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.
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We thank B. Andersen, L. Benfatto, S. Borisenko, D. Chowdhury, L. Classen, F. Hardy, P. Hirschfeld, A. Kreisel, M. Eschrig, L. Rhodes, and M. Watson for fruitful discussions. J.K. was supported by the National High Magnetic Field Laboratory through NSF Grant No. DMR-1157490 and the State of Florida. R.M.F. and A.V.C. were supported by the U.S. Department of Energy, Office of Science, Basic Energy Sciences, under Awards No. DE-SC0012336 (R.M.F.) and No. DE-SC0014402 (A.V.C.).
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