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

TL;DR

This paper investigates the gap structure and specific heat behavior of nematic superconductors like FeSe, analyzing how nematic order influences superconducting properties and potential phase transitions.

Contribution

It provides a detailed analysis of the angular dependence of the gap and specific heat in nematic superconductors, considering different nematic order scenarios and their effects on superconducting characteristics.

Findings

Nematicity affects the angular gap dependence and low-energy excitations.

Nematic order influences the specific heat jump at T_c and its temperature dependence.

Potential transition from an s+d to an s+ie^{iη}d state breaking time-reversal symmetry.

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.