The proximity effect in the lead-copper system by electron tunnelling

Abstract

The electronic structure resulting from the proximity effect has been investigated by electron tunnelling techniques. A well-defined gap-like structure is found at the surface of copper films deposited on lead. The observed ‘energy gap’ decreases with increasing copper thickness as does the deviation in the density of states. The form of the tunnelling characteristics shows that in the developed superconducting state there is a localization of the states lying just above the gap in the copper. We do not find the states within the gap as predicted by De Gennes and Saint-James.

By tunnelling into both sides of sandwiches of thin superimposed films of copper and lead we estimate the interaction parameter in copper, N(O)V, to lie between + 0·06 and -0·10, indicating that if copper becomes superconducting it has a transition temperature below 20 μ °K.