Anisotropic pairing gap in FeSe*

Superconductivity in FeSe occurs deep inside the nematic phase, but in the absence of any form of magnetic order. As a result, FeSe provides a unique opportunity to investigate pairing in an electronic nematic state, which reconstructs the low-energy band dispersions. We solved the pairing problem within the low-energy model of FeSe, which includes the orbital content of low-energy excitations. We found deep minima of a superconducting gap on the distorted hole pocket. The location of the minima depends on two features: the relative magnitude of the intra-orbital and inter-orbital pairing interactions, and the relative sign of the nematic order parameters on hole and electron pockets. We also discuss a possibility that superconducting order evolves below T_c and breaks time-reversal symmetry below a certain T<T_c. We compare our results with recent experiments in stoichiometric FeSe and S-doped FeSe.