Crystal growth and superconductivity of new Fe-Te base materials

A number of Fe-base superconducting materials with critical superconducting temperature up to 56K have been discovered recently. Because As and its oxide of the Fe-As base superconducting materials is poison, it is a serious safety issue for researchers to make the bulk materials. The new Fe-Te base superconducting materials with Se doping are less toxic and safe to handle. We have grown a number of the FeTe$_{1-x}$Se$_{x}$ single crystals (x = 0$\sim $0.5) by using a modified floating-zone growth technique. The effects of the growth condition and the composition of a feed rod on the single crystal growth of FeTe$_{1-x}$Se$_{x}$ have been studied by using a floating zone machine. The single crystals of the PbO-type tetragonal structure FeTe$_{1-y}$Fe$_{y}$ (y = 0.04 to 0.08) 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.