Tunnel diode oscillator measurements of the upper critical magnetic field of FeTe0.5Se0.5

Temperature dependence of the upper critical magnetic field (H-c2) of single crystalline FeTe0.5Se0.5(T-c = 14.5 K) have been determined by tunnel diode oscillator-based measurements in magnetic fields of up to 55 T and temperatures down to 1.6 K. The Werthamer-Helfand-Hohenberg model accounts for the data for magnetic field applied both parallel (H parallel to ab) and perpendicular (H parallel to c) to the iron conducting plane, in line with a single band superconductivity. Whereas Pauli pair breaking is negligible for H parallel to c, Pauli contribution is evidenced for H parallel to ab with Maki parameter alpha = 1.4, corresponding to Pauli field H-P = 79 T. As a result, the H-c2 anisotropy (gamma = H-c2(ab)/H-c2(c)) which is already rather small at T-c (gamma = 1.6) further decreases as the temperature decreases and becomes smaller than 1 at liquid helium temperatures.