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 λ and the defect scattering rate ρ_w, being uniquely determined by their ratio λ/ρ_w. Other factors such as film thickness and irradiation do not produce additional effects beyond these two parameters.