We show that spin-singlet $s$-wave multiband 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.

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• Multiband superconductivity
• Symmetry protected topological states
• Topological superconductors
• Iron-based superconductors
• Unconventional superconductors
• Discrete symmetries in condensed matter
• Superconductivity
• Tight-binding model