Superconductivity in Cu and Pt by Means of Superimposed Films with Lead

The study of normal-superconducting sandwiches provides information on the type of electron-electron interaction taking place in the normal metal. The magnitude and sign of the interaction is obtained by comparing a characteristic length which measures the depth of penetration of electron pairs in the normal metal with the effective coherence length. In order to study the electron-electron interaction in copper and platinum, superimposed films of lead-platinum and of lead-copper were deposited on glass substrates kept at 77°K. It has been shown that the effective coherence length should be calculated from the coefficient of normal electronic specific heat and the residual resistivity. The data were then plotted in such a way as to exhibit all the temperature dependence in one term, and the slope of such a plot is the depth of penetration of electron pairs in the normal metal. With the experimental accuracy presently available, the electron-electron interaction in platinum can be bracketed between -0.25 (repulsive interaction) and an attractive interaction of $+0.09(T_c=2\mathrm{}\times {10}^{-3\mathrm{}}°\mathrm{K})$. In the case of copper, the data correspond to a transition temperature of about 6 mdeg.