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TT: Fachverband Tiefe Temperaturen

TT 9: Focus Session: Anomalous Quantum Oscillations

TT 9.4: Hauptvortrag

Montag, 18. März 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