Bulletin of the American Physical Society
APS March Meeting 2020
Volume 65, Number 1
Monday–Friday, March 2–6, 2020; Denver, Colorado
Session X60: Weyl semimetal, Theory IIFocus

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Sponsoring Units: DMP Room: Mile High Ballroom 4A 
Friday, March 6, 2020 11:15AM  11:51AM 
X60.00001: Topological semimetals: from classification to material realization Invited Speaker: Jennifer Cano The field of topological semimetals continues to reveal new insights. I will discuss recent developments in the field, starting with the classification of nodal fermions in both magnetic and nonmagnetic space groups. I will then introduce new aspects of the bulkedge correspondence, which elucidate the topological nature of certain nonchiral fermions. Finally, I will discuss routes to material realizations. 
Friday, March 6, 2020 11:51AM  12:03PM 
X60.00002: Topological Vortices in Superconducting TimeReversal Symmetric Weyl Semimetals RAUF GIWA, Pavan Hosur Recent years have shown that superconducting vortices in topological insulators and strongly spinorbit coupled semiconductors are natural platforms for obtaining Majorana zero modes. We ask the question, "under what conditions do superconducting vortices in Weyl semimetals trap protected Majorana zero modes on the surface?" We show that sufficient conditions for a timereversal symmetric type I Weyl semimetal (TWSM) to host Majorana modes as above when ordinary BCS superconductivity is induced in them, are (i) the TWSM forms by perturbing a Dirac or a nodal line semimetal, so that kz=0,pi planes contain pairs of opposite chirality Weyl nodes with a small kspace separation (where z is the vortex axis), and (ii) the number of quadruplets of Weyl nodes in these planes is odd. We analytically calculate the topological invariant in this limit using Kitaev's Pfaffian criterion and support it with numerics on a cubic lattice model. Using our criteria, we predict TaAs and its isovalent counterparts to form topological trivial vortices, devoid of Majorana modes. Also, we propose Na(3x)KxBi with broken inversion symmetry, where doping occurs via interstitial substitution, to be an ideal platform for realizing a nontrivial superconducting vortex with Majorana zero modes at its ends. 
Friday, March 6, 2020 12:03PM  12:15PM 
X60.00003: FiniteSize Effects in the Dynamic Conductivity and Faraday Effect of Quantum Anomalous Hall Insulators Junjie Zeng, Tao Hou, Zhenhua Qiao, Wang Kong Tse We theoretically study the finitesize effects in the dynamical response of a quantum anomalous Hall insulator in the disk geometry. Semianalytic and numerical results are obtained for the wavefunctions and energies of the disk within a continuum Dirac Hamiltonian description subject to a topological infinite mass boundary condition. Using the Kubo formula, we obtain the frequencydependent longitudinal and Hall conductivities and find that optical transitions between edge states contribute dominantly to the real part of the dynamic Hall conductivity for frequency values both within and beyond the bulk bandgap. We also find that the topological infinite mass boundary condition changes the lowfrequency Hall conductivity to e^{2}/h in a finitesize system from the wellknown value e^{2}/2h in an extended system. The magnetooptical Faraday rotation is then studied as a function of frequency for the setup of a quantum anomalous Hall insulator mounted on a dielectric substrate, showing both finitesize effects of the disk and FabryPerot resonances due to the substrate. Our work demonstrates the important role played by the boundary condition in the topological properties of finitesize systems through its effects on the electronic wavefunctions. 
Friday, March 6, 2020 12:15PM  12:27PM 
X60.00004: Interplay of nonAbelian band topology with crystalline symmetry Tomas Bzdusek, QuanSheng Wu, Alexey Soluyanov, Adrien Bouhon, RobertJan Slager We discuss the recently discovered nonAbelian topological invariant that characterizes band nodes inside the momentum space of certain noninteracting metals. This nonAbelian topology prominently arises in systems with PT symmetry (spacetime inversion) or with C_{2}T symmetry (composition of πrotation with timereversal). Given the prevalence of these symmetries in most space groups, one expects important implications of the nonAbelian invariant for real materials. 
Friday, March 6, 2020 12:27PM  12:39PM 
X60.00005: Strain engineered higher order topological phases for spin3/2 Luttinger fermions Andras Szabo, Roderich Moessner, Bitan Roy Recently, the notion of topological phases of matter has been extended to higher order incarnations, supporting gapless modes on even lower dimensional boundaries, such as corners and hinges. We here identify a collection of cubic spin3/2 fermions with biquadratic touching of Kramers degenerate valence and conduction bands as a platform to strainengineer higherorder topological (HOT) phases: external uniaxial strain gives birth to a HOT Dirac semimetal or an insulator, depending on its sign, featuring topological \emph{hinge} modes in the strain direction. The insulator in fact exhibits \emph{mixed} topology, and in addition supports edge modes on orthogonal planes. These outcomes are germane when the external strain is applied along one of the $C_{4v}$ or coordinate axes, as well as $C_{3v}$ or bodydiagonal, directions. Our findings place HgTe, graySn, 227 pyrochlore iridates and halfHeusler compounds at the frontier of strain engineered electronic HOT phases. 
Friday, March 6, 2020 12:39PM  12:51PM 
X60.00006: WiedemannFranz law and Mott relation for transport coefficients in the nonlinear regime Chuanchang Zeng, Snehasish Nandy, Arghya Taraphder, Sumanta Tewari In contrast to their counterparts in the linear regime, the nonlinear anomalous Hall, Nernst, and thermal Hall effects can survive in timereversal symmetric system in the presence of inversion symmetry breaking. Within the framework of semiclassical Boltzmann theory, we calculate the analytical expressions for the nonlinear anomalous transport coefficients, namely, the nonlinear anomalous Hall, Nernst, and thermal Hall coefficients. With these expressions, we predict the fundamental relations between the nonlinear anomalous electric and thermoelectric transport coefficients, which are the analog of the WiedemannFranz law and the Mott relation in the nonlinear regime. We also make several experimental predictions for nonlinear anomalous Nernst Hall effect in bilayer WTe2 and for nonlinear anomalous thermal Hall effect for monolayer transition metal dichalcogenide which can be verified in experiments. 
Friday, March 6, 2020 12:51PM  1:03PM 
X60.00007: Phonon induced topological phase transition in ZrTe_{5} Niraj Aryal, Xilian Jin, Weiguo Yin Layered crystalline material ZrTe_{5} is a unique material to study topological phase transitions; it is very close to the phase boundary between a weak and strong topological insulator (TI) and one can switch from one phase to another by small perturbations like strain. Recently, photoinduced topological transition from the TI regime to the Dirac semimetallic phase has been reported in this material. Motivated by the aforementioned work, using firstprinciples calculations and effective model Hamiltonian, we study the transition from the TI regime to the Dirac and Weyl semimetallic phases induced by various phonon 
Friday, March 6, 2020 1:03PM  1:15PM 
X60.00008: Diagnosing and observing topological degeneracies in AI class systems Tiantian Zhang, Hu Miao, Ling Lu, Shuichi Murakami, Zhong Fang, Mark Dean, Hongming Weng, Chen Fang Topological states have been widely studied in Fermionic systems for many years, wherever, they are sporadically explored in Bosonic systems, even both Fermions and Bosons are fundamental particles in condensed matter physics. Actually, Bosons can offer a better platform to get spinless band structures where nodal lines can be realized with only paritytime reversal (PT) symmetry, such as phonons. By combining firstprinciples calculations and meVresolution inelastic xray scattering, we demonstrate the first realization of PT symmetry protected helical nodal lines of phonons in a real material. The combination of theoretical analysis and experimental measurements demonstrate the PTprotected helical nodal lines for the first time in phononic materials. This nodal line is robust because of the spinless nature of phonons, which will open a new route to explore exotic topological states in crystalline materials. 
Friday, March 6, 2020 1:15PM  1:27PM 
X60.00009: Effects of Van der Waals interaction on the phonon and electron band structures of ZrTe_{5} Xilian Jin, Niraj Aryal, Weiguo Yin Since the theoretical prediction of the existence of the topological insulating phase, ZrTe_{5} has attracted a lot of interests in its topological nature, which may lead to the lowenergy transport edge channels and the quantum spin Hall (QSH) effect in its twodimensional form. Possible strain or photon induced topological transition from the topological insulator (TI) to the Dirac semimetal in ZrTe_{5} has been recently suggested by experiments. Due to the layered structure of ZrTe_{5}, the van der Waals (vdW) interaction becomes an essential factor to study atomic vibrational behavior in the topological transition. In this theoretical work, phonon vibration modes and the corresponding electron band structures are explored systematically without and with vdW force correction to reveal the microscopic mechanism of the phase transition in ZrTe_{5}. 
Friday, March 6, 2020 1:27PM  1:39PM 
X60.00010: Bulk and surface polaritons in Weyl semimetals Alexey Belyanin, Qianfan Chen, Ryan Kutayiah, Maria Erukhimova, Ivan Oladyshkin, Mikhail Tokman

