Bulletin of the American Physical Society
APS March Meeting 2022
Volume 67, Number 3
Monday–Friday, March 14–18, 2022; Chicago
Session Y59: Dirac and Weyl Semimetals: Theory IRecordings Available

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Sponsoring Units: DCMP Chair: Angkun Wu, Rutgers University Room: Hyatt Regency Hotel DuSable AB 
Friday, March 18, 2022 8:00AM  8:12AM 
Y59.00001: Weyl Semimetal Path to Valley Filtering in Graphene Ahmed M Khalifa, Ribhu K Kaul, Efrat Shimshoni, Herb Fertig, Ganpathy N Murthy We propose a device in which a sheet of graphene is coupled to a Weyl semimetal, allowing for the physical access to the study of tunneling from two to threedimensional massless Dirac fermions. Because of the reconstructed band structure, we find that this device acts as a robust valley filter for electrons in the graphene sheet. We show that, by appropriate alignment, the Weyl semimetal draws away current in one of the two graphene valleys, while allowing current in the other to pass unimpeded. In contrast to other proposed valley filters, the mechanism of our proposed device occurs in the bulk of the graphene sheet, obviating the need for carefully shaped edges or dimensions. 
Friday, March 18, 2022 8:12AM  8:24AM 
Y59.00002: When do Dirac points have higher order Fermi arcs? Yuan Fang, Jennifer Cano Dirac semimetals lack a simple bulkboundary correspondence. 
Friday, March 18, 2022 8:24AM  8:36AM 
Y59.00003: A generic approach to extract geometry from nonadiabatic (gapclosing) processes and its consequences: geometric pumping and dephasing at topological phase transitions in topological insulators or semimetals. Boqun Song, Jonathan Smith, Liang Luo, JIGANG Wang Geometry in physics arises from adiabaticity via an analog to “parallel transport”. Locally, geometry is reflected by Berry curvatures, from which one can deduce various observables (e.g., adiabatic pump, electric polarization). 
Friday, March 18, 2022 8:36AM  8:48AM 
Y59.00004: Probing NiehYan Anomaly through phonon dynamics in the KramersWeyl semimetals of Chiral Crystals Chaoxing Liu NiehYan anomaly describes the nonconservation of chiral charges induced by the coupling between Dirac fermions and torsion fields. Since the torsion field is beyond general relativity, this effect remains hypothetical and its relevance to our universe is unclear in the context of highenergy physics. In this work, we propose that the phonons can induce a torsion field for the KramersWeyl fermions through electronphonon interaction in a nonmagnetic chiral crystal, thus leading to the occurrence of the NiehYan anomaly. As a consequence, the NiehYan term can strongly influence the phonon dynamics and lead to the helicity of acoustic phonons, namely, two transverse phonon modes mix with each other to form a circular polarization with a nonzero angular momentum and the phonon angular momentum forms a hedgehog texture in the momentum space. The phonon helicity can be probed through measuring the total phonon angular momentum driven by a temperature gradient. 
Friday, March 18, 2022 8:48AM  9:00AM 
Y59.00005: Anomalous surface conductivity of Weyl semimetals Pavan R Hosur, Osakpolor Obakpolor, Hridis Pal We calculate the surface dc conductivity of Weyl semimetals and show that it contains an anomalous contribution in addition to a regular, Drudelike contribution from the Fermi arc. The anomalous part is independent of the surface scattering time and appears at nonzero temperature and chemical potential, increasing quadratically with both. It is found that the nontrivial coupling between the surface and the bulk leads to an effective description on the surface that mimics an interacting twodimensional fluid in certain regimes of energy, even in the absence of any explicit scattering on the surface, and is responsible for the anomalous contribution. Remarkably, in the case of a layered Weyl semimetal, the temperaturedependent part of the surface conductivity is dominated by the anomalous response which can be probed experimentally to unravel this unusual behavior. 
