Bulletin of the American Physical Society
APS March Meeting 2019
Volume 64, Number 2
Monday–Friday, March 4–8, 2019; Boston, Massachusetts
Session F03: Novel Transport Properties of Weyl and Dirac SemimetalsFocus

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Sponsoring Units: DCMP Chair: Dmytro Pesin, University of Virginia Room: BCEC 107B 
Tuesday, March 5, 2019 11:15AM  11:27AM 
F03.00001: Fermi arc induced vortex structure in Weyl beam shifts Udvas Chattopadhyay, Likun Shi, Baile Zhang, Justin Song, Yidong Chong In periodic media, despite the close relationship between geometrical effects in the bulk and topological surface states, the two are typically probed separately. We have recently found that when beams in a Weyl medium reflect off an interface with a gapped medium, the trajectory is influenced by both bulk geometrical effects and the presence of Fermi arc surface states. The reflected beam experiences a displacement, analogous to the GoosHänchen or ImbertFedorov shifts, that forms a halfvortex in the twodimensional surface momentum space. The halfvortex structure is centered at the point where the Fermi arc of the reflecting surface touches the Weyl cone, and the magnitude of the shift scales as the inverse square root of distance from the touchingpoint. This striking feature provides a way to use bulk transport to probe the topological characteristics of a Weyl medium. 
Tuesday, March 5, 2019 11:27AM  11:39AM 
F03.00002: Magnetotransport in Weyl nanowires Vardan Kaladzhyan, Jens Bardarson We study longitudinal magnetotransport in Weyl semimetal nanowires. We show that depending on radii of nanowires and magnetic field amplitudes there exist two qualitatively and quantitavely different regimes of transport. It is demonstrated that in the strong magnetic field regime (magnetic length much smaller than the radius of the nanowire), Landau level spectrum contains the chiral 0th Landau level, and thus the transport properties resemble that of a bulk Weyl semimetal. On the contrary, in the weak magnetic field regime (magnetic length much larger than the radius of the nanowire), the lowestenergy band is a Fermi arc surfacebulk state with a nonchiral dispersion, hence there appear transport features distinct from those attained at larger radii or larger magnetic fields. We argue that both regimes are relevant for the ongoing experiments. We also demonstrate that the contribution of Fermi arc surface states is salient and, therefore, crucial for understanding transport properties of finitesize Weyl semimetal systems. 
Tuesday, March 5, 2019 11:39AM  11:51AM 
F03.00003: Chirality in Weyl semimetals in strong circularlypolarized electric field Fatemeh Nematollahi, Vadym Apalkov, JhihSheng Wu, Mark I Stockman We theoretically study the interaction of threedimensional topological Weyl semimetals with a single oscillation femtosecond long circularlypolarized pulse. The chiral pulse causes a finite electron population of the conduction band which is highly structured and is determined by the topological resonance. These textures can be probed by a linear pulse applied after the circularlypolarized pulse. The response of the system to the linear probe pulse is highly sensitive to the chirality of the circular pulse. Also, the pulse which consists of two oscillations of different chiralities causes conduction band population distribution in the reciprocal space which is highly chiral and is related to the intrinsic chirality of Weyl semimetals. 
Tuesday, March 5, 2019 11:51AM  12:03PM 
F03.00004: Topological Nodal Lines from Crystalline Symmetry Heqiu Li, Chen Fang, Kai Sun Topological nodal lines protected by the timereversal symmetry T and spaceinversion symmetry P can be classified into two categories, depending on the value of the Z_{2} monopole topological index. Nodal lines with trivial (nontrivial) monopole charge can (cannot) be gapped out via shrinking the nodal line into a point. In this study, we show that for fourband models with T^{ 2}=+1, there are two special points in phase space where the system has a Z_{2} symmetry, and any Hamiltonian can be adiabatically connected to one of these points. Utilizing this result, we provide a general principle for finding topological nodal lines with nontrivial monopole charge based on the interplay between lattice crystalline symmetries and the timereversal symmetry. In addition, this study also reveals a new family of topological nodal lines, protected by the timereversal symmetry and point group symmetries. 
