Approximate symmetries and conservation laws in topological insulators and associated $\mathbb{Z}$-invariants

Abstract: Solid state systems with time reversal symmetry and/or particle-hole symmetry often only have $\mathbb{Z}_2$-valued strong invariants for which no general local formula is known. For physically relevant values of the parameters, there may exist approximate symmetries or almost conserved observables, such as the spin in a quantum spin Hall system with small Rashba coupling. It is shown in a general setting how this allows to define robust integer-valued strong invariants stemming from the complex theory, such as the spin Chern numbers, which modulo $2$ are equal to the $\mathbb{Z}_2$-invariants. Moreover, these integer invariants can be computed using twisted versions of the spectral localizer.