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 thin films of comparable thickness. In this work, we characterize the London penetration depth and Ginzburg-Landau coherence length of S/N superlattices using polarized neutron reflectometry and electrical transport. Despite the normal metal spacer layer being significantly thinner than the coherence length, we find that the introduction of a thin N spacer between S layers leads to a dramatic increase in the London penetration depth and a decrease in the coherence length. Furthermore, the temperature dependence of the upper critical field suggests that the superlattices act as a layered 2D superconductor, despite the N thickness being more than a factor of two smaller than when such a 2D state is expected to occur. Using the measured values for the penetration depth and coherence length, we experimentally determine the Ginzburg-Landau parameter to quantify the superconducting disorder of the superlattice samples and compare to a single Nb thin film sample.