Phonon structure in the tunneling characteristics of thin proximity effect sandwiches

The tunneling spectra of Al-oxide-N-S proximity effect junctions show phonon structure not only from the superconducting layer S, but also from the normal metal layer N. We report experimental results for junctios with Ag, Cu, and Al normal metal layers (100-400 Å thick) and Pb or Sn superconducting layers (∼3000 Å thick). We compare these experimental results with the McMillan geometrical resonance model of the proximity effect and with an extended McMillan tunneling model. The extension includes the effect of the frequency-dependent electron-phonon interaction in the normal metal. Finally, we estimate the electron-phonon coupling constant λ for the normal metals from the size of their phonon structure in the tunneling spectra.