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

Abstract

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 $s + e^{i\eta} d$ 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.