Compound geometrical resonances in superconducting Zn-Pb film sandwiches

Thick, clean Zn films, when backed by clean Pb films of sufficient thickness, produce first-derivative tunneling spectra that differ markedly from the predictions of Wolfram's theory in the central virtual-state region. Gallagher has recently extended Wolfram's theory to allow for quasiparticle geometrical resonances in both metal layers, and, using his results, we are able to explain the anomalous portions of our spectra, as well as those portions which continue to agree with Wolfram's theory. For the $c$-axis orientation of Zn and the [111] orientation of Pb, we obtain Fermi velocities of (1.31 ±0.020) × ${10}^6$ and (0.78 ± 0.04) × ${10}^6$ m/s, respectively. Our estimate of the Zn electron-phonon coupling parameter, $\lambda =Z_N\mathrm{}\left(0\right)-1=0.42\mathrm{}$, agrees with theoretical estimates by Tomlinson and Swihart, but away from the Fermi surface our estimates of the complex renormalization function generally exceed theirs.