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

TL;DR

This study investigates the proximity effect in ultrathin Pb/Ag multilayers, demonstrating oscillations in critical temperature and energy gap consistent with the Cooper limit model, and revealing deviations in very thin layers likely due to inverse proximity effects.

Contribution

It provides experimental validation of the Cooper limit model in multilayers and introduces the concept of inverse proximity effect in ultrathin superconducting layers.

Findings

Critical temperature and energy gap oscillate with layer addition.

Agreement with theory for layers thicker than 30Å.

Deviations occur in layers thinner than 30Å, indicating inverse proximity effects.

Abstract

We report on transport and tunneling measurements performed on ultra-thin 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\AA . Samples with single layers thinner than 30\AA 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.