Bulletin of the American Physical Society
APS March Meeting 2020
Volume 65, Number 1
Monday–Friday, March 2–6, 2020; Denver, Colorado
Session A55: Dirac and Weyl Semimetals: Theory I 
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Sponsoring Units: DCMP Chair: Herbert Fertig, Indiana Univ  Bloomington Room: Mile High Ballroom 2B 
Monday, March 2, 2020 8:00AM  8:12AM 
A55.00001: Chiral Qubit: Implementing a Qubit Using Chiral Charge and Chiral Anomaly Evan Philip, Sahal Kaushik, Dmitri E Kharzeev Dirac and Weyl semimetals are capable, due to chiral anomaly, of a nearly dissipationless current in the presence of a chirality imbalance and magnetic field (Chiral Magnetic Effect). We propose an implementation of the qubit based on this. Superconducting qubits require low operating temperatures and are limited in operating frequency by the superconducting gap—two limitations that the "chiral qubit" could potentially overcome. I will present our theoretical model and estimates regarding the chiral qubit. 
Monday, March 2, 2020 8:12AM  8:24AM 
A55.00002: Twobody interaction induces axion/phason field in Weyl semimetals David Schmeltzer, Avadh Saxena Following our results that the twobody interaction can induce a space and time dependent topological axion term \theta(z,t)[(e^2)/(2\pi \hbar)](E.B), we show that by applying this theory to a Weyl semimetal with two nodes in a magnetic field a onedimensional sliding charge density wave (CDW) emerges which replaces the asymptotic effective topological representation, in agreement with the recent experimental finding of J. Gooth et al. [Nature, https://doi.org/10.1038/s4158601916304 (2019)]. 
Monday, March 2, 2020 8:24AM  8:36AM 
A55.00003: Chiral vector fieldinduced collective fermionic excitations in Weyl semimetals Alberto Cortijo, Erik van der Wurff We study the singleparticle fermionic spectrum of Weyl semimetals in three dimensions taking into account the axial vector character of the electronphonon interaction. We find that, together the expected standard quasiparticle peak, a second peak appears in the spectral function. The main features of this new fermionic excitation are to be intrinsically anisotropic and gapped. We will discuss also the potential impact of these collective fermionic modes on the transport phenomena in Weyl semimetals. 
Monday, March 2, 2020 8:36AM  8:48AM 
A55.00004: Theoretical Study of Resonant Inelastic Xray Scattering for Monopole Densitywave Order and Superconductivity in Weyl Semimetals Eric Bobrow, Peter Abbamonte, Yi Li Resonant Inelastic Xray Scattering (RIXS) spectra can provide useful information for phasesensitive bulk detection. A difference of RIXS spectra for crosspolarized Xrays has been previously proposed to detect the topological windings of singleparticle Weyl nodes in, for example, TaAs. We generalize this idea to investigate various topological excitations near the emergent Weyl nodes in exotic nodal superconducting and densitywave ordered Weyl semimetal systems characterized by monopole harmonic symmetry. 
Monday, March 2, 2020 8:48AM  9:00AM 
A55.00005: Nonlinear Planar Hall as Another Signature of Chiral Anomaly in Weyl Semimetals Ruihao Li, Shulei Zhang, Anton Burkov, Olle Heinonen Weyl semimetals (WSMs) are a newly discovered class of quantum materials which can host a number of exotic quasiparticles called Weyl fermions. One of the defining properties of WSMs is chiral anomaly – a pair of Weyl nodes with opposite chirality act as source and drain of electrons in the presence of electric and magnetic fields that are not perpendicular to each other. To date, the most remarkable phenomenon induced by chiral anomaly is the longitudinal negative magnetoresistance, which is a linear response effect. In this work, we theoretically investigate the transport properties of WSMs in the nonlinear regime, and predict a nonlinear planar Hall effect as another manifestation of chiral anomaly in transport. Intuitively, a steadystate density difference between a pair of Weyl nodes may be established when the chiral pumping and internode relaxation reach a balance, which conspires with anomalous velocity to give rise to the nonlinear Hall effect. In contrast to the intrinsic quantum nonlinear Hall effect, this effect does not rely on a finite Berry curvature dipole. 
