Bulletin of the American Physical Society
APS March Meeting 2022
Volume 67, Number 3
Monday–Friday, March 14–18, 2022; Chicago
Session Z59: Dirac and Weyl Semimetals: Theory IIRecordings Available

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Sponsoring Units: DCMP Chair: Nitin Prasad, University of Texas Room: Hyatt Regency Hotel DuSable AB 
Friday, March 18, 2022 11:30AM  11:42AM 
Z59.00001: Vortical effects in chiral band structures using Kubo's formula Swadeepan Nanda, Pavan R Hosur Recently, transport phenomena in Weyl semimetals have attracted much research attention in condensed matter physics. The chiral vortical effect  a chiral anomalyinduced transport phenomenon characterized by an axial current in a rotating chiral fluid  is wellstudied in high energy physics, but its realization in condensed matter is much less understood. In this work, we derive the response of electrons in a general band structure to rotation at nonzero temperature T, wavevector q, and frequency Ω. Unlike the hydrodynamic and Boltzmannequationbased approaches used in highenergy physics, we use the Kubo formula which is more quantummechanical and easier to extend to finite q and Ω and general band structures. For a Weyl fermion at T=0, we recover the highenergy physics results in the static limit (Ω→0 before q→0$) but not in the uniform limit (q→0 before Ω→0), and trace the difference in results to vanishing contribution from intraband terms in the latter limit. 
Friday, March 18, 2022 11:42AM  11:54AM 
Z59.00002: Interacting Chiral Anomaly Alireza Parhizkar, Colin Rylands, Anton Burkov, Victor M Galitski The chiral anomaly is a fundamental quantum mechanical phenomenon which is of great importance to both particle physics and condensed matter physics alike. In the context of QED, it manifests as the breaking of chiral symmetry in the presence of electromagnetic fields. It is also known that anomalous chiral symmetry breaking can occur through interactions alone, as is the case for interacting onedimensional systems. We will discuss the interplay between these two modes of anomalous chiral symmetry breaking in the context of interacting Weyl semimetals. Using Fujikawa’s path integral method, we show that the chiral charge continuity equation is modified by the presence of interactions which can be viewed as including the effect of the electric and magnetic fields generated by the interacting quantum matter. This can be understood further using dimensional reduction and a Luttinger liquid description of the lowest Landau level. These effects manifest themselves in the nonlinear response of the system. In particular, we find an interactiondependent density response due to a change in the magnetic field as well as a contribution to the nonequilibrium and inhomogeneous anomalous Hall response while preserving its equilibrium value. 
Friday, March 18, 2022 11:54AM  12:06PM 
Z59.00003: Anomalous electromagnetic field transmission and reflection in Weyl and Dirac semimetals Pavlo Sukhachov, Leonid Glazman The current response to an electromagnetic field in a Weyl or Dirac semimetal becomes nonlocal due to the chiral anomaly activated by an applied magnetic field. The nonlocality develops under the conditions of the normal skin effect and is related to the valley charge imbalance generated by the joint effect of the electric field of the impinging wave and the static magnetic field. The length scale for the nonlocality is determined by the diffusion length of the valley charge imbalance, which does not violate the local electric charge neutrality. We predict that the signatures of this nonlocality can be found in the transmission and reflection of electromagnetic waves. In view of a weaker decay of the anomalous components in the nonlocal regime, it is possible to achieve an enhancement of the electromagnetic wave penetration depth. In the reflection of electromagnetic waves, the chiral anomaly leads to the decrease of the dissipative part of the surface impedance with the magnetic field; these anomalous effects are reduced in the nonlocal regime. 
Friday, March 18, 2022 12:06PM  12:18PM 
Z59.00004: Higher Rank Chiral Fermions in 3D Weyl Semimetals Oleg Dubinkin, Taylor L Hughes, Fiona J Burnell We report on exotic response properties in 3D timereversal invariant Weyl semimetals with mirror symmetry. Despite having a vanishing anomalous Hall coefficient, we find that the momentumspace quadrupole moment formed by four Weyl nodes determines the coefficient of a mixed electromagnetic chargestress response, in which momentum flows perpendicular to an applied electric field, and electric charge accumulates on certain types of lattice defects. This response is described by a mixed ChernSimonslike term in 3 spatial dimensions, which couples a rank2 gauge field to the usual electromagnetic gauge field. On certain 2D surfaces of the bulk 3D Weyl semimetal, we find what we will call rank2 chiral fermions, with ω = k_{x}k_{y} dispersion. The intrinsically 2D rank2 chiral fermions have a mixed chargemomentum anomaly which is cancelled by the bulk of the 3D system. 
