Superconducting and magnetic properties of Fe-Se-Te compounds

The discovery of a new Fe based superconductor $\alpha $-FeSe$_{x}$ with a T$_{c}$ of 8~K, hot on the heels of the discovery of superconductivity in LaOFeP/As compounds, has triggered a fresh interest in the study of Fe based superconductors. Se can be replaced with Te in FeSe$_{1-x}$Te$_{x}$ and this results in an increase in T$_{c}$ from 8~K for x = 0 to 15~K for x = 0.5 while compounds for x $>$ 0.8 are no longer superconducting. We report the synthesis and characterization of the compounds FeSe$_{1-x}$Te$_{x}$ covering the entire solid solution range. The superconducting transition in resistivity measurements does not show any broadening in magnetic fields up to 9~T, but shifts to lower temperatures linearly with a value $\sim $ -0.22~K/T. This results in extremely high upper critical fields (H$_{c2})$ of the order of 70-80~T in these compounds. The superconducting properties are also sensitive to applied pressure and exhibit a positive dT$_{c}$/dP of around 0.41~K/kbar for the x = 0.5 composition. We observe a jump in specific heat at T$_{c}$ corresponding to a superconducting gap of 1.8~meV, indicating the bulk nature of superconductivity. Detailed investigations through magnetization, transport and specific heat measurements are presented. A study of the magnetic properties of the non superconducting end compound, FeTe is also presented to gain insight into the onset of superconductivity in the doped systems.