Bulletin of the American Physical Society
APS March Meeting 2020
Volume 65, Number 1
Monday–Friday, March 2–6, 2020; Denver, Colorado
Session F55: Dirac and Weyl Semimetals: Theory II 
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Sponsoring Units: DCMP Room: Mile High Ballroom 2B 
Tuesday, March 3, 2020 8:00AM  8:12AM 
F55.00001: Nonlocal annihilation mechanism in momentum space for correlated Weyl fermions. Giorgio Sangiovanni, Adriano Amaricci, Lorenzo Crippa, Niklas Wagner, Jan Budich, Massimo Capone We discuss the effects of interaction in Weyl semimetals with broken timereversal symmetry. The spin degeneracy of the gapless Dirac point at the topological quantum phase transition is resolved resulting in two gapless Weyl nodes separated in momentum space. For free fermions, the topological transition requires the two Weyl nodes to annihilate continuously in momentum space. Yet, we show that in presence of a stron local Coulomb repulsion this paradigm breaks down, opening a nonlocal annihilation channel for the Weyl cones. 
Tuesday, March 3, 2020 8:12AM  8:24AM 
F55.00002: Quantum Lifshitz criticality of topological semimetals Shouvik Sur, Alexander Tyner, Pallab Goswami In the absence of particlehole symmetry, all clean topological semimetals can be realized in two thermodynamically distinct forms: a powerlaw, incompressible phase and a compressible phase, respectively possessing zero and constant density of states at the band touching energy. Therefore, the density of states can serve as an order parameter for describing quantum Lifshitz transitions between two thermodynamically distinct but topologically equivalent states of matter. We address the nature of such quantum Lifshitz transitions for different topological semimetals, and show how they can strongly influence thermodynamic, spectroscopic, and transport properties of many quantum materials. 
Tuesday, March 3, 2020 8:24AM  8:36AM 
F55.00003: Intrinsic Plasmon damping in DiracFermi liquids Prachi Sharma, Dmitrii Maslov A DiracFermi liquid (DFL) —a doped system with Dirac spectrum—is a special and important subclass of nonGalileaninvariant Fermi liquids (FLs), which includes, e.g., monolayer graphene and surface states of threedimensional topological insulators. The lack of Galilean invariance leads to some interesting features not encountered in conventional Fermi liquids. Namely, the dissipative part of the conductivity of a DFL stays finite at q→0, whereas for a Galileaninvariant FL it vanishes as q^{2}. We explore the consequences of this fundamental difference for the intrinsic damping of plasmons in DFL. Charge density fluctuation leads to a collective mode, plasmon, in a twodimensional (2D) system with q^{1/2 }dispersion. The imaginary part of charge susceptibility, χ''(q, ω), is directly related to the damping rate of the plasmon mode. We obtain the explicit form of χ''(q, ω) for DFL, by going beyond the randomphase approximation. We calculate the selfenergy, MakiThompson, and AslamazovLarkin diagrams for a dynamically screened Coulomb potential and find that χ''(q, ω) scales as q^{2}ω and the damping rate scales as q^{2}. We show that the same result follows from the Einstein relation between the conductivity and charge susceptibility. 
Tuesday, March 3, 2020 8:36AM  8:48AM 
F55.00004: Internodal tunnelling and magnetotransport in Weyl semimetals Sergey Syzranov, Grigory Bednik, Konstantin Tikhonov Internodal dynamics of quasiaprticles in Weyl semimetals manifests itself in hydrodynamic, transport and thermodynamic phenomena and is essential for potential valleytronic applications of these systems. Charge transfer between the nodes in a Weyl semimetal is often considered as an inelastic process, with the charge carriers quickly thermalising at each node. In an external magnetic field, however, coherent quasipaticle tunnelling between the nodes may lead to a significant modification of the quasiparticle dispersion in a clean Weyl semimetal for certain directions of the magnetic field. In particular, it results in the opening of a gap in the quasiparicle dispersion, whose magnitude depends exponentially on the magnetic field. We study the interplay of such tunnelling with magnetotransport in a Weyl semimetal. We compute microscopically the longitudal resistivity of a disordered Weyl semimetal with two nodes in a strong magnetic field and demonstrate that it has a strong angular dependence ρ(η)∼C_{1} + C_{2} cos^{2}η, where η is the angle between the field and the line connecting the nodes. The first term is determined by the coherent internodal coupling and depends exponentially on the magnetic field, while the second term is independent of this coupling. 
