Variation of transition temperatures and residual resistivity ratio in vapor-grown FeSe

Abstract

The study of the iron-based superconductor FeSe has blossomed with the availability of high-quality single crystals, obtained through flux/vapor-transport growth techniques below the structural transformation temperature of its tetragonal phase, $T\ensuremath{\approx}450{\phantom{\rule{0.16em}{0ex}}}^{\ensuremath{\circ}}\mathrm{C}$. Here, we report on the variation of sample morphology and properties due to small modifications in the growth conditions. A considerable variation of the superconducting transition temperature ${T}_{\mathrm{c}}$, from 8.8 K to 3 K, which cannot be correlated with the sample composition, is observed. Instead, we point out a clear correlation between ${T}_{\mathrm{c}}$ and disorder, as measured by the residual resistivity ratio. Notably, the tetragonal-to-orthorhombic structural transition is also found to be quite strongly disorder dependent $({T}_{\mathrm{s}}\ensuremath{\approx}72--90\phantom{\rule{4pt}{0ex}}\text{K})$ and linearly correlated with ${T}_{\mathrm{c}}$.