Tunnel-Effect Measurements on Superimposed Layers of Lead and Aluminum

Superimposed Al and Pb films are studied using the tunnel effect as an indication of the influence of a normal metal (Al) on a superconductor (Pb). The structures are composed of four layers, Al/${\mathrm{Al}}_2$ ${\mathrm{O}}_3$/Pb/Al. The Pb and top Al layers were vacuum-evaporated onto 77°K substrates to eliminate diffusion problems. Curves are presented which show the effect of an overlayer on the tunnel-junction $I-V$ characteristic and density-of-states function. The film thicknesses are varied over a wide range. The variation in energy gap with Pb thickness is plotted for a constant Al overlayer of 3100 Å. In addition there are three samples made to determine the critical temperature by resistive measurements with the same structure as a check on the reliability of the tunnel-function results. The energy-gap transition occurs between 300 and 4500 Å of Pb. Comparison with Hauser and Theuerer's results on superimposed Al and Pb films is included in the discussion.