Tunnel diode oscillator measurements of the upper critical magnetic field of FeTe(0.5)Se(0.5).

Temperature dependence of the upper critical magnetic field (Hc2) of single crystalline FeTe0.5Se0.5(Tc = 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 ‖ ab) and perpendicular (H ‖ c) to the iron conducting plane, in line with a single band superconductivity. Whereas Pauli pair breaking is negligible for H ‖ c, Pauli contribution is evidenced for H ‖ ab with Maki parameter α = 1.4, corresponding to Pauli field HP = 79 T. As a result, the Hc2 anisotropy [Formula: see text] which is already rather small at Tc (γ = 1.6) further decreases as the temperature decreases and becomes smaller than 1 at liquid helium temperatures.