Tunneling investigation of Cu-Pb proximity sandwiches. Barrier transmission effects

We have measured the tunneling characteristics (I, dV/dI, and d²V/dI²vs. voltage) for a series of junctions of the form Al-Ox-Cu-Pb for Cu thicknesses ranging from 400 to 3000 ». From the second derivative characteristics we observe two distinct contributions, one from an electron interference effect and another from the interaction of the superconducting electrons with phonons in the Cu. These contributions may be separated by the application of a magnetic field less than the critical field of the sandwich. The electron interference effect provides a direct measurement of the Fermi velocity in the normal metal, while the interactions with the Cu phonons provide a measurement of the electron-phonon coupling constant. Contamination of the interface between Cu and Pb by oxidation of the Cu allows us to decouple the two films and observe the effects of changing the transmission coefficient t. We find that the electron interference is reduced as t is reduced but its energy dependence is unchanged, so that the evaluation of V_F is unaffected. The phonon structure from the Cu is also decreased with t. The measured energy gap is proportional to the amplitude of the interference effect, indicating that both measure the pair amplitude at the free surface of the Cu. The phonon structure in Cu is proportional to the induced gap squared, independent of whether the gap is varied by a change in thickness or in transition coefficient. For Cu we find V_F = 1.06 × 10⁸ cm/sec and the dimensionless electron-phonon coupling constant λ = 0.11 ± 0.03.