Friday, March 18, 2022 9:00AM  9:12AM 
Y59.00006: Surface Luttinger arcs in Weyl Semimetals Osakpolor E Obakpolor, Pavan R Hosur Weyl semimetals – 3D topological materials in which nondegenerate conduction and valence bands cross at certain points in the Brillouin zone – have garnered attention in recent years. Despite the surface hosting Fermi arcs, fundamental sumrules obeyed by general Fermi liquids, such as Luttinger’s theorem that equates particle density to the Fermi volume, are naïvely inapplicable to the surface metal since it lacks a closed Fermi surface. In this work, we show that the surface of a Weyl semimetal is endowed with another feature, the Luttinger arc, defined as the momentum space contour where the electron Green’s function vanishes. We show that the Luttinger arcs, like the Fermi arcs, also connect surface projections of Weyl nodes of opposite chirality. As a result, the Fermi and Luttinger arcs together form closed loops in undoped Weyl semimetals. Furthermore, we show that the closed loops fulfill Luttinger’s theorem in a physical limit and the equivalence obtained survives interactions if the system remains in the Weyl limit. Finally, we propose a direct method for detecting Luttinger arcs experimentally in bilayered Weyl semimetals and apply this idea to determine Luttinger arcs in Co_{3}Sn_{2}S_{2} using existing photoemission data. 
Friday, March 18, 2022 9:12AM  9:24AM 
Y59.00007: Broken symmetry and competing orders in Weyl semimetal interfaces with Fermi arc states Ritajit Kundu, Arijit Kundu, Herb Fertig Weyl semimetal (WSM) slabs hosting Fermi arc surface states in proximity can give rise to broken symmetry phases, due to emergent excitonic order, in presence of Coulomb interactions. In such a phase, correlations between lowenergy interlayer fermions become nonzero even without any tunneling processes. The recently found WSM Co_{3}Sn_{2}S_{2} and the proposed WSM 42 Spinel compound, which respectively hosts two and three Fermi arcs in the surface Brillouin zone, provide platforms to examine the effects of multiple Fermi arcs in such phases. We find that such a system hosts different excitonic orders, such as zero momentum exciton modes as well as finite momentum exciton modes and chargedensitywave orders. These orders break a number of U(1) symmetries, whereas charge density wave orders also break translation symmetry. Broken U(1)symmetries give rise to Goldstone modes with interesting transport signatures and potential applications in quantum technologies. 
Friday, March 18, 2022 9:24AM  9:36AM 
Y59.00008: Emergent anomalies and generalized Luttinger theorems in metals and semimetals Anton Burkov, Chong Wang, Alexander Hickey, Xuzhe Ying Luttinger's theorem connects a basic microscopic property of a given metallic crystalline material, the number of electrons per unit cell, 
Friday, March 18, 2022 9:36AM  9:48AM 
Y59.00009: Bulk and surface photocurrents in topological chiral crystals Guoqing Chang, Zahid M Hasan, Arun Bansil, Hsin Lin, Suyang Xu, Daniel S Sanchez, Ilya Belopolski, Jiaxin Yin The nonlinear optical responses from topological semimetals are crucial in both understanding the fundamental properties of quantum materials and designing nextgeneration light sensors or solar cells. Using the ideal topological chiral semimetal RhSi as a representative, we, for the first time, theoretically determine the quantized photocurrent from bulk multifold chiral fermions. We further propose a new surfaceonly photocurrent response from their chiral Fermi arcs. We quantitatively compute the photogalvanic currents from bulk "Weyl" cone and surface Fermi arcs. We demonstrate that Fermi arc photogalvanic currents can be perpendicular to the bulk injection currents regardless of the choice of materials surface. We then generalize this finding to other cubic chiral space groups and predict material candidates. Our theory reveals a powerful notion where common crystalline symmetry can be used to completely disentangle bulk and surface optical responses in many conducting material families. 
Friday, March 18, 2022 9:48AM  10:00AM 
Y59.00010: SymmetryEnforced Weak Topology in Dirac Semimetals Moritz M Hirschmann, Andreas Leonhardt, Niclas P Heinsdorf, Xianxin Wu, Douglas H Fabini, Andreas P Schnyder Conventional topological insulators result from band inversions. As such their existence is dependent on the details of the system. In the present work we discuss certain space groups that lead to a nontrivial arrangement of parity eigenvalues in a 2D subset of the 3D Brillouin zone. Consequently, all materials with the corresponding crystalline symmetries exhibit a nontrivial weak topological invariant. While, as a whole, these systems are fillingenforced semimetals, the nontrivial gapped subsystems imply the existence of surface states. We discuss several examples of such enforced topology among orthorhombic and tetragonal space groups using general symmetry arguments and tightbinding models. Supported by DFT calculations we propose two example materials, Sr_{2}Bi_{3} and Ir_{2}Si that realize the enforced weak topology in the vicinity of the Fermi energy. 
