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

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 ($T_{\mathrm{n}}$) 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 ($T_{\mathrm{S}\mathrm{F}}$) and $T_{\mathrm{n}}$ is small for FeSe but significantly increases with S substitution. Below $T_{\mathrm{S}\mathrm{F}}$ 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${\rm x}$.