Effects of Iron Contents on the Vortex State in Fe x Se 0.5 Te 0.5

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 Se 0.5 Te 0.5 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 Se 0.5 Te 0.5 superconductor.