Surface superconductivity as the primary cause of broadening of superconducting transition in Nb-films

TL;DR

This study investigates the broadening of the superconducting transition in Nb films, revealing that surface superconductivity significantly influences the transition behavior even in disordered and perpendicular magnetic fields.

Contribution

It demonstrates that surface superconductivity is the primary cause of transition broadening in Nb films, with detailed measurements linking Hc2 and Hc3 to the transition features.

Findings

Hc2 corresponds to the bottom of the transition

Hc3 occurs near the top of the transition

Surface superconductivity persists in disordered and perpendicular fields

Abstract

We study the origin of broadening of superconducting transition in sputtered Nb films. From simultaneous tunneling and transport measurements we conclude that the upper critical field Hc2 always corresponds to the bottom of transition R~0, while the top R~Rn occurs close to the critical field for destruction of surface superconductivity Hc3 ~ 1.7 Hc2. The two-dimensional nature of superconductivity at H>Hc2 is confirmed by cusp-like angular dependence of magnetoresistance. Our data indicates that surface superconductivity is remarkably robust even in disordered polycrystalline films and, surprisingly, even in perpendicular magnetic fields.