Superconducting proximity effect in copper-lead sandwiches

Tunnelling measurements have been made at temperatures of about 0.1 K of the electronic densities of states on the copper side of copper—lead proximity sandwiches. We have investigated the effect on the energy gap and density of states of varying the coupling between the copper and lead films and also of varying only the thickness of the lead film. In both cases we have compared our results with the predictions of McMillan's theory. Our measurements of the energy gaps are in quantitative agreement with McMillan's predictions provided reasonable assumptions are made concerning the magnitude of the gap in the lead film. Some qualitative agreement also occurs concerning the form of the density of states, particularly in our most strongly decoupled specimens. We have also observed deviations from the theoretical predictions. Experimental evidence is presented which confirms that our results were not significantly affected by intermetallic diffusion in the samples.