Bulletin of the American Physical Society
APS March Meeting 2022
Volume 67, Number 3
Monday–Friday, March 14–18, 2022; Chicago
Session D59: Signatures of Topological PhasesRecordings Available

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Sponsoring Units: DCMP Chair: Ioannis Petrides, Harvard University Room: Hyatt Regency Hotel DuSable AB 
Monday, March 14, 2022 3:00PM  3:12PM 
D59.00001: NonHermitian Higherorder Dirac Semimetals Sayed Ali Akbar Ghorashi, Tianhe Li, Masatoshi Sato, Taylor L Hughes We study 3D nonHermitian higherorder Dirac semimetals (NHHODSMs) [1]. Our focus is on C_{4}symmetric nonHermitian systems where we investigate inversion (I) or timereversal (T) symmetric models of NHHODSMs having real bulk spectra. We show that they exhibit the striking property that the bulk and surfaces are antiPT and PTsymmetric, respectively, and so belong to two different topological classes realizing a novel nonHermitian topological phase which we call a hybridPT topological phases. Interestingly, while the bulk spectrum is still fully real, we find that exceptional Fermirings (EFRs) appear connecting the two Dirac nodes on the surface. This provides a route to probe and utilize both the bulk Dirac physics and exceptional rings/points on equal footing. Moreover, particularly for TNHHODSMs, we also find real hingearcs connecting the surface EFRs. We show that this higherorder topology can be characterized using a biorthogonal realspace formula of the quadrupole moment. Furthermore, by applying Hermitian C_{4}symmetric perturbations, we discover various novel phases, particularly: (i) an intrinsic INHHODSM having hinge arcs and surface exceptional Fermiarcs, and (ii) a novel Tsymmetric skintopological HODSM which possesses both topological and skin hinge modes. This work paves the way toward uncovering rich phenomena and hybrid functionality that can be readily realized in experiment. We have also recently studied the nonHermitian higherorder Weyl semimetals in a separate work (see [2]). 
Monday, March 14, 2022 3:12PM  3:24PM 
D59.00002: Nonlinear Nanoelectrodynamics of a Weyl Metal Yinming Shao, Ran Jing, Sang Hoon Chae, Chong Wang, Zhiyuan Sun, Eve Emmanouilidou, Suheng Xu, Dorri Halbertal, Baichang Li, Anjaly Rajendran, Frank Ruta, Lin Xiong, Yinan Dong, Alexander S McLeod, Sai S Sunku, James C Hone, Joel E Moore, Joseph Orenstein, James G Analytis, Andrew J Millis, Ni Ni, Di Xiao, Dmitri N Basov Chiral Weyl fermions with linear energymomentum dispersion in the bulk accompanied by Fermiarc states on the surfaces prompt a host of enticing optical effects. While new Weyl semimetal materials keep emerging, the available optical probes are limited. In particular, isolating bulk and surface electrodynamics in Weyl conductors remains a challenge. We devised a novel approach to the problem based on nearfield photocurrent imaging at the nanoscale and applied this technique to a prototypical Weyl semimetal TaIrTe_{4}. As a first step, we visualized nanophotocurrent patterns in realspace and demonstrated their connection to bulk nonlinear conductivity tensors through extensive modeling augmented with density functional theory calculations. Notably, our nanoscale probe gives access to not only the inplane but also the outofplane electric fields so that it is feasible to interrogate all allowed nonlinear tensors including those that remained dormant in conventional farfield optics. Surface and bulkrelated nonlinear contributions are distinguished through their “symmetry fingerprints” in the photocurrent maps. Robust photocurrents also appear at mirrorsymmetry breaking edges of TaIrTe_{4} single crystals that we assign to nonlinear conductivity tensors forbidden in the bulk. Nanophotocurrent spectroscopy at the boundary reveals a strong resonance structure absent in the interior of the sample, providing evidence for elusive surface states. 
