We present ${}^{77}\mathrm{Se}$-NMR measurements on ${\mathrm{FeSe}}_{1-x}{\mathrm{S}}_x$ samples with sulfur content $x=0\%$, 9%, 15%, and $29\%$. Twinned nematic domains are observed in the NMR spectrum for all samples except $x=29\%$. The NMR spin-lattice relaxation rate shows that antiferromagnetic (AFM) fluctuations are initially enhanced between $x=0\%$ and $x=9\%$, but are strongly suppressed for higher $x$ values. The observed behavior of the AFM fluctuations parallels the superconducting transition temperature $T_{\mathrm{c}}$ in these materials, providing strong evidence for the primary importance of AFM fluctuations for superconductivity, despite the presence of nematic quantum criticality in the ${\mathrm{FeSe}}_{1-x}{\mathrm{S}}_x$ system.

• Received 9 February 2018
• Revised 12 July 2018

DOI:https://doi.org/10.1103/PhysRevB.98.020507