Specific Heat and the gap structure of a Nematic Superconductor, application to FeSe

We report the results of our in-depth analysis of spectroscopic  and  thermodynamic  properties of a multi-orbital metal, like FeSe, which first develops a nematic order and then undergoes a transition into a superconducting state, which co-exists with nematicity. We analyze the angular dependence of the gap function and specific heat C_V(T) of such nematic superconductor. We specifically address three issues: (i) angular dependence of the gap in light of the competition between nematicity-induced s-d mixture and orbital transmutation of low-energy excitations in the nematic state, (ii) the effect of nematicity on the magnitude of the jump of the specific heat C_V(T) at T_c and the temperature dependence of C_V(T) below T_c, and (iii) a potential transition at T_c1 < T_c from an s+d state to an s + e^i η d state that breaks time-reversal symmetry. We consider two scenarios for a nematic order: scenario A, in which this order develops between d_xz and d_yz orbitals on hole and electron pockets  and scenario B, in which there is an additional component of the nematic order for d_xy fermions on the two electron pockets.