Degradation Mechanism of the Superconducting Transition Temperature in Nb Thin Films

Systemic measurements show that there is no 3D to 2D crossover in the reduction of the superconducting transition temperature T-c in Nb thin films. This result is consistent with all previous measurements while it is contrary to the prevailing understanding based on the interplay between proximity, localization, and lifetime broadening. Our study indicates that the decrease of T-c can be interpreted by the combined effects of electron-phonon coupling parameter lambda and the defect scattering rate rho(w), being uniquely determined by their ratio lambda/rho(w). Other factors such as film thickness and irradiation do not produce additional effects beyond these two parameters.