Inversion of the upper critical field anisotropy in FeTeS films

We present the complete superconducting upper critical field (H c2) - temperature (T) diagram of FeTeS films measured at three crystalline orientations (H ∥ c, 45° and ab). We find that Hc2 is almost isotropic in magnetic field orientation with μ0Hc2 (T = 0) ∼ 30 T and a transition temperature of Tc ∼ 7 K. A small but clear Hc2 angular anisotropy is observed, with a crossover around T = 0.7Tc, from Hc2 (∥ c) < Hc2 (∥ ab) for T > 0.7Tc to H c2 (∥ c) > Hc2 (∥ ab) for T < 0.7T c. This change in the anisotropy is similar to that observed in FeTeS and FeTeSe single crystals but occurs at a higher T/Tc for our film. We analyze the Hc2 (T) in terms of pair-breaking mechanisms and two-band superconductor theory. Understanding the inversion of Hc2 opens the possibility to adjust the effective anisotropy of superconductors for different applications. © 2014 IOP Publishing Ltd.