Superconducting Proximity Effect of Nb

Previous investigators have studied the proximity effect of a number of nontransition-metal superconductors. In this investigation the proximity effect of Nb (a transition-metal superconductor) has been studied using the Nb-Cu and Nb-Ni systems. The films were sputtered sequentially without breaking vacuum. The sandwiches were maintained at temperatures below 200°K until after $T_c$ had been measured, to preclude chemical or metallurgical interactions. Sandwich $T_c$ has been obtained as a function of Nb film thickness for thick overlayer films. The Nb-Cu data are in agreement with the de Gennes-Werthamer theory. The Nb-Ni data follow the form of the adjustable-parameter ($\beta \equiv \frac{\hslash }{2\pi k_B{\tau }_s}$) theory proposed by Werthamer. However, Ni is found to have less effect on Nb than was reported for Pb: a value of $\beta =7\mathrm{}$ is obtained for Nb compared to $\beta =43\mathrm{}$ for Pb. Possible explanations of the observed behavior are discussed.