Bulletin of the American Physical Society
APS March Meeting 2022
Volume 67, Number 3
Monday–Friday, March 14–18, 2022; Chicago
Session G68: Topological Semimetals IIFocus Recordings Available

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Sponsoring Units: DMP Chair: Leigh Smith, University of Cincinnati Room: Hyatt Regency Hotel Hyde Park B 
Tuesday, March 15, 2022 11:30AM  12:06PM 
G68.00001: Optical fingerprints of unconventional carriers in kagome metals Invited Speaker: Ece Uykur Kagome metals are the newly emerging class of quantum materials, where the peculiar kagome structure along with the itinerant character of the electrons give rise to a nontrivial combination of entangled magnetic states, electronic correlations, and topological orders. Two dissimilar features of massive, localized carriers at dispersionless flat bands and the massless Dirac fermions at the linearly dispersing bands coexist in their electronic structure, giving rise to various exotic phenomena. While the magnetic kagome metals are at the center of interest due to the proximity of the flatband magnetism, newly discovered nonmagnetic counterparts possess a peculiar density wave order along with the unconventional superconductivity. 
Tuesday, March 15, 2022 12:06PM  12:18PM 
G68.00002: Spin waves in graphene with Zeemantype spin splitting Matthew J Anderson, Florent Perez, Carsten A Ullrich We present a computational study of spin waves in doped paramagnetic singlelayer graphene with adjustable Zeemantype band splitting. The spin waves are described using timedependent spindensityfunctional response theory, treating dynamical exchangecorrelation effects within the Slater and SingwiTosiLandSjolander approximations. We obtain spinwave dispersions and spin stiffnesses as a function of doping and spin polarization, and discuss prospects for their experimental observation. 
Tuesday, March 15, 2022 12:18PM  12:30PM 
G68.00003: Superconductor vortex spectrum of the Fermi arc metal in Weyl semimetals RAUF O GIWA, Pavan R Hosur The surface of a Weyl semimetal hosts open Fermi arcs that form closed contours with Fermi arcs on the opposite surface. The resulting 2D Fermi surface, when subjected to a magnetic field, exhibits characteristic quantum oscillations stemming from semiclassical cyclotron orbits that involve both bulk and surface travel and obey a subtle form of BohrSommerfeld quantization conditions. In this work, we use similar semiclassical ideas to derive the superconductor vortex spectrum of this 2D metal. We first recall that each Weyl node produces a chiral Majorana fermion in the bulk inside the vortex. We then show that the vortex spectrum is governed by the total Berry phase acquired by a wavepacket traversing a closed orbit consisting of Fermi arcs on opposite surfaces and the bulk chiral modes. In particular, the vortex gap closes and reopens when the total Berry phase sweeps through $(2n+1)\pi$ . The Berry phase contributed by the chiral modes depends on the vortex orientation relative to the separation between the Weyl nodes. We, therefore, predict characteristic features in the vortex tiltdependence of the local density of states that can be probed by standard scanning tunneling measurements. 
Tuesday, March 15, 2022 12:30PM  12:42PM 
G68.00004: Evolution of Charge Order in Ba_{1x}Sr_{x}Al_{4} Prathum Saraf, Eleanor M Clements, Danila Sokratov, Christopher Eckberg, Daniel J Campbell, Thomas W Heitmann, Jeffrey W Lynn, Peter Y Zavalij, Johnpierre Paglione The BaAl_{4} type structure also hosts a variety of interesting and exotic properties. Recent results have shown an emergence of CDW order in SrAl_{4} at 243K, together with a hysteretic structural transition at 87K that is thought to be a structural transition to a monoclinic system. The tetragonal structure of BaAl_{4} has also recently been shown to have topologically nontrivial bands and has lattice parameters very close to SrAl_{4}. In order to study the interplay between these features we have done a chemical substitution study to track the reported transitions. We have performed electrical resistivity measurements, along with xray and neutron scattering on the Ba_{1x}Sr_{x}Al_{4} family. We show a suppression of the CDW order with increased Ba substitution. Neutron scattering and xray measurements show the nature of the CDW. 
