Proximity Effect-Induced Superconductivity and NMR Relaxation in Nb-Cu Multilayers

The nuclear spin lattice relaxation rate, T1-l, for 63Cu in zero field from 8 K down to 0.3 K, has been measured in Nb-Cu multilayered thin films where both the Nb- and Cu-layer thicknesses are much smaller compared to the coherence lengths. A BCS-like temperature dependence of T1-1 replaced gradually by a Korringa-law-like one at low temperatures has been found. A dependence of the relaxation on the Cu-layer thickness at low temperatures has also been found. It has been proposed and confirmed by a series of experiments that superconductivity in the Cu-layer induced by the proximity effect has an anisotropic energy gap, reflecting the geometrical feature of a thin film.