Friday, March 18, 2022 12:18PM  12:30PM 
Z59.00005: Quantumlimit Hall effect with large carrier density in topological semimetals Guang Yang The quantumlimit Hall effect at ν = nh/eB ∼ O(1) that hosts a variety of exotic quantum phenomena requires demanding strong magnetic field B and low carrier density n. We propose to realize the quantumlimit Hall effect even in the presence of large carrier density residues n_e and n_h in a relatively weak and variable magnetic field B in topological semimetals, where a single Fermisurface contour allows both electrontype and holetype carriers and approaches charge neutrality when n_e ≈ n_h. The underlying filling factor ν = n_e − n_hh/eB also offers an example that violates Onsager's relation for quantum oscillations. 
Friday, March 18, 2022 12:30PM  12:42PM 
Z59.00006: Quantum Hall effect in Dirac semimetal films Junyi Zhang, Jinwoong Kim, David Vanderbilt, Yi Li To understand quantum Hall effects in films of 3D Dirac semimetals observed in recent experiments, we studied the contributions of various topological boundary states contributions. We found that there can be a Lifshitzlike transition with reconnection of surface Fermi arcs. Particularly, as the chemical potential varies, open Fermi arcs may loop around with its two ends connecting the same surface projected Dirac point. This allows a doubled frequency in the quantum oscillations via Möbius Weyl orbits as the electron loops twice at the top and bottom surfaces respectively and tunnels through the bulk at the Dirac point in between, which traces the boundary of a Möbius strip. In addition, we also discussed the interplay between the contributions from the edge states and bulk subbands to the quantum Hall effect. 
Friday, March 18, 2022 12:42PM  12:54PM 
Z59.00007: Intrinsic Optical Absorption in Dirac Metals Adamya P Goyal, Prachi Sharma, Dmitrii Maslov In an ideal Dirac metal, optical absorption is absent for frequencies below the Pauli threshold (twice the Fermi energy). In real systems, however, e.g., in doped graphene, both optical absorption [1] and Raman scattering [2] find a very broad transition region around the Pauli threshold. While a number of extrinsic damping mechanisms were proposed to explain this observation in the past, we argue that the effect can be explained by an intrinsic mechanism  Augerlike recombination of optically excited minority carriers with equilibrium majority carriers. The idea goes back to a similar mechanism proposed for doped gapped semiconductors by Gavoret et al [3]. We also discuss certain electronhole processes that give a T^{2} scaling to the d.c. conductivity and could possibly be detected in 3dimensional Dirac systems. 
Friday, March 18, 2022 12:54PM  1:06PM 
Z59.00008: Orbital physics in metals and insulators Armando P Pezo, Aurelien Manchon, Diego Ovalle The investigation of novel means to control the orbital degree of freedom is currently gaining momentum. Among the various effects that are under scrutiny, we focus on the orbital Hall effect, i.e., the development of a chargeneutral current, transverse to the direction of injection and carrying orbital angular momentum. To date, most theories have focused on the intraatomic orbital degree of freedom, associated with the motion of the electron around the atom. In this work, we show that the orbital Hall effect does not only arise from intraatomic contribution but also possesses a substantial interatomic contribution, associated with the gyrotropic selfrotation of the electron wavepacket. We emphasize the connection of this contribution to the Berry curvature dipole and computed both intraatomic and interatomic contributions of the orbital Hall effects in selected systems. Using firstprinciples calculations and Kubo formula, we demonstrate that the interatomic orbital Hall effect can be substantial and sometimes even exceed the intraatomic contribution. We believe that this work is of great importance in the emergent field of orbitronics as wells as in spintronics where the interplay between spin and orbital degrees of freedom plays an essential role. 
Friday, March 18, 2022 1:06PM  1:18PM 
Z59.00009: Gravitational anomaly in nonlinear charge transport Tobias Holder, Daniel Kaplan, Roni Ilan, Binghai Yan We show that the flow of electrons necessarily involves a deformation of the semiclassical wavepacket, and argue that the effective quasiparticle motion therefore takes place in nonEuclidian space. As an important application, we discuss the semiclassical equations of motion for materials lacking timereversal symmetry, which exhibit a mixed axialgravitational anomaly. We predict an anomaly current at second order in the electric field and without applied magnetic field. 
Friday, March 18, 2022 1:18PM  1:30PM 
Z59.00010: Artificial event horizons in Weyl semimetal heterostructures Christophe De Beule, Solofo Groenendijk, Tobias Meng, Thomas L Schmidt Heterostructures comprising typeI and typeII Weyl semimetals give rise to effective spacetimes featuring black or white hole event horizons [1] with the prospect of electronic analogs of Hawking radiation and gravitational lensing, among others. We investigate transport in this system using a minimal model for a Weyl semimetal with a positiondependent tilt of the Weyl cone. We provide an exact solution to the scattering problem at normal incidence for energies close to the Weyl node, both for a fast and a slow varying tilt profile relative to the Fermi wavelength. In the latter case, we find analogous Hawking radiation in two transport channels which cancel one another in equilibrium. We show that one can favor the contribution of one channel using a nonequilibrium state, either by irradiating the typeII region or by injecting a spinpolarized current. This in turn gives rise to a characteristic peak in the twoterminal differential conductance which can serve as an experimental indicator of the artificial event horizon. 
