Universal scaling behavior of the upper critical field in strained FeSe0.7Te0.3 thin films

Revealing the universal behaviors of iron-based superconductors (FBS) is important to elucidate the microscopic theory of superconductivity. In this work, we investigate the effect of in-plane strain on the slope of the upper critical field H c2 at the superconducting transition temperature T c (i.e. −dH c2/dT) for FeSe0.7Te0.3 thin films. The in-plane strain tunes T c in a broad range, while the composition and disorder are almost unchanged. We show that −dH c2/dT scales linearly with T c, indicating that FeSe0.7Te0.3 follows the same universal behavior as observed for pnictide FBS. The observed behavior is consistent with a multiband superconductivity paired by interband interaction such as sign change s ± superconductivity.