Monday, March 14, 2022 3:24PM  3:36PM 
D59.00003: Disorder effects in topological insulator nanowires Yi Huang, Boris I Shklovskii Threedimensional topological insulator (TI) nanowires with quantized surface subband spectra are studied as a main component of Majorana bound states (MBS) devices. However, such wires are known to have large concentration N ~ 10^{19} cm^{3} of Coulomb impurities. It is believed that a MBS device can function only if the amplitude of longrange fluctuations of the random Coulomb potential Γ is smaller than the subband gap Δ. Here we calculate Γ for recently experimentally studied largedielectricconstant (Bi_{1x}Sb_{x})_{2}Te_{3} wires in a smalldielectricconstant environment (no superconductor). We show that provided by such a dielectricconstant contrast, the confinement of electric field of impurities within the wire allows more distant impurities to contribute into Γ, leading to Γ ~ 3Δ. We also calculate a TI wire resistance as a function of the Fermi level and carrier concentration due to scattering on Coulomb and neutral impurities, and do not find observable discrete subbandspectrum related oscillations at N ≥ 10^{18} cm^{3}. 
Monday, March 14, 2022 3:36PM  3:48PM 
D59.00004: Anomalous Interference of FloquetBloch States in the Magnetic Topological Insulator MnBi_{2}Te_{4} Nina Bielinski, Soyeun Kim, Chandra Shekhar, Anuva Aishwarya, Vidya Madhavan, Claudia Felser, Fahad Mahmood The coherent optical manipulation of solids via Floquet engineering has emerged as a promising way to understand nonequilibrium physics and generate novel quantum phases of matter. So far, Floquet engineering of materials has been limited to Dirac materials without strong correlations. Here we use time and angleresolved photoemission spectroscopy (trARPES) with midIR pump pulses to observe FloquetBloch states in the intrinsic antiferromagnetic topological insulator MnBi_{2}Te_{4}. This system is an ideal candidate for studying Floquet phenomena in the presence of both magnetic correlations and topology. We observe an anomalous interference between certain FloquetBloch states which is distinct from the well characterized FloquetBloch response of the 3D topological insulator Bi_{2}Se_{3}. We will discuss this observation in relation to the intrinsic antiferromagnetic ordering and topological character of MnBi_{2}Te_{4}. 
Monday, March 14, 2022 3:48PM  4:00PM 
D59.00005: Optical conductivity and magnetic properties of lightinduced Floquet vortex states Iman Ahmadabadi, Hossein Dehghani, Mohammad Hafezi In a recent paper (arxiv:2106.08515), it was demonstrated that irradiating light with a nonzero orbital angular momentum on a massive Dirac fermionic insulator can create localized vortex states. In this work, we study physical observables that can reveal the existence of vortex states in these systems. Specifically, the longitudinal and transverse dynamical conductivities are calculated and we separate the contributions of the vortex states to conductivities from those of bulk and edge states. Additionally, the selfinduced current density due to the nonvanishing orbital angular momentum of light is evaluated and we calculate its resulting orbital magnetization. We also discuss experimental methods for measuring this orbital magnetization. 
Monday, March 14, 2022 4:00PM  4:12PM 
D59.00006: Topological valleytronics in monolayer 2D Xenes Koustav Jana, Bhaskaran Muralidharan We propose a realizable device design for an allelectrical robust valley filter that utilizes spin protected topological interface states hosted on monolayer 2DXene materials with large intrinsic spinorbit coupling. In contrast with conventional quantum spinHall edge states localized around the Xpoints, the interface states appearing at the domain wall between topologically distinct phases are either from the K or K′ points, making them suitable prospects for serving as valleypolarized channels. We show that the presence of a large bandgap quantum spin Hall effect enables the spatial separation of the spinvalley locked helical interface states with the valley states being protected by spin conservation, leading to a robustness against shortrange nonmagnetic disorder. By adopting the scattering matrix formalism on a suitably designed device structure, valleyresolved transport in the presence of nonmagnetic shortrange disorder for different 2DXene materials is also analyzed in detail. Our numerical simulations confirm the role of spinorbit coupling in achieving an improved valley filter performance with a perfect quantum of conductance attributed to the topologically protected interface states. Our analysis further elaborates clearly the right choice of material, device geometry and other factors that need to be considered while designing an optimized valleytronic filter device. 