Friday, March 6, 2020 1:39PM  1:51PM 
X60.00011: Hourglass Weyl loops in two dimensions: Theory and material realization in monolayer GaTeI family Weikang Wu, Yalong Jiao, Si Li, XianLei Sheng, zhiming yu, Shengyuan Yang Nodal loops in twodimensional (2D) systems are typically vulnerable against spinorbit coupling (SOC). Here, we explore 2D systems with a type of doubly degenerate nodal loops that are robust under SOC and feature an hourglass type dispersion. We present symmetry conditions for realizing such hourglass Weyl loops, which involve nonsymmorphic lattice symmetries. Depending on the symmetry, the loops may exhibit different patterns in the Brillouin zone. Based on firstprinciples calculations, we identify the monolayer GaTeIfamily materials as a realistic material platform to realize such loops. These materials host a single hourglass Weyl loop circling around a highsymmetry point. Interestingly, there is also a spinorbit Dirac point enabled by an additional screw axis. We show that the hourglass Weyl loop and the Dirac point are robust under a variety of applied strains. By breaking the screw axis, the Dirac point can be transformed into a second Weyl loop. Furthermore, by breaking the glide mirror, the hourglass Weyl loop and the spinorbit Dirac point can both be transformed into a pair of spinorbit Weyl points. Our work offers guidance and realistic material candidates for exploring fascinating physics of several novel 2D emergent fermions. 

X60.00012: Fermionic analogue of Hawking radiation in bilayer black phosphorous Hang Liu, Jiatao Sun, Huaqing Huang, Feng Liu, Sheng Meng Topological matters upon the periodic driving of laser can lead to nonequilibrium topological states, which is not accessible in equilibrium topological materials. Here we propose that the periodically driven twodimensional black phosphorous (BP) thin film with a spatial gradient of FloquetDirac states can mimic the “gravity” felt by fermionic quasiparticles as that for a Schwarzschild black hole (SBH). Quantum tunneling of electrons from a typeII Dirac cone (inside BH) to a typeI Dirac cone (outside) emits a SBHlike Hawking radiation spectrum. The obtained T_{H} of a fermionic analog of BH in BP is approximately 3 K, which is several orders of magnitude higher than previous works. Our work sheds some light on increasing the T_{H} from the perspective of nonequilibrium topological states under periodic driving of laser. The obtained SBH accessible in nonequilibrium BP provides some clues to access the more Fermionic analogues of astronomy phenomena in solids. 
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