# Highly Anisotropic Superconducting Gap in Nematically Ordered and Tetragonal Phases of FeSe$_{1-x}$S$_x$

## TL;DR

This study investigates how the superconducting gap structure in FeSe$_{1-x}$S$_x$ evolves with nematic order suppression, revealing persistent anisotropic gaps with different nodal structures in nematic and tetragonal phases.

## Contribution

It provides the first detailed comparison of superconducting gap anisotropy in nematic and tetragonal phases of FeSe$_{1-x}$S$_x$, highlighting the evolution of gap minima and nodal positions.

## Key findings

- Superconducting gap exhibits deep minima or line nodes in both phases.
- Gap minima/nodes differ between nematic and tetragonal regimes.
- Superconducting pairing interaction remains highly anisotropic across phases.

## Abstract

FeSe has a unique ground state in which superconductivity coexists with a nematic order without long-range magnetic ordering at ambient pressure. Here, to study how the pairing interaction evolves with nematicity, we measured the thermal conductivity and specific heat of FeSe$_{1-x}$S$_x$, where the nematicity is suppressed by isoelectronic sulfur substitution. We find that in the whole nematic ($0\leq x \leq 0.17$) and tetragonal ($x=0.20$) regimes, the application of small magnetic field causes a steep increase of both quantities. This indicates the existence of deep minima or line nodes in the superconducting gap function, implying that the pairing interaction is significantly anisotropic in both the nematic and the tetragonal regimes. Moreover, the present results indicate that the position of gap minima/nodes in the tetragonal regime appears to be essentially different from that in the nematic regime. These results place an important constraint on current theories.