Variation of transition temperatures and residual resistivity ratio in   vapor-grown FeSe
  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~450 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 Tc, 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 Tc 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 (Ts =72 - 90 K), and linearly correlated with Tc. 
A. E. B\"ohmer, V. Taufour, W. E. Straszheim, T. Wolf, and P. C.   Canfield
cond-mat.supr-con