Time-reversal symmetry-breaking nematic superconductivity in FeSe

FeSe is a unique member of the family of iron-based superconductors, not only because of the high values of Tc in FeSe monolayer, but also because in bulk FeSe superconductivity emerges inside a nematic phase without competing with long-range magnetic order. Near Tc, superconducting order necessarily has s+d symmetry, because nematic order couples linearly the s-wave and d-wave harmonics of the superconducting order parameter. Here we argue that the near-degeneracy between s-wave and d-wave pairing instabilities in FeSe, combined with the sign-change of the nematic order parameter between hole and electron pockets, allows the superconducting order to break time-reversal symmetry at a temperature T∗<Tc. The transition from an s+d state to an s+eiαd state should give rise to a peak in the specific heat and to the emergence of a soft collective mode that can be potentially detected by Raman spectroscopy.