Superfluid density from magnetic penetration depth measurements in Nb-Cu 3D nano-composite films

Superconductivity in 3D Nb-Cu nanocomposite granular films have been studied with varying thickness for two different compositions, Nb rich with 88 at% of Nb and Cu rich with 46 at% of Nb. For both compositions, the superconducting transition temperature (T_c) decreases with decreasing film thickness. For any thickness, doubling the Cu content in the films decreases the T_c by about 2 K. To explore if phase fluctuations play any role in superconductivity in these 3D films, the superfluid stiffness (J_S) of the films was measured using low frequency two-coil mutual inductance (M) technique. Interestingly, the measurement of M in magnetic fields showed two peaks in the imaginary component of M for both Nb rich and Cu rich films. The two peaks were associated with the pair-breaking effect of the magnetic field on the intra and inter-granular coupling in these films consisting of random network of superconductor (S) and normal metal (N) nano-particles. Furthermore, J_S was seen to decrease with decreasing film thickness and increasing Cu content. However, for all films studied J_S remained higher than the superconducting energy gap (∆) indicating that phase fluctuations do not play any role in superconductivity in the film thickness and composition range investigated. Our results indicate that an interplay of quantum size effects (QSE) and superconducting proximity effect (SPE) controls the T_c with composition in these 3D nano-composite films.