Proximity-Effect Tunneling in Pd-Pb Sandwiches

Tunneling characteristics of junctions of the form $\mathrm{Al}{\mathrm{O}}_x$-Pd-Pb are reported. From the thickness dependences of $T_c$ and the induced gaps we find that the total BCS interaction ($\mathrm{NV}$) in Pd is 0±0.05. We find a Fermi velocity of ∼ 0.2×${10}^8$ cm/sec indicating that the narrow $d$-band electrons dominate our measurements. Pd phonon structure is not resolved, setting an upper limit of $\lambda \lesssim 0.2$ for the effective electron-phonon interaction. These results indicate that paramagnon effects are substantially less important for superconductivity than was previously envisioned.