Tuesday, March 15, 2022 12:42PM  12:54PM 
G68.00005: Electric quantum oscillations in Weyl semimetals Kyusung Hwang, WooRam Lee, Kwon Park Electronic transport in Weyl semimetals is quite extraordinary due to the topological property of the chiral anomaly generating the charge pumping between two distant Weyl nodes with opposite chiralities under parallel electric and magnetic fields. Here, we develop a full nonequilibrium quantum transport theory of the chiral anomaly, based on the fact that the chiral charge pumping is essentially nothing but the Bloch oscillation. Specifically, by using the Keldysh nonequilibrium Green function method, it is shown that there is a rich structure in the chiral anomaly transport, including the negative magnetoresistance, the nonOhmic behavior, the EsakiTsu peak, and finally the resonant oscillation of the dc electric current as a function of electric field, called the electric quantum oscillation. We argue that, going beyond the usual behavior of linear response, the nonOhmic behavior observed in BiSb alloys can be regarded as a precursor to the occurrence of electric quantum oscillation, which is both topologically and energetically protected in Weyl semimetals. 
Tuesday, March 15, 2022 12:54PM  1:06PM 
G68.00006: Discovering Dirac materials with quadhelicoid surface states Daisuke Hara, Shuichi Murakami, Tiantian Zhang Surface states of Dirac materials consist of two opposite helicoid surfaces, which cross each other along a line in the Brillouin zone and will hybridize with each other even against symmetryallowed perturbations [1]. However, recent studies show that Dirac points with nonzero Z2 monopole charge can host gapless surface states under certain conditions [13], and such Dirac points can even have topologically protected quadhelicoid surface states which have never been explored in electronic and phononic systems. 
Tuesday, March 15, 2022 1:06PM  1:18PM 
G68.00007: Electronic structures of twisted fewlayer black phosphorus Taesik Nam, Hangyu Kim, Hyoung Joon Choi Fewlayer black phosphorus (BP), a gapped twodimensional (2D) semiDirac material, has pseudospin features even in its semiconducting phase [1,2]. When its band gap is closed, fewlayer BP has a semiDirac point, and when its band gap is inverted, it has two anisotropic Dirac cones [13]. Twisted bilayers of gapped 2D semiDirac materials may have interesting emergent phenomena due to directional mismatch of inplane anisotropy of each layer. In our present work, we perform density functional theory (DFT) calculations of 90degree twisted fewlayer BP. We control the band gap of twisted fewlayer BP by changing structural parameters and analyze electronic band structures near the chemical potential. We construct effective Hamiltonian based on DFT results and obtain relevant pseudospin representations. We also discuss effects of externally applied electric fields. 
Tuesday, March 15, 2022 1:18PM  1:30PM Withdrawn 
G68.00008: Large enhancement of Berrycurvature dipole in monolayer W_{0.5}Mo_{0.5}Te_{2} Madhav P Ghimire, Deergh B Shahi, Dipak Bhattarai, Shiang Fang, JhihShih You, Sobhit Singh WTe_{2} and MoTe_{2} monolayers provide a rich atomically thin platform to explore topological physics, electrically switchable circular photogalvanic effect, rectification, and quantum nonlinear Hall effects. For realization of the nonlinear quantum phenomena, a nonzero Berrycurvature dipole (BCD) is required, which has been demonstrated in pristine WTe_{2} and MoTe_{2} monolayers [14]. In this work, we theoretically design a new monolayer W_{0.5}Mo_{0.5}Te_{2} using the pristine T_{d}WTe_{2} and T_{d}MoTe_{2} monolayers [5]. We confirm the dynamical, elastic, and mechanical stability of W_{0.5}Mo_{0.5}Te_{2} monolayer by phonon and elastic constants calculations. Our electronic band structure calculations reveal the presence of a relatively much smaller bandgap near the Fermi energy compared to that of in pristine T_{d}WTe_{2} and T_{d}MoTe_{2} monolayers. Such a narrow bandgap leads to divergence of Berry curvature near the Fermi energy yielding a large enhancement of BCD in W_{0.5}Mo_{0.5}Te_{2} monolayer, as predicted by our firstprinciples calculations. 
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