Diamagnetic Response of Normal-Superconducting Double Layers

The diamagnetism of a normal metal (N: Cu or Au), which is induced by the proximity effect of a superconductor (S: Nb), has been investigated for N-S double layers, which are formed by a thin-film deposition process. Detailed studies of samples, which have different electronic mean-free path \ell_N in N, suggest that \ell_N should be controlled by the impurity concentration rather than the mechanical imperfections in the lattice in order to clarify the \ell_N dependence of the proximity effect. Both the screening distance \rho in N and the parameter \nu in \rho \propto T^-\nu increase with an increase in \ell_N. This result can be understood on the assumption that the normal metal changes its behavior from the "dirty" limit (\xi_N>\ell_N) to the "clean" limit (\xi_N<\ell_N), where \xi_N is the coherence length in N.