Tuesday, March 5, 2019 12:03PM  12:15PM 
F03.00005: Strong disorder in nodal semimetals: SchwingerDyson–Ward approach Bjoern Sbierski The selfconsistent Born approximation quantitatively fails to capture disorder effects in semimetals. We present an alternative, simpletouse nonperturbative approach to calculate the disorder induced selfenergy. It requires a sufficient broadening of the quasiparticle pole and the solution of a differential equation on the imaginary frequency axis. We demonstrate the performance of our method for various paradigmatic semimetal Hamiltonians and compare our results to exact numerical reference data. For intermediate and strong disorder, our approach yields quantitatively correct momentum resolved results. It is thus complementary to existing RG treatments of weak disorder in semimetals. 
Tuesday, March 5, 2019 12:15PM  12:27PM 
F03.00006: Parity anomaly in the nonlinear response of nodalline semimetals Alberto MartinRuiz, Alberto Cortijo Nodalline semimetals (NLSM) are topological semimetals where conduction and valence bands touch each other at one dimensional lines in the Brillouin zone. We will focus on the the PTsymmetric version of NLSM and see how adding and removing a PT symmetrybreaking parameter leaves scars in the electromagnetic response of these systems. Tilting breaks P and T but leaves their product invariant, not modifying the symmetry breaking procedure, but allows the parity anomaly to appear both in the linear and nonlinear response. In the case of the linear response, we will show that the tilted NLSM display an axionic electromagnetic response, but with a different origin of the one appearing in Weyl semimetals. 
Tuesday, March 5, 2019 12:27PM  12:39PM 
F03.00007: Topological Amorphous Metals Yanbin Yang, Tao Qin, DongLing Deng, Luming Duan, Yong Xu A Weyl semimetal, a crystalline material with translational symmetry, possesses pairs of Weyl points in momentum space band structures and its topology is characterized by the first Chern number defined over a closed surface in momentum space. Here, we study amorphous systems with completely random sites and find that, through constructing and exploring a concrete model Hamiltonian, such a system can host an exotic phase of topological amorphous metal in three dimensions. In contrast to the traditional Weyl semimetals, topological amorphous metals break translational symmetry, and thus cannot be characterized by the first Chern number defined based on the momentum space band structures. Instead, their topological properties will manifest in the Bott index and the Hall conductivity as well as the surface states. Moreover, by studying the energy band and quantum transport properties, we find that topological amorphous metals exhibit a diffusive metal behavior. Our results open a door for exploring topological gapless phenomena in amorphous systems. 
Tuesday, March 5, 2019 12:39PM  12:51PM 
F03.00008: Adiabatic dechiralisation and thermodynamics of Weyl semimetals Sergey Syzranov, Yaroslav I Rodionov, Brian Skinner We study thermodynamic manifestations of the chiral anomaly in disordered Weyl semimetals. We focus, in particular, on the effect which we call "adiabatic dechiralisation", the phenomenon in which a change in temperature and/or an absorption or release of heat results from changing external electric and magnetic fields that change the imbalance of quasiparticles with different chiralities (at different Weyl nodes). This effect is similar to that of adiabatic demagnetisation, which is commonly used as a method of lowtemperature refrigeration. We describe this phenomenon quantitatively and discuss experimental conditions favourable for its observation. A related phenomenon, which we analyse and which is readily observable in experiment, is the dependency of the heat capacity of a Weyl semimetal on parallel electric and magnetic fields. 
Tuesday, March 5, 2019 12:51PM  1:03PM 
F03.00009: Dynamical density response and optical conductivity in topological metals Anton Burkov Topological metals continue to attract attention as novel gapless states of matter. While there by now exists an exhaustive classification of possible topologically nontrivial metallic states, their observable properties, that follow from the electronic structure topology, are less well understood. Here we present a study of the electromagnetic response of threedimensional topological metals with Weyl or Dirac nodes in the spectrum, which systematizes and extends earlier pioneering studies. In particular, we argue that a smokinggun feature of the chiral anomaly in topological metals is the existence of propagating chiral density modes even in the regime of weak magnetic fields. We also demonstrate that the optical conductivity of such metals exhibits an extra peak, which exists on top of the standard metallic Drude peak. The spectral weight of this peak is transferred from high frequencies and its width is proportional to the chiral charge relaxation rate. 
