Single crystal growth and superconductivity of Fe-Te-Se materials*

A number of Fe-base superconducting materials with critical superconducting temperature up to 56K have been discovered in 2008. Fe-Te-Se is ``11'' system superconductor which has the simplest crystal structure. We have grown a number of the FeTe$_{1-x}$Se$_{x}$ single crystals (x = 0$\sim $0.7) by using a Bridgman growth technique. The effects of the growth condition and the composition of a feed rod on the single crystal growth of Fe$_{1+y}$Te$_{1-x}$Se$_{x}$ have been studied. The single crystals of the PbO-type tetragonal structure Fe$_{1+y}$Te$_{1-x}$Se$_{x}$ with high extra Fe (\textit{ie} y$>$0) are not superconducting. When Se substitutes for Te in FeTe$_{1-x}$Se$_{x}$ single crystals, the superconducting transition temperature increases with increasing Se content. The maximum Tc of FeTe$_{0.5}$Se$_{0.5 }$is 15K.