Disorder-sensitive node-like small gap in FeSe

TL;DR

This study shows that a small, disorder-sensitive, node-like small gap exists in FeSe, and its presence or absence explains previous conflicting reports on the material's gap structure.

Contribution

It reveals the disorder sensitivity of the small gap and clarifies the controversy over FeSe's gap structure by linking it to sample quality.

Findings

Small disorder has little effect on band structure and nematic state.

Higher quality samples exhibit a small gap with vertical-line nodes.

Disorder smears out the nodes, leading to an isotropic gap in lower quality samples.

Abstract

We investigate the band structure, nematic state and superconducting gap structure of two selected FeSe single crystals containing different amount of disorder. Transport and angle-resolved photoemission spectroscopy measurements show that the small amount of disorder has little effect to the band structure and the nematic state of FeSe. However, temperature and magnetic field dependencies of specific heat for the two samples are quite different. Wave-vector-dependent gap structure are obtained from the three dimensional field-angle-resolved specific heat measurements. A small gap with two vertical-line nodes or gap minima along the $k_z$ direction is found only in the sample with higher quality. Such symmetry-unprotected nodes or gap minima are found to be smeared out by small amount of disorder, and the gap becomes isotropic in the sample of lower quality. Our study reveals that the reported controversy on the gap structure of FeSe is due to the disorder-sensitive node-like small gap.