Friday, March 18, 2022 1:30PM  1:42PM Withdrawn 
Z59.00011: Electron scattering by magnetic impurity in Weyl semimetals Álvaro Díaz Fernández, Francisco DominguezAdame, Óscar de Abril Weyl semimetals are prominent examples of topologically protected quantum matter. These materials are the threedimensional counterparts of graphene and great efforts are being devoted to achieve a thorough understanding of their fundamental physics. In this work, we aim at contributing to this end by discussing the effect of a single magnetic impurity in Weyl semimetals as a first step towards considering a larger number of pointlike impurities. We find that resonances appear in the local density of states (LDOS) with a Friedellike behaviour, oscillating as a function of distance. By studying the spinresolved LDOS, we can observe nontrivial and anisotropic spin textures where the spin components perpendicular to the spin of the impurity wind around the latter, until the spin becomes completely parallel to the impurity right at the impurity's location. Friedel oscillations also play a relevant role in the form of the spin textures, forming an oscillatory pattern. We believe our results can pave the way to further studies which consider the presence of a large number of random magnetic impurities. 
Friday, March 18, 2022 1:42PM  1:54PM 
Z59.00012: The robustness of triple point fermions against disorder HsiuChuan Hsu, Ion C Fulga, JhihShih You Triple point fermions (TPFs) are fermionic quasiparticles in solids with pseudospin degree of freedom 1 which do not have the counterpart in the standard model. A TPF node is a topological monopole with Chern number ±2. In this study, we show that the TPF nodes and Fermi arcs are robust against disorder, in contrast to the quadratic Weyl nodes shown in previous studies. Comparing the white noise and correlated disorder, we find that the internode scattering is the main mechanism for destroying the TPF nodes. Moreover, we compute the Berry curvature carried by the Fermi arcs and discuss the disorder effects on the surface Berry curvature. 
Friday, March 18, 2022 1:54PM  2:06PM 
Z59.00013: Less is more: Vacancyengineered nodalline semimetals Mariana Malard Sales Andrade A nodalline semimetal phase which is enforced by the symmetries of the material is interesting from fundamental and application standpoints. We demonstrate that such a phase of matter can be engineered by the following method: Introducing vacancies in certain configurations in common symmorphic materials leads to nonsymmorphic polymorphs with symmetryenforced nodal lines which are immune to symmetrypreserving perturbations, such as spinorbit coupling. Furthermore, the spectrum acquires also accidental nodal lines with enhanced robustness to perturbations. These phenomena are explained on the basis of a symmetry analysis of a minimal effective twodimensional model which captures the relevant symmetries of the proposed structures, and verified by firstprinciples calculations of vacancyengineered borophene polymorphs, both with vanishing and with strong Rashba spinorbit coupling. Our findings offer an alternative path to using complicated nonsymmorphic compounds to design robust nodalline semimetals; one can instead remove atoms 
Friday, March 18, 2022 2:06PM  2:18PM 
Z59.00014: A general framework for emergent Lorentz violation in Dirac and Weyl semimetals Babak Seradjeh, Alan Kostelecky, Navin McGinnis, Ralf Lehnert, Marco Schreck We use a general framework of Lorentz violation in quantum electrodynamics to study the breaking of emergent Lorentz symmetry in Dirac and Weyl materials. In addition to Weyl semimetals already reported in the literature, we also find Dirac nodal spectra with nodal lines and rings depending on the pattern of Lorentz violation. We further utilize this framework to characterize transport coefficients and report on the electromagnetic response due to all possible effects of Lorentz violation at leading order. 
Friday, March 18, 2022 2:18PM  2:30PM 
Z59.00015: NonAbelian bosonization in a (3+1)d Kondo semimetal via quantum anomalies Colin Rylands, Alireza Parhizkar, Victor M Galitski Kondo lattice models have established themselves as an ideal platform for studying the interplay between topology and strong correlations such as in topological Kondo insulators or WeylKondo semimetals. The nature of these systems requires the use of nonperturbative techniques which are few in number, especially in high dimensions. Motivated by this we study a model of Dirac fermions in 3 dimensions coupled to an arbitrary array of spins via a generalization of functional nonAbelian bosonization. We show that there exists an exact transformation of the fermions which allows us to write the system as decoupled free fermions and interacting spins. This decoupling transformation consists of a local chiral, Weyl and Lorentz transformation. Owing to its chiral and Weyl components this transformation is anomalous and generates a contribution to the action. From this we obtain the effective action for the spins and expressions for the anomalous transport in the system. 
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