# Anomalous Knight shift and low-energy spin dynamics in the nematic state of FeSe$_{\rm 1-x}$S$_{\rm x}$

## TL;DR

This study explores how sulfur substitution affects the nematic transition and low-energy spin fluctuations in FeSe, revealing that spin dynamics are not directly coupled to nematic order and involve multiple spin degrees of freedom.

## Contribution

It provides new insights into the relationship between nematic order and spin fluctuations in FeSe$_{1-x}$S$_{x}$, showing the emergence of spin fluctuations below the nematic transition and their complex behavior with S substitution.

## Key findings

- Low-energy spin fluctuations emerge below the nematic transition.
- The difference between the onset temperature of spin fluctuations and the nematic transition increases with S substitution.
- The violation of the Korringa relation and sign change of hyperfine coupling indicate complex spin dynamics.

## Abstract

The interplay between the nematic order and magnetism in FeSe is not yet well understood. There is a controversy concerning the relationship between orbital and spin degrees of freedom in FeSe and their relevance for superconductivity. Here we investigate the effect of S substitution on the nematic transition temperature ($T_{\rm n}$) and the low-energy spin fluctuations (SF) in FeSe single crystals. We show that the low-energy SF emerge below the nematic transition. The difference between the onset temperature for the critical SF ($T_{\rm SF}$) and $T_{\rm n}$ is small for FeSe but significantly increases with S substitution. Below $T_{\rm SF}$ the Korringa relation is violated and the effective muon hyperfine coupling constant changes a sign. Our results exclude a direct coupling of the low-energy SF to the electronic nematic order indicating a presence of multiple spin degrees of freedom in FeSe$_{\rm 1-x}$S$_{\rm x}$.