Superconducting Tunneling in Ultrathin Nb Films

Good-quality tunnel junctions have been fabricated on ultrathin films of Nb whose thicknesses ranged from 9 to 900 {}. As the film thickness is reduced below 50 {}, Tc decreases, 2ΔkTc increases, and mode softening is observed through d²VdI² measurements. These results imply that as the thickness decreases below 50 {} the electron-phonon constant, λ, becomes larger than expected from the drop in Tc.