Quasi-symmetry-protected topology in a semi-metal

Chunyu Guo, Lunhui Hu, Carsten Putzke, Jonas Diaz, Xiangwei Huang, Kaustuv Manna, Feng Ren Fan, Chandra Shekhar, Yan Sun, Claudia Felser, Chaoxing Liu, B. Andrei Bernevig, Philip J.W. Moll

Research output: Contribution to journalArticlepeer-review

18 Scopus citations


The crystal symmetry of a material dictates the type of topological band structure it may host, and therefore, symmetry is the guiding principle to find topological materials. Here we introduce an alternative guiding principle, which we call ‘quasi-symmetry’. This is the situation where a Hamiltonian has exact symmetry at a lower order that is broken by higher-order perturbation terms. This enforces finite but parametrically small gaps at some low-symmetry points in momentum space. Untethered from the restraints of symmetry, quasi-symmetries eliminate the need for fine tuning as they enforce that sources of large Berry curvature occur at arbitrary chemical potentials. We demonstrate that quasi-symmetry in the semi-metal CoSi stabilizes gaps below 2 meV over a large near-degenerate plane that can be measured in the quantum oscillation spectrum. The application of in-plane strain breaks the crystal symmetry and gaps the degenerate point, observable by new magnetic breakdown orbits. The quasi-symmetry, however, does not depend on spatial symmetries and hence transmission remains fully coherent. These results demonstrate a class of topological materials with increased resilience to perturbations such as strain-induced crystalline symmetry breaking, which may lead to robust topological applications as well as unexpected topology beyond the usual space group classifications.

Original languageEnglish (US)
Pages (from-to)813-818
Number of pages6
JournalNature Physics
Issue number7
StatePublished - Jul 2022

All Science Journal Classification (ASJC) codes

  • General Physics and Astronomy


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