Evidence for attractive pair interaction in diffusive gold films deduced from studies of the superconducting proximity effect with aluminium

Abstract

An experimental study of the proximity effect of superconductor-normal metal films with the help of low-temperature scanning tunneling spectroscopy is reported. The behaviors of bilayers of the noble metals gold and silver in contact with the superconductor aluminum are compared for various thicknesses of the normal metal. Although the normal conducting properties of Au and Ag are very similar to each other, the measured differential conductance spectra from which the quasiparticle density of states is deduced differ markedly. While the behavior of the Al/Ag system follows the quasiclassical theory of the proximity effect for diffusive systems, differences exist for the Al/Au system. The absolute value of the induced minigap in Au is larger than predicted by theory, and its suppression with increasing temperature is weaker. These observations are quantitatively accounted for by including a finite interaction parameter for Au of (N0V)Au=0.10±0.03. The third investigated metal is palladium, which is close to ferromagnetism. The method presented here enables one to detect small superconducting correlations by investigating a spectroscopic property rather than the supercurrent or the critical temperature.