Upper Critical Fields of the 11-System Iron-Chalcogenide Superconductor FeSe0.25Te0.75

We have performed electrical resistivity measurements of a polycrystalline sample of FeSe0.25Te0.75, which exhibits superconductivity at Tc ~ 14 K, in magnetic fields up to 55 T to determine the upper critical field μ₀Hc2. In this compound, very large slopes of μ₀Hc2 at the onset, the mid-point, the zero-resistivity temperatures on superconductivity are determined to be -13.7, -10.1, and -6.9 T/K, respectively. The observed μ₀Hc2(T)s of this compound are considerably smaller than those expected from the Werthamer-Helfand-Hohenberg model, manifesting the Pauli limiting behavior. These results suggest that this compound has a large Maki parameter, but it is smaller than that calculated for a weak-coupling superconductor, indicating a large superconducting gap of this compound as a strong-coupling superconductor.