Tuesday, March 3, 2020 8:48AM  9:00AM 
F55.00005: Lowfield anomalous Hall effect in nonmagnetic metals by Wannier interpolation. Stepan Tsirkin, Ivo Souza In ferromagnets, the Bloch states acquire a Berry curvature that produces a Hall effect at B = 0: the anomalous Hall effect (AHE). In nonmagnetic metals the Hall effect only appears at linear order in B. Interestingly, the lowfield Hall conductivity of nonmagnetic metals has an anomalous (Berrycurvature) contribution in addition to the ordinary (Lorentzforce) one. In noncentrosymmetric crystals it goes as σ_{AHE }∝ ∫ d^{3}k Ω_{k} (m_{k°}B)f_{0}', where Ω_{k} and m_{k} are the Berry curvature and intrinsic magnetic moment (spin plus orbital) imparted on the Bloch states by the broken inversion symmetry; since Ω_{k} and m_{k} are both odd under time reversal, k and −k contribute equal amounts to σ_{AHE}. In centrosymmetric cystals the bands are Kramers degenerate, and the expression for σ_{AHE }involves the trace of the product of 2 × 2 matrices describing the Berry currvature and magnetic moment of the degenerate states; in this case TrΩ_{k} = Trm_{k} = 0, but Tr(Ω_{k}m_{k}) = Tr(Ω_{−k}m_{−k}) ≠ 0. Working in this nonAbelian setting, we develop a Wannierinterpolation scheme to calculate the lowfield anomalous Hall conductivity from first principles. As a byproduct, we obtain the anomalous g factors of the Bloch states. The lowfield AHE is present in all metals, but is more pronounced in Weyl and Dirac semimetals. 
Tuesday, March 3, 2020 9:00AM  9:12AM 
F55.00006: Ultrafast topological phenomena in Weyl semimetals Fatemeh Nematollahi, JhihSheng Wu, Vadym Apalkov, Mark I Stockman For Weyl semimetals, we predict that an intense ultrafast (singleoscillation) circularly polarized optical pulse can induce a large population of the conduction band. The response of Weyl points to an ultrafast laser field is different and so the conduction population is highly textured which we call it topological resonance. The topological resonance is due to the Bloch motion of electrons in the reciprocal space where electron population textures are formed due to nonAbelian Berry curvature. 
Tuesday, March 3, 2020 9:12AM  9:24AM 
F55.00007: Nodal Chain Semimetal in Geometric Frustrated Acoustic metamaterial Meng Xiao, XiaoQi Sun, Shanhui Fan Geometrically frustrated systems and topological semimetals have both attracted much interest and been studied in various systems in recent years. Here we study the interplay between these two systems. We show that a Weyl point can be extended to a chain of degeneracy (a nodal chain) with nonzero charge of Berry flux through geometrical frustration. We propose to realize such a charged nodal chain in an acoustic metamaterial, based on both tightbinding and fullwave numerical simulations. Moreover, we observe a fanlike surface state spectrum, whose dispersion is controlled by the bulk band properties. Our work points to a new class of band degeneracy that carries nonzero Berry flux. The resulting topological metamaterial may be useful for controlling the flow of sound and light. 
Tuesday, March 3, 2020 9:24AM  9:36AM 
F55.00008: Quantum distance and anomalous Landau levels of flat bands JunWon Rhim, BohmJung Yang Semiclassical quantization of electronic states under magnetic field, advocated by Onsager, can successfully describe not only the Landau level spectrum but also the geometric responses under magnetic field in metals. Even in graphene with relativistic energy dispersion, for instance, Onsager’s rule correctly described the πBerry phase as well as the unusual Landau level spectrum of Dirac particles. Here we show that, however, the semiclassical quantization rule completely breaks down for a class of dispersionless flat bands, dubbed the singular flat bands. The singular flat band has a band crossing with another dispersive band enforced by band flatness. In contrast to the conventional isolated flat band which does not respond to external magnetic field, the singular flat band shows anomalous Landau level structures and magnetic responses. We discuss the novel quantum geometric interpretations of them regarding the quantum distance, and how to measure them experimentally. 
Tuesday, March 3, 2020 9:36AM  9:48AM 
F55.00009: Superconductivity in interacting Weyl models: A combined auxiliaryfield quantum Monte Carlo and meanfield study Peter Rosenberg, Niraj Aryal, Efstratios Manousakis The discovery of superconducting Weyl semimetals has sparked an intense effort to understand the connection between topology and correlations in these materials, specifically, the pairing behavior of emergent Weyl fermions. Here we study a simple 2D Weyl model with attractive interactions using a combination of numerically exact auxiliaryfield quantum Monte Carlo calculations and meanfield theory [1]. We focus on the rich pairing behavior that emerges from the interplay of the spin and sublattice degrees of freedom, as well as the bonddensity order. Finally, we present preliminary results on a 3D extension of the model, with direct relevance to the case of MoTe_{2}. These highaccuracy results are an important step towards a manybody description of stronglycorrelated Weyl materials and topological superconductors. 
