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TT: Fachverband Tiefe Temperaturen
TT 9: Focus Session: Anomalous Quantum Oscillations
TT 9.4: Invited Talk
Monday, March 18, 2024, 16:45–17:15, H 0104
Simplicity of quantum oscillations in CoSi from its hidden quasi-symmetry — •Philip J.W. Moll1, Chunyu Guo1, Lunhui Hu2, Carsten Putzke1, Jonas Diaz3, Xiangwei Huang3, Kaustuv Manna4, Feng-Ren Fan4, Yan Sun4, Chandra Shekhar4, Claudia Felser4, Chaoxing Liu2, and Andrei B Bernevig5 — 1MPI for the Structure & Dynamics of Matter — 2The Pennsylvania State University — 3EPFL — 4MPI for Chemical Physics of Solids — 5Princeton University
The crystal symmetry dictates the type of topological band structures it may host, hence it is the principle guiding the search for topological materials. Here we present a twist on this idea, materials in which approximate symmetries stabilize near-degeneracies of bands. Specifically, we coin quasi-symmetry as a term for an exact symmetry of a Hamiltonian to lower-order yet is broken by higher-order perturbation terms. This enforces finite but parametrically small gaps at low-symmetry k-points across the whole Brillouin zone, eliminating the need for fine-tuning as the sources of large Berry curvature will occur at any arbitrary chemical potentials. We demonstrate that in the eV-bandwidth semi-metal CoSi an internal quasi-symmetry stabilizes gaps below 2 meV on eight large near-degenerate planes (2D) [1]. These quasi-degeneracies connect continuously to thetrue, symmetry-protected topological ones in CoSi. Depending on spatial symmetry, these are easily gapped by weak strain which is evidenced by new magnetic breakdown orbits [2]. In contrast, the quasi-symmetry has no spatial character and thus is resilient to strain.
Keywords: CoSi; quasi-symmetry; topological semi-metal