Electron tunneling study of the superconducting proximity effect in Pb-Cd

Abstract

Electron tunneling has been used to study the superconducting state in superimposed Pb-Cd thin films in which the Pb thickness varied from 950 to 330 Å. The measured transition temperature $T_c$ varied from 7.22 to 4.86 K in the samples, and the observed energy gaps varied from 1.38 to 0.43 meV. The tunneling density of states as recorded on either side of the sandwich resembles those predicted by the McMillan model, but there are some quantitative differences. The observed phonon structure in the density of states occurs at the same energy as that in bulk Pb, and the magnitude of the structure scales with $T_c^2$. The shape of the calculated phonon spectral function remains essentially unchanged throughout this series.