Nematic Spin Correlations Pervading the Phase Diagram of ${\mathrm{FeSe}}_{1-x}{S}_x$

We use resonant inelastic x-ray scattering (RIXS) at the $\mathrm{Fe}\text{−}{L}_3$ edge to study the spin excitations of uniaxial-strained and unstrained ${\mathrm{FeSe}}_{1-x}{S}_x$ ($0\le x\le 0.21$) samples. The measurements on unstrained samples reveal dispersive spin excitations in all doping levels, which show only minor doping dependence in energy dispersion, lifetime, and intensity, indicating that high-energy spin excitations are only marginally affected by sulfur doping. RIXS measurements on uniaxial-strained samples reveal that the high-energy spin-excitation anisotropy observed previously in FeSe is also present in the doping range $0<x\le 0.21$ of ${\mathrm{FeSe}}_{1-x}{S}_x$. The spin-excitation anisotropy persists to a high temperature up to $T>200K$ in $x=0.18$ and reaches a maximum around the nematic quantum critical doping ($x_c\approx 0.17$). Since the spin-excitation anisotropy directly reflects the existence of nematic spin correlations, our results indicate that high-energy nematic spin correlations pervade the regime of nematicity in the phase diagram and are enhanced by the nematic quantum criticality. These results emphasize the essential role of spin fluctuations in driving electronic nematicity and highlight the capability of uniaxial strain in tuning spin excitations in quantum materials hosting strong magnetoelastic coupling and electronic nematicity.