On the electron tunneling study of the superconducting proximity effect in Pb-Cd

Abstract

We show that the recent experimental data of Toplicar and Finnemore on the superconducting proximity efiect in P&Cd thin films can be understood within the McMillan tunneling model by using our new transition temperature i'r,j formula and determining the parameters self-consistently. Agreement between the theoretical and the experimental values of T,, the gap, and the general shape and the height of the electronic density of state N,(E) curves is very good. In agreement with experiment, we find no low-energy peak in the density of states.Recently, Toplicar and Finnemore' (TF) published their results on the superconducting proximity effect in Pb-Cd thin films. They tried to understand their data in terms of the McMillan tunneling model' (MTM). However, the following discrepancies were found: (1) the theoretical electronic density of states curves for the ds = 330 A and ds = 670 A films (ds = thickness of Pb) were peaked much more than the experimental ones;(2) the theoretical value of the energy gap R for the ds = 670 A film was much lower than the experimental value; (3) for the ds = 670 '4 film, theory predicted a low-energy peak in the density of states which was absent in the data. The above authors could not find a plausible explanation for the discrepancies.In this paper we wish to point out that the discrepancies can be understood reasonably well by (1) using our new transition temperature (T,) formula, which is valid for the case of TzN # 0 (for Cd, TcN = 0.56 K) and (2) finding the parameters of the MTM self-consistently (whcih was not done by TF). In fact, point by point agreement between our theoretical and the experimental values of T, and R is excellent and, in agreement with *