Superconducting density of states induced by the proximity effect

Abstract

We have measured the excitation spectra induced by the superconducting proximity effect in superimposed thin films of copper and lead. The experiments were done at temperatures in the range 0·06–0·11 K, so that an energy resolution of about 15 μeV was obtained. On the copper side of specimens with copper films less than 1500 Å thick, the scale of the gap structure was in agreement with McMillan's theory using no adjustable parameters. However, the detailed form of the density of states was different. In particular we observed a smeared gap edge, a dip in the density of states at an energy close to the lead gap for a certain range of copper thicknesses, and evidence for the existence of states at zero energy. On the lead side of the sandwich the excitation spectra were gapless for thin lead films (< 1000 Å) backed by 2000 Å copper. As predicted by McMillan, thicker lead films exhibited an energy gap equal to that on the remote side of the copper, and many states at energies less than the lead gap.