Correlation between Superconductivity and Magnetic Fluctuations in Fe(Se1-xSx) Revealed by 77Se NMR

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