Representation-protected topology of spin-singlet $s$-wave superconductors

TL;DR

This paper demonstrates that spin-singlet s-wave multi-band superconductors can host a topological phase protected by symmetry, characterized by a Z2 index, with implications for surface states and iron-based superconductors.

Contribution

It introduces a new symmetry-protected topological phase in s-wave superconductors, linking the Z2 index to Fermi surface properties and topological invariants.

Findings

Identification of a Z2 topological index in s-wave superconductors

Existence of gapless surface states with quadratic dispersion

Connection to iron-based superconductors

Abstract

We show that spin-singlet $s$-wave multi-band superconductors have a topological phase protected by rotation symmetry and time-reversal symmetry without spin-orbit coupling in two and three dimensions. This topological phase, an example of a representation-protected topological phase, has a $\mathbb{Z}_2$ topological index and is stable as long as the bands at the Fermi energy are formed by a doublet of orbital states with finite angular momenta. In the limit of weak superconducting pair potential, the $\mathbb{Z}_2$ index gives a Fermi-surface formula and is related to the winding number of three-dimensional strong topological superconductors of class CI. We present a model of a topological $s_{\pm}$-wave superconductor that has gapless surface states with a quadratic dispersion and suggest a connection with iron-based superconductors.