Friday, March 18, 2022 10:00AM  10:12AM 
Y59.00011: Linear and nolinear optical responses of chiral multifold semimetals Miguel Ángel Sánchez Martínez, Bing Xu, Zhenyao Fang, Zhuoliang Ni, Baozhu Lu, Jörn W Venderbos, Fernando De Juan, Eugene J Mele, Andrew M Rappe, Adolfo G Grushin, Darius H Torchinsky, Liang Wu The chiral topological semimetals RhSi and CoSi exhibit band degeneracies near the Fermi level enforced by the crystal symmetries. The lowenergy quasiparticles emerging near these band degeneracies, referred to as multifold fermions, have no counterpart as elementary fermionic particles. We calculate the linear optical conductivity of all chiral multifold fermions[1] and show that it provides an experimental fingerprint for each type of multifold fermion. We use a tightbinding model for space group 198, where RhSi and CoSi crystallize, revealing that the location of the chemical potential is crucial to understand the optical response seen in experiments[2,3], determined at low energies by the threefold fermion at the Γ point in both materials, and providing signatures of the existence of a spin3/2 fourfold fermion in CoSi. Finally, we study the secondharmonic generation of RhSi [4]. We analyze the experimental results using a secondorder k·p Hamiltonian and compare our results with density functional theory calculations to provide a comprehensive description of the origin of the different features in the secondharmonic response and their relation to the topological character of the bands in RhSi. 
Friday, March 18, 2022 10:12AM  10:24AM 
Y59.00012: Threedimensional flat bands in nodalline semimetals Alexander Lau, Giuseppe Cuono, Carmine Autieri, Dmitry I Pikulin, Timo Hyart Sparked by the discovery of unconventional superconductivity in twisted bilayer graphene, the study of flatband physics has become a driving force in the search for new phases of matter. In a recent paper [1], we demonstrated how to lift this study into the third dimension by employing strain engineering in nodalline semimetals. We further showed that the emerging threedimensional (3D) flat bands lead to correlated phases, such as superconducting and magnetic phases. In this talk, we will discuss the mechanism leading to 3D flat bands with a focus on the celebrated HgTe materials class, which is wellknown for realizing quantumspin Hall insulators, 3D topological insulators, and Weyl semimetals. More specifically, we will present new results demonstrating that alloys of HgTe and structurally similar materials are another promising platform for the study of 3D flat bands and the associated correlated phases. 
Friday, March 18, 2022 10:24AM  10:36AM 
Y59.00013: Topological acoustic triple point Yoonseok Hwang, Sungjoon Park, Hong Chul Choi, BohmJung Yang Acoustic phonon in a crystalline solid is a wellknown example of elementary excitation with a triple degeneracy in the band structure. Because of the NambuGoldstone theorem, this triple degeneracy is always present in the phonon band structure. Here, we show that the triple degeneracy of acoustic phonons can be characterized by a topological charge q that is a property of threeband systems with spacetimeinversion symmetry. We therefore call triple points with nontrivial q the topological acoustic triple point (TATP). The topological charge q can equivalently be characterized by the skyrmion number of the longitudinal mode, or by the Euler number of the transverse modes, and this strongly constrains the nodal structure around the TATP. The TATP can also be symmetryprotected at highsymmetry momenta in the band structure of phonons and spinless electrons by the O_{h} and T_{h} groups. The nontrivial wavefunction texture around the TATP can induce anomalous thermal transport in phononic systems and orbital Hall effect in electronic systems. Our theory demonstrates that the gapless points associated with the NambuGoldstone theorem are an avenue for discovering new classes of degeneracy points with distinct topological characteristics. 
Friday, March 18, 2022 10:36AM  10:48AM 
Y59.00014: Multigap topological phases: geometrical properties and realizations. RobertJan Slager With the recent advent of powerful classification schemes a significant step has been made towards characterizing and classifying topological band theory. Within this talk I will address a new direction, entailing phases that go beyond these wisdoms, thriving on the concept of multigap topology. In particular, such phases conspire into new types of invariants and a rich novel interplay with geometry [13]. I will highlight some of these new aspects that we have obtained this year and point out a variety of experimental settings, including phonon spectra, metamaterials, cold atoms and electronic band systems, where these concepts culminate in readily observable experimental signatures [4,5]. 
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