Tuesday, March 5, 2019 1:03PM  1:15PM 
F03.00010: Twoparticle collisional coordinate shifts and hydrodynamic anomalous Hall effect in systems without Lorentz invariance Dmytro Pesin We show that electrons undergoing a twoparticle collision in a crystal experience a coordinate shift that depends on their singleparticle Bloch wave functions, and derive a gaugeinvariant expression for such shift, valid for arbitrary band structures, and arbitrary twoparticle interaction potentials. As an application of the theory, we consider twoparticle coordinate shifts for Weyl fermions in space of three spatial dimensions. We also demonstrate that such shifts in general contribute to the anomalous Hall conductivity of a clean electron liquid. 
Tuesday, March 5, 2019 1:15PM  1:27PM 
F03.00011: Optical responses of chiral multifold fermions Miguel Angel SanchezMartinez, Fernando De Juan, Adolfo Grushin Multifold fermions are generalizations of twofold degenerate Weyl fermions with three, four, six or eightfold degeneracies protected by crystal symmetries, of which only the last type is necessarily nonchiral. Their low energy degrees of freedom can be described as higher spin generalizations of Weyl semimetals. In this talk I will discuss their linear and non linear optical responses including optical activity, linear optical conductivity and photogalvanic effects. These are typically enhanced compared to Weyl fermions of the same Fermi velocity and can be quantized in the case of the circular photogalvanic effect. We use these results to qualitatively predict optical responses in multifold materials such as RhSi or CoSi. 
Tuesday, March 5, 2019 1:27PM  1:39PM 
F03.00012: Intrinsic magnetoresistance in threedimensional Dirac materials with low carrier density Huanwen WANG, Bo Fu, Shunqing Shen Negative longitudinal and positive inplane transverse magnetoresistance have been observed in most topological Dirac/Weyl semimetals and some other topological materials. Here we present a quantum theory of intrinsic magnetoresistance for threedimensional Dirac fermions at a finite and uniform magnetic field B. In the semiclassical regime, it is shown that the longitudinal magnetoresistance is negative and quadratic of a weak field B while the inplane transverse magnetoresistance is positive and quadratic of B. The relative magnetoresistance is inversely quartic of the Fermi wave vector and only determined by carrier density, irrelevant to the external scatterings in the weak scattering limit. This intrinsic anisotropic magnetoresistance is measurable in systems with low carrier density and high mobility. In the quantum oscillation regime, a formula for the phase shift in Shubnikov–de Haas oscillation is present as a function of the mobility and the magnetic field, which is helpful for experimental data analysis 
Tuesday, March 5, 2019 1:39PM  1:51PM 
F03.00013: Novel Planar Hall Effect in Tilted Weyl Semimetals Da Ma, Hua Jiang, Haiwen Liu, Xincheng Xie In addition to the chiral anomaly, the planar Hall effect has been proposed in the magnetotransport of the Weyl semimetals. We investigate the transport of the tilted Weyl semimetals semiclassically in an inplane magnetic field. The results show that the longitudinal and the Hall conductivities in this system each has a linear term in magnetic field besides the quadratic terms and the angular dependence of them are different from the case without the tilt. The unusual magnetotransport is induced by the Berry curvature, either through the axial chemical potential or the anomalous velocity. The correction of the phase space volume factor is also presented, which has effects on both the angular and the magnetic field dependence of the conductivities. 
Tuesday, March 5, 2019 1:51PM  2:03PM 
F03.00014: WITHDRAWN ABSTRACT

Tuesday, March 5, 2019 2:03PM  2:15PM 
F03.00015: Spintocharge conversion in magnetic Weyl semimetals. Shulei Zhang, Anton Burkov, Ivar Martin, Olle Heinonen Weyl semimetals (WSM) are a newly discovered class of quantum materials which can host a number of exotic transport properties in bulk, such as the chiral magnetic effect, negative magnetoresistance, and the anomalous Hall effect. In this work, we investigate theoretically the inverse Edelstein effect (IEE) in a magnetic WSM, i.e., a charge current induced by spin accumulation at the interface between a magnetic WSM and a normal metal. Formally, the induced current is obtained by solving for the scattering wave functions and the nonequilibrium distribution functions in each layer. We find that both surface and bulk states contribute to the IEE, and that the induced current exhibits strong anisotropy: It vanishes along the direction parallel to the line connecting the two Weyl nodes, regardless of the direction of the injected spin. We also determined the dependence of the current on the position of the Fermi level as well as the separation between the two Weyl nodes, which provide extra knobs for controlling the spincharge conversion in these topological materials. 
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