Effects of Iron Contents on the Vortex State in Fe x Se0.5Te0.5

We investigate the effects of iron content on the upper critical field (H c2) and the activation energy U(T) in thermally activated flux flow in Fe x Se0.5Te0.5 near the superconducting transition temperature T c . The variations in H c2(T) with temperature are analyzed using Ginzburg-Landau (GL), Werthamer-Helfand-Hohenberg (WHH) models along with the empirical relation (ER). The obtained values of H c2(0) depend strongly on the model and the criteria used to determine the transition temperature. However, the general trend is that that H c2(0) increases with the increasing Fe content. The activation energy U(T) is maximum for x = 1 and rapidly suppressed by excess or deficiency of iron. The low values of U(T) (∼10 meV) reflect the low vortex-pinning nature (due to defects, vacancies, etc.) in the Fe x Se0.5Te0.5 superconductor.