Variation of transition temperatures and residual resistivity ratio in vapor-grown FeSe and Fe(Se,S)

We report on the vapor growth and physical properties of single-crystalline FeSe and Fe(Se,S). Significant variations of sample morphology resulting from slight modifications of the growth conditions are observed. The superconducting transition temperature, $T_{\mathrm c}$, of different FeSe samples varies between 8.8 K and 3 K, and the structural transition temperature, $T_{\mathrm s}$, varies between 90 K and 72 K, respectively. We find that those variations cannot be correlated with the sample composition, measured by WDS. Instead, we point out a clear correlation between $T_{\mathrm s}$, $T_{\mathrm c}$ and disorder, as measured by the residual resistivity ratio. Notably, $T_{\mathrm s}$ and $T_{\mathrm c}$ are linearly correlated with each other\footnote{B\"ohmer et al., PRB 94, 024526 (2016).}. On the other hand, a small amount of sulfur-substitution suppresses $T_{\mathrm s}$ but enhances $T_{\mathrm c}$. We will compare and discuss the effects of various perturbations---disorder, chemical substitution and hydrostatic pressure---on the transition temperatures and properties of FeSe.