Distortions to the penetration depth and coherence length of superconductor/normal-metal superlattices

Superconducting (S) thin film superlattices composed of Nb and a normal-metal spacer (N) have been extensively utilized in Josephson junctions given their favorable surface roughness compared to Nb films of comparable thickness. In this work, we characterize the London penetration depth and Ginzburg-Landau coherence lengths of S/N superlattices using polarized neutron reflectometry and electrical transport. Despite the normal-metal spacer layers being only approximately 8% of the total superlattice thickness, we surprisingly find that the introduction of these thin N spacers between S layers leads to a dramatic increase in the measured London penetration depth compared to that of a single Nb film of comparable thickness. Using the measured values for the effective in- and out-of-plane coherence lengths, we quantify the induced anisotropy of the superlattice samples and compare to a single Nb film sample. From these results, we find that the superlattices behave similarly to layered 2D superconductors.