Bulk superconductivity at 14 K in single crystals of Fe1+yTexSe1−x

Resistivity, magnetic susceptibility, and heat-capacity measurements are reported for single crystals of ${text{Fe}}_{1+y}{text{Te}}_{x}{text{Se}}_{1ensuremath{-}x}$ grown via a modified Bridgeman method with $0<y<0.15$ and $x=1$, 0.9, 0.75, 0. 67, 0.55, and 0.5. Although resistivity measurements show traces of superconductivity near 14 K for all $x$ except $x=1$, only crystals grown with compositions near $x=0.5$ exhibit bulk superconductivity. The appearance of bulk superconductivity correlates with a reduction in the magnitude of the magnetic susceptibility at room temperature and smaller values of $y$, the concentration of Fe in the Fe(2) site.