Unusual band splitting and superconducting gap evolution with sulfur substitution in FeSe

TL;DR

This study uses high-resolution photoemission to explore how sulfur substitution affects band splitting and superconducting gaps in FeSe, revealing unusual band behavior and gap evolution linked to nematicity.

Contribution

It provides new insights into the relationship between nematicity and superconducting gap evolution in sulfur-doped FeSe, highlighting unusual band splitting phenomena.

Findings

Unusual two hole bands observed near Brillouin-zone center.

Superconducting gaps show high anisotropy with resolution-limited nodes.

Superconducting gap on the outer hole band decreases linearly with sulfur substitution.

Abstract

High-resolution angle-resolved photoemission measurements were taken on FeSe$_{1-x}$S$_x$ (x=0, 0.04, and 0.08) superconductors. With an ultrahigh energy resolution of 0.4 meV, unusual two hole bands near the Brillouin-zone center, which was possibly a result of additional symmetry breaking, were identified in all the sulfur-substituted samples. In addition, in both of the hole bands highly anisotropic superconducting gaps with resolution limited nodes were evidenced. We find that the larger superconducting gap on the outer hole band is reduced linearly to the nematic transition temperature while the gap on the inner hole is nearly S-substitution independent. Our observations strongly suggest that the superconducting gap increases with enhanced nematicity although the superconducting transition temperature is not only governed by the pairing strength, demonstrating strong constraints on theories in the FeSe family.