Nodal gaps in the nematic superconductor FeSe from heat capacity

TL;DR

This study investigates the superconducting gap structure of FeSe through heat capacity measurements, revealing nodal gaps consistent with prior quasiparticle interference data and highlighting unresolved questions about its Fermi surface topology.

Contribution

It provides experimental evidence for nodal gaps in FeSe and discusses implications for its electronic structure and superconducting properties.

Findings

Heat capacity data supports nodal gap structure in FeSe.

Results align with previous quasiparticle interference studies.

The second electron pocket's existence remains uncertain.

Abstract

Superconductivity in FeSe has recently attracted a great deal of attention because it emerges out of an electronic nematic state of elusive character. Here we study both the electronic normal state and the superconducting gap structure using heat-capacity measurements on high-quality single crystals. The specific-heat curve, from 0.4 K to Tc = 9.1 K, is found to be consistent with a recent gap determination using Bogoliubov quasiparticle interference [P. O. Sprau et al., Science 357, 75 (2017)], however only if nodes are introduced on either the electron or the hole Fermi-surface sheets. Our analysis, which is consistent with quantum-oscillation measurements, relies on the presence of only two bands, and thus the fate of the theoretically predicted second electron pocket remains mysterious.