Suppression of the superconducting transition temperature and magnetic-field-induced quantum critical behavior in three-dimensional polycrystalline niobium films

We have investigated the superconducting transition temperature T-c depression and magnetic-field-induced quantum critical behavior in three-dimensional (3D) polycrystalline niobium (Nb) films. For superconducting films with 26.8 nm <= D <= 72.9 nm, the T-c value increases from similar to 4.94 to similar to 8.74 K. Through comparing our experimental data with proximity effect and weak localization effect, the T-c depression can be explained by the proximity effect predicted by Cooper. Pure size effect could not induce T-c depression because the average energy level spacing of grains is much smaller than the bulk Nb superconducting energy gap. The reason for the proximity effect is the metallic niobium monoxide NbO surrounding Nb grains. The superconductor-insulator transition (SIT) induced by magnetic field has been observed in T-s = 433 and 523 K films. The critical field B-c in SIT satisfies a scaling relationship, similar to that caused by quantum phase transition in a two-dimensional (2D) cleaning system. The reason for that is the average polycrystalline grain size is smaller than the bulk Nb coherence length. The quantum effect is probably related to the Nb oxides.