Monday, March 14, 2022 4:12PM  4:24PM 
D59.00007: Quantum Oscillations in the Zeroth Landau Level: Serpentine Landau Fan and the Chiral Anomaly Trithep Devakul, Yves H Kwan, Shivaji Sondhi, Siddharth A Parameswaran We identify an unusual mechanism for quantum oscillations in nodal semimetals, driven by a single pair of Landau levels periodically closing their gap at the Fermi energy as a magnetic field is varied. These “zero Landau level” quantum oscillations (ZQOs) appear in the nodal limit where the zerofield Fermi volume vanishes and have distinctive periodicity and temperature dependence. We link the Landau spectrum of a twodimensional (2D) nodal semimetal to the Rabi model, and show by exact solution that, across the entire Landau fan, pairs of oppositeparity Landau levels are intertwined in a “serpentine” manner. We propose 2D surfaces of topological crystalline insulators as natural settings for ZQOs. In certain 3D nodal semimetals, ZQOs lead to oscillations of anomaly physics. We propose a transport measurement capable of observing such oscillations, which we demonstrate numerically. 
Monday, March 14, 2022 4:24PM  4:36PM 
D59.00008: Resistivity scaling in topological semimetal CoSi Hsin Lin, ShangWei Lien, Ion Garate, ChengYi Huang, Utkarsh Bajpai, ChuangHan Hsu, YiHsin Tu, TayRong Chang, Nicholas A Lanzillo, Gengchiau Liang, Arun Bansil, ChingTzu Chen Topological Weyl physics associated with anomalous transport properties provides a potential solution to the resistance bottleneck in metalinterconnect scaling. The Fermiarc surface states in topological semimetals can contribute substantially to conductivity even when the system size is large. We use the chiral topological semimetal CoSi as a model system to demonstrate the decreasing resistivity with scaling. Wannierfunctionbased tightbinding models derived from firstprinciples calculations are used to calculate both the surface and bulk scattering rates due to the vacancies. Thickness dependence of resistivity in (001) CoSi slabs is investigated theoretically via a semiclassical approach. For sufficiently thin slabs, the surface current dominates. As a result, the film resistivity decreases with decreasing thickness even in the presence of defects, in sharp contrast to the resistivity scaling in conventional metals. Our study shows the promise of considering topological semimetals as candidate materials for the beyondCu scaled interconnects. 
Monday, March 14, 2022 4:36PM  4:48PM 
D59.00009: Anomaly indicators and bulkboundary correspondences for threedimensional interacting topological crystalline phases with mirror and continuous symmetries Binbin Mao, Chenjie Wang, ShangQiang Ning, Zhengqiao Li We derive a series of quantitative bulkboundary correspondences for 3D bosonic and fermionic symmetry protected topological (SPT) phases under the assumption that the surface is gapped, symmetric, and topologically ordered, i.e., a symmetryenriched topological (SET) state. We consider those SPT phases that are protected by the mirror symmetry and continuous symmetries that form a group of U(1), SU(2), or SO(3). In particular, the fermionic cases correspond to a crystalline version of 3D topological insulators and topological superconductors in the famous tenfoldway classification, with the timereversal symmetry replaced by the mirror symmetry and with strong interaction taken into account. Based on the previously proposed dimensional reduction and folding approaches, we rederive the classification of bulk SPT phases and define a complete set of bulk topological invariants for every symmetry group under consideration and then derive explicit expressions of the bulk invariants in terms of surface topological properties and symmetry properties. These expressions are our quantitative bulkboundary correspondences. Meanwhile, the bulk topological invariants can be interpreted as anomaly indicators for the surface SETs which carry 't Hooft anomalies of the associated symmetries whenever the bulk is topologically nontrivial. Hence, the quantitative bulkboundary correspondences provide an easy way to compute the 't Hooft anomalies of the surface SETs. Moreover, our anomaly indicators are complete. Our derivations of the bulkboundary correspondences and anomaly indicators are explicit and physically transparent. The anomaly indicators obtained in this work can be straightforwardly translated to their timereversal counterparts that apply to the usual topological insulators and topological superconductors, due to a known correspondence between mirror and timereversal topological phases. 