Tuesday, March 3, 2020 9:48AM  10:00AM 
F55.00010: The optical conductivity of strongly interacting Dirac fermions: a bosonization approach to the KadanoffBaym selfconsistent resummation Sebastian Mantilla, Inti Sodemann The optical conductivity of 2D Dirac fermions at low energies is controlled by fundamental constants of nature σ_{0}=e^{2}/16 \hbar. However, Coulomb interactions produce a nontrivial dependence of the conductivity with the frequency. We use a bosonization approach to implement exactly a selfconsistent KadanoffBaym resummation of the electronhole propagator by mapping the momentum space lattice onto a Heisenbergtype model of interacting spins and employ this approach to determine the frequency dependence of the optical conductivity for Coulomb repulsions. We recover the perturbative renormalization group results at small coupling and extend its predictions to strong coupling. We discuss the relevance of our results to Dirac materials such as graphene and 3D topological inusulator surafce states. 
Tuesday, March 3, 2020 10:00AM  10:12AM 
F55.00011: SU(3) fermions in a threeband graphenelike model Sumiran Pujari, Ankur Das Twodimensional graphene is fascinating because of its unique electronic properties. From a fundamental perspective, one among them is the geometric phase structure near the Dirac points in the Brillouin zone, owing to the SU(2) nature of the Dirac cone wave functions. We ask if there are geometric phase structures in two dimensions that go beyond that of a Dirac cone. Here we write down a family of threeband continuum models of noninteracting fermions that have more intricate geometric phase structures. This is connected to the SU(3) nature of the wave functions near threefold degeneracies. We also give a tightbinding free fermion model on a twodimensional graphenelike lattice where the threefold degeneracies are realized at finetuned points. Away from them, we obtain new “threeband” Dirac cone structures with associated nonstandard Landau level quantization, whose organization is strongly affected by the nonSU(2) or beyondDirac geometric phase structure of the finetuned points. 
Tuesday, March 3, 2020 10:12AM  10:24AM 
F55.00012: Topological Semimetals with Butterflylike FourPoint Intersecting Ellipses XIAOTING ZHOU, Hugo Aramberri, ChengYi Huang, Maia G Vergniory, Hsin Lin, Nicholas Kioussis Recent years, the exotic properties of topological semimetals have garnered great attention and efforts in seeking for new topological phases and material realization. In this work, we introduce a new type of nodal line, the butterflylike fourpointintersectingellipses (FPIE) residing in a plane. We identify the criteria for the existence of the FPIE in a time reversal invariant spinless fermion system with negligible spinorbital coupling (SOC). In addition we demonstrate that its emergence is possible in 7 out of 230 space groups, and identify the locations it would emerge in the Brillouin zone (BZ). Using firstprinciples band structure calculations, we predict a family of compounds as candidates hosting the FPIE in the Fermi surface (FS) with vanishing SOC. 
Tuesday, March 3, 2020 10:24AM  10:36AM 
F55.00013: Unconventional Superconductivity and Mott Transition in a SignFree Electronic Model Xiao Yan Xu, Tarun Grover It is widely believed that repulsive onsite interactions between electrons can lead to unconventional superconductivity. Further, one expects that proximity of an unconventional superconductor to a Mott transition can lead to a quantum spinliquid ala resonating valence bond (RVB) picture. Motivated by such folklore, here we construct a signproblem free model that has unconventional fwave superconductivity, and a proximate Mott transition to a Neel ordered phase. We present preliminary results on the nature of transition between these two phases, and potential spinliquid phases in its vicinity. We also show using fieldtheoretic methods that a direct deconfined quantum critical point is allowed between the superconductor and the Neel phase. 

F55.00014: Cyclotron orbit knot and tunablefield quantum Hall effect Yi Zhang The BohrSommerfeld quantization of the cyclotron orbit in a magnetic field gives rise to discrete Landau levels and a series of fascinating quantum Hall phenomena. Here we consider topologically nontrivial physics from a distinct origin, where the cyclotron orbits take nontrivial knotting structure. We present a scenario of a Weyl semimetal slab, where the Fermi arcs on the opposing surfaces can cross without interfering with each other and form a knot together with the bulk chiral Landau levels. We provide a microscopic lattice model with cyclotron orbits of trefoilknot geometry and study the corresponding quantum oscillations. Interestingly, unlike the conventional ringshaped cyclotron orbit, a trefoil knot is selfthreading, allowing the magnetic field line along the cyclotron orbit to contribute to the overall Berry phase and therefore altering the external magnetic field for each quantization level. The cyclotron orbit knot offers an arena of the nontrivial knot theory in three spatial dimensions and its subsequent physical consequences. 
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