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

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 Z₂ 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 Z₂ 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±-wave superconductor that has gapless surface states with a quadratic dispersion and suggest a connection with iron-based superconductors.