Monday, March 2, 2020 9:00AM  9:12AM 
A55.00006: Viscosity as a probe of nodal topology Marianne Moore, Piotr Surowka, Vladimir Juricic, Bitan Roy Electronic materials can sustain a variety of unusual, but symmetry protected touchings of valence and conduction bands, each of which is identified by a distinct topological invariant. Well known examples include linearly dispersing pseudorelativistic fermions in monolayer graphene, Weyl and nodalloop semimetals, biquadratic (cubic) band touching in bilayer (trilayer) graphene, as well as mixed dispersions in multiWeyl systems. We here show that depending on the underlying band curvature, the shear viscosity in the collisionless regime displays unique powerlaw scaling with frequency at low temperatures, bearing the signatures of the band topology, which are distinct from the ones when the system resides at the brink of a topological phase transition into a band insulator. Therefore, besides density of states (governing specific heat, compressibility) and dynamic conductivity, shear viscosity can be instrumental to pin nodal topology in electronic materials. 
Monday, March 2, 2020 9:12AM  9:24AM 
A55.00007: Large enhancement of thermopower at low magnetic field in compensated Dirac/Weyl semimetals Xiaozhou Feng, Brian Skinner We theoretically study the thermoelectric properties of compensated Dirac/Weyl semimetals subjected to a magnetic field. Previous work has shown that Dirac/Weyl semimetals without compensation can exhibit a large enhancement of thermopower in the extreme quantum limit of magnetic field. Here we show that in compensated systems a huge enhancement of thermopower can be obtained even at much smaller magnetic field, provided that ω_{c}τ » 1. We discuss our results in light of recent measurements on the compensated Weyl semimetal tantalum phosphide, in which an enormous magnetothermoelectric effect was observed. 
Monday, March 2, 2020 9:24AM  9:36AM 
A55.00008: Charged Domain Wall of Electric Polarizations in Topological NodalLine Semimetals Akihiko Sekine, Naoto Nagaosa We study theoretically the electronic structure of threedimensional topological nodalline semimetals. We show that, in the presence of a gapopening spatially dependent mass term that forms a domain wall, an ingap charged localized mode emerges at the domain wall. It turns out that such a domain wall is realized by headtohead (or tailtotail) bulk electric polarizations. The localized mode has a topological origin, i.e., a topological confinement is realized, which is understood by a semiclassical topological number defined in the semiclassical momentumreal space. Namely, a stable charged domain wall of electric polarizations is realized in nodalline semimetals. In sharp contrast to the wellknown band bending mechanism, the origin of the charged domain wall in this study is purely electronic, i.e., due to the band topology. Moreover, in contrast to previous studies, our study demonstrates a topological confinement at the interface between two insulators without bulk topological numbers. The dispersion of the localized mode evolves from gapless to gapped as the bulk bandgap increases, which means that its conductivity is externally tunable. We discuss a possible experimental realization of the stable, electricallytunable charged domain wall. 
Monday, March 2, 2020 9:36AM  9:48AM 
A55.00009: Spin and PseudoSpin Collective Modes in Doped Graphene Abhishek Kumar, Dmitrii Maslov Collective modes in graphene has been an active area of research during the last decade. As any twodimensional electron system, graphene harbors a gapless plasmon mode which disperses as q^{1/2}. Gapped collective modes in graphene are, however, less explored. The goal of this work is twofold: a comprehensive study of collective modes in graphene (a) without spinorbit coupling (SOC) and (b) in the presence of Rashba and valleyZeeman SOC. For a shortrange electronelectron interaction, we find that in case (a) there exists a collective mode below the continuum, corresponding to oscillations of pseudospin. In case (b), we show that, in graphene with Rashba SOC, there exists a collective spin mode consisting of three branches: a doubledegenerate branch, corresponding to oscillations of inplane magnetization, and a nondegenerate branch corresponding to oscillations of out of plane magnetization. For graphene with valleyZeeman SOC, there exists only a single branch of the collective mode corresponding to out of plane oscillation of the magnetization. We also study the case of an unscreened Coulomb interaction, solved by reducing the sum of ladder diagrams for a vertex function to a Dirac equation with nonlocal interaction. 
Monday, March 2, 2020 9:48AM  10:00AM 
A55.00010: Quantum oscillations in Dirac magnetoplasmons Johannes Hofmann The plasmon frequency in standard electron gases with a parabolic singleparticle dispersion is a purely classical quantity that is not sensitive to electron interactions or the equation of state. We demonstrate that this canonical result no longer holds for plasmons in threedimensional semimetals, which can thus be used to probe manybody effects. In particular, we show that the plasmon frequency in an external magnetic field displays quantum oscillations, which is not the case for the electron gas. Using the random phase approximation, results are presented for the magnetoplasmon dispersion and the loss function in Dirac semimetals. We include a full discussion of the loss function in a magnetic field as a function of the direction of propagation with respect to the magnetic field direction and discuss the transition from large magnetic fields to the lowfield limit. 