Monday, March 14, 2022 4:48PM  5:00PM 
D59.00010: Correlated disorderinduced anomalous transport in timereversal symmetry breaking topological insulator Tanay Nag Much having explored in the area of random uncorrelated disorder driven topological phase transitions (TPTs), we uncover that the topological Anderson insulator (TAI) phases can be induced by correlated disorder in magnetically doped topological insulator thin film. We undergo a large scale transport simulation to compute the conductance through a central disordered region coupled to reservoirs in the quantum spin Hall (QSH) regime. The quasiperiodic nature of the potential allows us to explore rich phase diagrams, consisting of normal insulator, quantum anomalous Hall (QAH), quantum spin Chern (QSC), QSH and AI phases, in terms of magnetic exchange field, the Fermi level, and the initial nontopological/ topological phase, for its various orientations in the twodimensional plane. For isotropic quasiperiodic potential and correlated disorder, we find quantized conductance from extended bulk bands before entering into QAH and QSC phases. We obtain uninterrupted edge transport for transverse quasiperiodicity as well as correlated disorder while reentrant QAH phase exists (vanishes) for longitudinal quasiperiodicity (correlated disorder) in the moderate amplitude. We successfully explain the TPTs by the renormalized negative mass via the self consistent Born approximation. 
Monday, March 14, 2022 5:00PM  5:12PM 
D59.00011: Adding Doublons to a FloquetTopological Insulator Helena Drueeke, Dieter Bauer We characterize a Floquettopological insulator on a finite square lattice with a linear defect in the form of an additional onsite potential along the diagonal. 
Monday, March 14, 2022 5:12PM  5:24PM 
D59.00012: Fermi arc reconstruction at the interface of twisted Weyl semimetals Faruk Abdulla, Sumathi Rao, Ganpathy N Murthy Threedimensional Weyl semimetals have pairs of topologically protected Weyl nodes whose projections onto the surface Brillouin zone are the end points of zeroenergy surface states called Fermi arcs. At the endpoints of the Fermi arcs, surface states extend into and are hybridized with the bulk. Here, we consider a twodimensional junction of two identical Weyl semimetals whose surfaces are twisted with respect to each other and tunnel coupled. Confining ourselves to commensurate angles (such that a larger unit cell preserves a reduced translation symmetry at the interface) enables us to analyze arbitrary strengths of the tunnel coupling. We study the evolution of the Fermi arcs at the interface, in detail, as a function of the twisting angle and the strength of the tunnel coupling. We show unambiguously that in certain parameter regimes, all surface states decay exponentially into the bulk, and the Fermi arcs become Fermi loops without endpoints. We show that changes in the connectivity of the Fermi arcs/loops have interesting signatures in the optical conductivity in the presence of a magnetic field perpendicular to the surface. 
Monday, March 14, 2022 5:24PM  5:36PM 
D59.00013: The generalized NielsenNinomiya Theorem of nonHermitian lattices for the 17 wallpaper groups ChingKai Chiu, Congcong Le, ZheSen Yang In a 2D Brillouin zone of a nonHermitian lattice, Fermi points and exceptional points can be topologically protected and immune from any perturbations. These two types of topological points separately obey the NielsenNinomiya doubling theorem since the total charges of the points must be neutralized. However, in the presence of crystalline symmetry, the additional symmetries limit the possible number of the topological points so that the minimal number of the topological points might be more than two; thus, the doubling theorem does not indicate doubling of the points. In this talk, we show the minimal numbers of the nonHermitian topological points for the 17 wallpaper groups and use this generalized NielsenNinomiya theorem to study the topology of the 3D nonHermitian bulks. 
Monday, March 14, 2022 5:36PM  5:48PM 
D59.00014: Lindhard function, optical conductivity and plasmon mode of a linear triple component fermionic system Bashab Dey We investigate the nature of density response of linear triple component fermions by computing the Lindhard function, dielectric function, plasmon mode and long wavelength optical conductivity of the system and compare the results with those of Weyl fermions and three dimensional free electron gas. Linear triple component fermions are the low energy quasiparticles of linear triple component semimetals, consisting of linearly dispersive and dispersionless (flat band) excitations. The presence of flat band brings about notable modifications in the response properties with respect to Weyl fermions such as induction of a new region in the particlehole continuum, reduced plasmon energy gap, shift in absorption edge, enhanced rate of increase in energy absorption with frequency and forbidden intercone transitions in the long wavelength limit. The plasmon dispersion follows the usual ω ≈ ω_{0}+ ω_{1}q^{2} nature as observed in most of the three dimensional electronic systems. 
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