in Weyl Semimetals Shengyuan Yang, Hui Pan, Fan Zhang We generalize a semiclassical theory and use the argument of angular momentum conservation to examine the ballistic transport in lightlydoped Weyl semimetals, taking into account various phasespace Berry curvatures. We predict universal transverse shifts of the wavepacket center in transmission and reflection, perpendicular to the direction in which the Fermi energy or velocities change adiabatically. The anomalous shifts are opposite for electrons with different chirality, and can be made imbalanced by breaking inversion symmetry. We discuss how to utilize local gates, strain effects, and circularly polarized lights to generate and probe such a chiralitydependent Hall effect. [Preview Abstract] 
Tuesday, March 15, 2016 1:27PM  1:39PM 
F26.00012: Full quantum theory of the chiral anomaly transport in a Weyl semimetal WooRam Lee, Kwon Park In relativistic field theory, the chiral anomaly means a violation of the number conservation of chiral fermions. In condensed matter physics, the chiral anomaly can be manifested in a Weyl semimetal as a negative magnetoresistance in the presence of parallel electric and magnetic fields. In this work, we use the KeldyshFloquet Greenâ€™s function formalism to develop a full quantum theory of the chiral anomaly transport, which can be valid in a broad range of both electric and magnetic field strengths. [Preview Abstract] 
Tuesday, March 15, 2016 1:39PM  1:51PM 
F26.00013: Effect of disorder in three dimensional layered Chern insulator Shang Liu, Tomi Ohtsuki, Ryuichi Shindou Critical nature of quantum phase transition between topological phase and nontopological phase is one of the most fundamental research issues in the studies of topological phases, where a bulk delocalized state is universally observed between distinct phases as a holographic requirement from the stable surface states. In this work, we studied the effects of disorder in a three dimensional layered Chern insulator, which, in the clean limit, is either a Chern insulator or Weyl semimetal (WSM) depending on the strength of an interlayer coupling. By calculating the localization length with the transfer matrix method and density of states with the kernel polynomial expansion, we found two distinct types of metallic phases between the Anderson insulator and Chern insulator phases; one is a diffusive metallic (DM) phase and the other is a renormalized WSM phase. We showed that the longitudinal conductivity at the zero energy state remains finite not only in the DM phase but also in the renormalized WSM phase, while goes to zero at the semimetalmetal quantum phase transition point. Based on the Einstein relation combined with the selfconsistent Born analysis, we also argue the conductivity scaling near the quantum transition point. [Preview Abstract] 
Tuesday, March 15, 2016 1:51PM  2:03PM 
F26.00014: The appearance of a switch in orbital texture and the resulting absence of complete spin polarization in a Rashba semiconductor Qihang Liu, Xiuwen Zhang, Alex Zunger We consider spinorbit coupling (SOC) induced spin splitting and spin polarization in nonmagnetic bulk materials lacking inversion symmetry, in which a pair of sidebyside bands that cross at some wavevector $K$ is formed. We find that (i) even though in the semiclassical singleelectron model the two bands manifest complete spin polarization, we find via density functional theory and \textit{k \textbullet p} modeling that SOCinduced spin splitting in real materials does not necessarily manifest complete spin polarization. (ii) Away from band crossing point $K$ and looking at the same wavevector, the spin polarization in different branches does not compensate each other. When considering the full pair of spinsplit bands this leads to a net spin texture. We explain this unexpected phenomenon and find that the key factor here is the complex interplay between spin and orbital textures entangled by SOC. (iii) In surprising analogy to the surface states of topological insulators (such as Bi$_{\mathrm{2}}$Se$_{\mathrm{3}})$, a bulk Rashba compound (such as BiTeI) also exhibits a switch of inplane orbital character between radial and tangential orbital at the critical band crossing point $K$. These observations provide a different thinking on the fundamental concept of SOCinduced spin polarization, and opens a new route for manipulating spin degree of freedom by the atomicorbital feature. [Preview Abstract] 
Tuesday, March 15, 2016 2:03PM  2:15PM 
F26.00015: Quantum Mechanics of Palladium Nanostructures Ajit Hira, James Mckeough, Bridget Ortiz, Juan Diaz We continue our interest in the chemisorption of different atomic and molecular species on small clusters of metallic elements, by examining the interactions of H, H$_{\mathrm{2,}}$ Li and O adsorbates with Pd$_{\mathrm{n}}$ clusters (n $=$ 2 thru 20). The study of clusters can reveal the effects of substrate geometry on the behavior of adsorbates. Transitionmetal clusters are especially suited for the study of quantum size effects and for formation of metallic states, and are ideal candidates for catalytic processes. Hybrid ab initio methods of quantum chemistry (particularly the DFTB3LYP model) are used to derive optimal geometries for the clusters of interest. We compare calculated binding energies, bondlengths, ionization potentials, electron affinities and HOMOLUMO gaps for the clusters. Of particular interest are the comparisons of binding strengths at the three important types of sites: edge (E), hollow (H), ontop (T), threefold sites and fourfold sites. Effects of crystal symmetries corresponding to the bulk structures are investigated. The capacity of Pd clusters to adsorb H atoms will be compared to Ni clusters. Admixture with Pt atoms will also be considered. [Preview Abstract] 
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