Monday, March 2, 2020 10:00AM  10:12AM 
A55.00011: Fermiarcs and nonlocal collective modes in thinfilm Weyl systems Sonu Verma, Debasmita Giri, Herbert Fertig, Arijit Kundu Fermiarcs in Weyl semimetals carry signatures of their topological origin. In a thinfilm geometry, in the presence of Coulomb interactions, particlehole excitations can take place either on the same surface, or between the two surfaces, giving rise to the possibility of unique nonlocal collective modes. We study, in particular, plasmonic modes that are nonlocally bound in a thinfilm Weyl semimetal carrying the signature of bulk and surface mixing and analyze the possibility of their application. 
Monday, March 2, 2020 10:12AM  10:24AM 
A55.00012: Effect of charge renormalization on electric and thermoelectric transport along the vortex lattice of a Weyl superconductor Gal Lemut, Michal Pacholski, Inanc Adagideli, Carlo W J Beenakker Building on the discovery that a Weyl superconductor in a magnetic field supports chiral Landau level motion along the vortex lines, we investigate its transport properties out of equilibrium. We show that the vortex lattice carries an electric current I=1/2(Q^{2}_{eff}/h)(Φ/Φ_{0})V between two normal metal contacts at voltage difference V, with Φ the magnetic flux through the system, Φ_{0 }the superconducting flux quantum, and Q_{eff}<e the renormalized charge of the Weyl fermions in the superconducting Landau level. Because the charge renormalization is energy dependent, a nonzero thermoelectric coefficient appears even in the absence of energydependent scattering processes. 
Monday, March 2, 2020 10:24AM  10:36AM 
A55.00013: Effect of Disorder on Edge States in Nodal Topological Materials Saumitran Kasturirangan, Xuzhe Ying, Fiona Burnell, Alex Kamenev Many nodal topological materials exhibit gapless flatband boundary modes which are protected by nontrivial topology in momentum space. Here we ask the question of whether, as is the case for weak topological insulators [1], these boundary flatbands maintain their integrity in the presence of disorder – and whether this leads to observable signatures in transport. To do this, we study the effects of various types of quenched shortrange disorder on the energy spectrum, localization, and transport properties of the boundary states of a 2D topological semimetal in symmetry class BDI. Our primary example is graphene with timereversal and particlehole symmetry. We discuss the implications of our results for experimentally detecting the boundary modes of nodal topological superconductors in 2D. References: [1] Z. Ringel, Y. E. Kraus, A. Stern, Phy. Rev. B, 86, 045102 (2012) 
Monday, March 2, 2020 10:36AM  10:48AM 
A55.00014: Topologically protected Landau level in the vortex lattice of a Weyl superconductor Michal Pacholski, Carlo W J Beenakker, Inanc Adagideli The question whether the mixed phase of a gapless superconductor can support a Landau level is a celebrated problem in the context of dwave superconductivity, with a negative answer: The scattering of the subgap excitations (massless Dirac fermions) by the vortex lattice obscures the Landau level quantization. Here we show that the same question has a positive answer for a Weyl superconductor: The chirality of the Weyl fermions protects the zeroth Landau level by means of a topological index theorem. As a result, the heat conductance parallel to the magnetic field has the universal value G=½g_{0}Φ/Φ_{0}, with Φ the magnetic flux through the system, Φ_{0} the superconducting flux quantum, and g_{0} the thermal conductance quantum. 

A55.00015: Nonlocal Response Mediated by Weyl Orbits Zhe Hou, QingFeng Sun Nonlocality is always an interesting topic in quantum physics and is usually mediated by some unique quantum states. Nonlocal phenomena in three dimensional systems mediated by trivial bulk states are rarely reported however. Here we investigate a Weyl semimetal slab and find an exotic nonlocal correlation effect when placing two potential wells merely on the top and bottom surfaces. This nonlocal response is totally carried by the bulk states inside the Weyl orbit, which is a result of the peculiar band structure of Weyl semimetals. A giant nonlocal transport signal and a body breakdown by the Weyl fermions are further uncovered, which can serve as signatures for verifying this nonlocal phenomenon experimentally. Our results extend a new member in the nonlocality family and have potential applications for designing new electric devices with fancy functions. 
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