Tunneling into Weakly Coupled Films of Tin and Aluminum in Proximity. II. From the Tin Side

Tunneling experiments have been performed into the tin side of aluminum-tin proximity sandwiches evaporated at room temperature onto an oxidized aluminum electrode $B$. The coupling between the aluminum and tin films in proximity was made weak by allowing slight oxidation of the tin to occur at the interface. Using the McMillan model of the proximity structure, values of the electron-transmission probability $\alpha$ at the interface may be derived from the experiments. The $\alpha$ value is related to the critical temperature of the sandwich and also to the positions of peaks in the McMillan density of states. These peaks are reflected in the tunneling currents when the electrode $B$ is superconducting. Values of $\alpha$ derived from these two kinds of measurements are found to be consistent and, qualitatively, to vary as expected with the oxidation conditions. Although the calculated curves of tunneling conductance as a function of applied voltage, when $B$ is superconducting, exhibit sharper structure than the experimental curves, they are otherwise in good agreement. In contrast to tunneling from the aluminum side of the proximity sandwich, the tunneling characteristics into the tin side, when $B$ is in the normal state, are similar to those into a BCS superconductor. On the whole, the experimental results are in good agreement with the predictions of the McMillan model.