Bulletin of the American Physical Society
APS March Meeting 2022
Volume 67, Number 3
Monday–Friday, March 14–18, 2022; Chicago
Session G44: Topological Physics in Charge Density Wave MaterialsInvited Live Streamed

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Sponsoring Units: DCMP Chair: Gregory Fiete, Northeastern University Room: McCormick Place W375C 
Tuesday, March 15, 2022 11:30AM  12:06PM 
G44.00001: Unconventional charge order and superconductivity in kagome materials AV_{3}Sb_{5} Invited Speaker: Titus Neupert Lattice geometry, topological electron behavior, and the competition between different possible ground states all have a role in determining the properties of materials with a kagome lattice structure. In particular, the compounds KV_{3}Sb_{5}, CsV_{3}Sb_{5} and RbV_{3}Sb_{5} all feature a kagome net of vanadium atoms. These materials have recently been shown to exhibit superconductivity at low temperature and charge order at high temperature, revealing a connection to the underlying topological nature of the band structure. I will summarize the experimental evidence, in particular regarding the nature of the charge order, and provide a theoretical interpretation of it. Particularly exciting is the potential realization of timereversal symmetry breaking charge order with finite relative angular momentum in this material class. Both of these are longsought properties in charge ordered ground states, which have important repercussions on the topology of the resulting ground state. 
Tuesday, March 15, 2022 12:06PM  12:42PM 
G44.00002: Axionic Band Topology and Beyond in WeylChargeDensity Waves: Theory and Material Realization in (TaSe_{4})_{2}I Invited Speaker: Benjamin J Wieder Over the past decade, researchers have linked the lowenergy field theory of a Weyl semimetal gapped with a chargedensity wave (CDW) to highenergy theories with axion electrodynamics. The possibility of realizing axioninsulating (AXI) phases in WeylCDWs has over the past two years returned to the forefront of condensedmatter physics, due to the discovery of a Weylsemimetalinsulator transition in the established CDW compound (TaSe_{4})_{2}I. In this talk, we will first present theoretical analysis and experimental data establishing that the roomtemperature state of (TaSe_{4})_{2}I is indeed a structurally chiral, quasi1D Weyl semimetal, and that the Weylsemimetal state becomes gapped by a CDW when cooled just below room temperature. We will next revisit the initial proposals of AXI phases in timereversalbreaking WeylCDWs from the perspective of Magnetic Topological Quantum Chemistry. We will demonstrate that when the lowenergy theory of the simplest, inversionsymmetric WeylCDW is latticeregularized in a tightbinding model, the bulk at static values of the CDW phase angle Φ is not an AXI, but in fact realizes one of two quantum anomalous Hall (QAH) phases that differ by a fractional translation in the modulated cell, analogous to the two phases of the SuSchriefferHeeger model of polyacetylene. Crucially, the two QAH phases still differ by an origindependent axion angle △θ=π, indicating that the axionic WeylCDW response in the first proposals originates from meanfield band topology. We will conclude by considering nonmagnetic Weyl and DiracCDWs, which are nonaxionic in the absence of emergent valley symmetry, as θ mod 2π=0 for all static values of Φ. In particular, we will elucidate the relationship between nonmagnetic semimetalCDWs and nonaxionic (helical) higherorder topological crystalline insulators, in which the nontrivial bulk topology emerges from a response coefficient distinct from the axion angle θ. 
Tuesday, March 15, 2022 12:42PM  1:18PM 
G44.00003: Band Engineering of Dirac Semimetals Using Charge Density Waves Invited Speaker: Leslie M Schoop We now know that topological materials are anything but rare – between 30 and 50 % of materials have been predicted to be topological [1]. However, a very common issue of and compound with an “interesting” band structure is that most of the time, the feature of interest is convoluted with trivial bands. In this talk, I will show that we can take advantage of a charge density wave (CDW) to gap out such interfering bands, while keeping a nonsymmorphic Dirac node intact. This way we could design the first nonsymmorphic Dirac semimetal in which the Dirac node lies at the Fermi level, while the interference of other bands is minimal [2]. The concept can be expanded to a family of materials in the LnSbxTe2x system. I will also discuss the physical properties of these materials, which range from unusual transport phenomena to complex magnetism, involving the possibility of skyrmions as well as a “devil’s staircase” [34]. 
Tuesday, March 15, 2022 1:18PM  1:54PM 
G44.00004: Monopole charge density wave states in Weyl semimetals Invited Speaker: Yi Li

Tuesday, March 15, 2022 1:54PM  2:30PM 
G44.00005: Charge density waves and electronic properties of superconducting kagome metals Invited Speaker: Binghai Yan Kagome metals AV3Sb5 (A=K, Rb, and Cs) exhibit intriguing superconductivity below 0.9∼2.5 K, a charge density wave (CDW) transition around 80∼100 K, and Z2 topological surface states. We investigate the electronic and structural properties of CDW by firstprinciples calculations. We reveal an inverse Star of David deformation as the 2×2 CDW ground state of the kagome lattice. The kagome lattice shows softening breathingphonon modes, indicating the structural instability. We investigate roles of Fermi surface nesting for the CDW transition via the electronic susceptibility. We further analyze impacts of hole doping to CDW and compare our results with recent experiments. 
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