Proximity effect in ultrathin Pb/Ag multilayers within the Cooper limit

We report on transport and tunneling measurements performed on ultrathin Pb/Ag (strong-coupled superconductor/normal metal) multilayers evaporated by quench condensation. The critical temperature and energy gap of the heterostructures oscillate with addition of each layer, demonstrating the validity of the Cooper limit model in the case of multilayers. We observe excellent agreement with a simple theory for samples with layer thickness larger than 30 Å. Samples with single layers thinner than 30 Å deviate from the Cooper limit theory. We suggest that this is due to the “inverse proximity effect” where the normal-metal electrons improve screening in the superconducting ultrathin layer and thus enhance the critical temperature.