Superconducting phonon structure in ultra-thin proximity-effect tunnel junctions

We report measurements of the phonon structure of superconducting Pb observed through ultra-thin layers (10-100 Å) of Cu for electrodes deposited onto liquid-helium-cooled substrates and annealed to 77 K, which show that the tunneling density of states is substantially changed even for the smallest practical normal-layer thicknesses. Comparison is made with theory and the implications for using proximity structures for the study of the phonon spectrum of those high-$T_c$ superconductors which do not form a native oxide suitable for high-quality tunneling studies are pointed out.