Penetration depth in proximity-effect superconductors

We have studied magnetic field penetration into the normal-metal side of proximity-effect sandwiches by measuring the periodic magnetic field dependence of the dc Josephson current in tunnel junctions containing the normal-metal/superconductor bilayers. The systems studied include the bilayers Ag/Pb, Al/Pb, Ag/Sn, and Sn/Pb and the trilayer sandwich Ag/Sn/Pb, in the temperature range down to 0.1 K. We find that the magnetic field is screened out of the normal metal in much the same way as for a type-I superconductor, with a penetration depth that becomes independent of the normal-metal thickness, or the temperature, at sufficiently low temperature. The magnitude of the induced normal-metal penetration depth is a function of the normal-metal parameters and the inverse of the gap in the backing superconductor.