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

Abstract

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