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: Non-local 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 time-reversal 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 non-local 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 particle-hole symmetry, all clean topological semimetals can be realized in two thermodynamically distinct forms: a power-law, 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 Dirac-Fermi liquids Prachi Sharma, Dmitrii Maslov A Dirac-Fermi liquid (DFL) —a doped system with Dirac spectrum—is a special and important subclass of non-Galilean-invariant Fermi liquids (FLs), which includes, e.g., monolayer graphene and surface states of three-dimensional 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 Galilean-invariant FL it vanishes as q2. 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 two-dimensional (2D) system with q1/2 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 random-phase approximation. We calculate the self-energy, Maki-Thompson, and Aslamazov-Larkin diagrams for a dynamically screened Coulomb potential and find that χ''(q, ω) scales as q2ω and the damping rate scales as q2. 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)∼C1 + C2 cos2η, 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: Low-field 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 low-field Hall conductivity of nonmagnetic metals has an anomalous (Berry-curvature) contribution in addition to the ordinary (Lorentz-force) one. In noncentrosymmetric crystals it goes as σAHE ∝ ∫ d3k Ωk (mk°B)f0', where Ωk and mk are the Berry curvature and intrinsic magnetic moment (spin plus orbital) imparted on the Bloch states by the broken inversion symmetry; since Ωk and mk are both odd under time reversal, k and −k contribute equal amounts to σAHE. In centrosymmetric cystals the bands are Kramers degenerate, and the expression for σAHE involves the trace of the product of 2 × 2 matrices describing the Berry currvature and magnetic moment of the degenerate states; in this case TrΩk = Trmk = 0, but Tr(Ωkmk) = Tr(Ω−km−k) ≠ 0. Working in this non-Abelian setting, we develop a Wannier-interpolation scheme to calculate the low-field anomalous Hall conductivity from first principles. As a by-product, we obtain the anomalous g factors of the Bloch states. The low-field 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, Jhih-Sheng Wu, Vadym Apalkov, Mark I Stockman For Weyl semimetals, we predict that an intense ultrafast (single-oscillation) 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 non-Abelian Berry curvature. |
Tuesday, March 3, 2020 9:12AM - 9:24AM |
F55.00007: Nodal Chain Semimetal in Geometric Frustrated Acoustic metamaterial Meng Xiao, Xiao-Qi 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 tight-binding and full-wave numerical simulations. Moreover, we observe a fan-like surface state spectrum, whose dispersion is controlled by the bulk band properties. Our work points to a new class of band degeneracy that carries non-zero 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 Jun-Won Rhim, Bohm-Jung 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 π-Berry 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 auxiliary-field quantum Monte Carlo and mean-field 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 auxiliary-field quantum Monte Carlo calculations and mean-field 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 bond-density order. Finally, we present preliminary results on a 3D extension of the model, with direct relevance to the case of MoTe2. These high-accuracy results are an important step towards a many-body description of strongly-correlated 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 Kadanoff-Baym self-consistent resummation Sebastian Mantilla, Inti Sodemann The optical conductivity of 2D Dirac fermions at low energies is controlled by fundamental constants of nature σ0=e2/16 \hbar. However, Coulomb interactions produce a non-trivial dependence of the conductivity with the frequency. We use a bosonization approach to implement exactly a self-consistent Kadanoff-Baym resummation of the electron-hole propagator by mapping the momentum space lattice onto a Heisenberg-type 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 three-band graphene-like model Sumiran Pujari, Ankur Das Two-dimensional 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 three-band 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 tight-binding free fermion model on a two-dimensional graphene-like lattice where the threefold degeneracies are realized at fine-tuned points. Away from them, we obtain new “three-band” Dirac cone structures with associated nonstandard Landau level quantization, whose organization is strongly affected by the non-SU(2) or beyond-Dirac geometric phase structure of the fine-tuned points. |
Tuesday, March 3, 2020 10:12AM - 10:24AM |
F55.00012: Topological Semimetals with Butterfly-like Four-Point Intersecting Ellipses XIAOTING ZHOU, Hugo Aramberri, Cheng-Yi 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 butterfly-like four-point-intersecting-ellipses (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 spin-orbital 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 first-principles 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 Sign-Free Electronic Model Xiao Yan Xu, Tarun Grover It is widely believed that repulsive on-site 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 spin-liquid ala resonating valence bond (RVB) picture. Motivated by such folklore, here we construct a sign-problem free model that has unconventional f-wave 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 spin-liquid phases in its vicinity. We also show using field-theoretic methods that a direct deconfined quantum critical point is allowed between the superconductor and the Neel phase. |
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F55.00014: Cyclotron orbit knot and tunable-field quantum Hall effect Yi Zhang The Bohr-Sommerfeld 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 trefoil-knot geometry and study the corresponding quantum oscillations. Interestingly, unlike the conventional ring-shaped cyclotron orbit, a trefoil knot is self-threading, 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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