Quasi-symmetry protected topology in a semi-metal

Abstract

The crystal symmetry of a material dictates the type of topological band structures 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 an exact symmetry at 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 will occur at arbitrary chemical potentials. We demonstrate that a 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.

Nat. Phys. (2022)

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• Atomically Sharp Internal Interface in a Chiral Weyl Semimetal Nanowire
• Quasi-symmetry Constrained Spin Ferromagnetism in Altermagnets
• Group Theory on Quasisymmetry and Protected Near Degeneracy
• Gapped nodal planes drive a large topological Nernst effect in a chiral lattice antiferromagnet
• Quantum spin Hall effect protected by spin U(1) quasisymmetry
• Duality,Hidden Symmetry and Dynamic Isomerism in 2D Hinge Structures
• Topological diffusive metal in amorphous transition metal monosilicides
• Chiral Floquet Engineering on Topological Fermions in Chiral Crystals
• Fermi surface of the chiral topological semimetal CoSi
• IrF4: From Tetrahedral Compass Model to Topological Semimetal
• Spin-orbit-coupled superconductivity with spin-singlet non-unitary pairing
• Quantum Geometric Origin of the Intrinsic Nonlinear Hall Effect
• Sub-symmetry Protected Topology in Topological Insulators and Superconductors
• Geometric semimetals and their simulation in synthetic matter
• Intrinsic defect intolerance in the ultra-pure metal PtSn${}_4$
• Inverse-current quantum electro-oscillations in a charge-density wave insulator
• Atomically thin obstructed atomic insulators with robust edge modes and quantized spin Hall effect
• Topological Charge Quadrupole Protected by Spin-Orbit U(1) Quasi-Symmetry in Antiferromagnet NdBiPt
• Physical Pictures for Quasisymmetry in Crystals
• Fate of Bosonic Topological Edge Modes in the Presence of Many-Body Interactions