Fermi velocity of silver from proximity effect tunnelling

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Abstract
We have measured the tunneling characteristics of a series of Al-oxide-(Ag1-xErx)Pb proximity effect sandwiches for normal metal thicknesses ranging from 500 A to 3 000 A and parallel magnetic fields of 0-1 000 gauss. Second derivative characteristics (d2V/dI2) reveal the Pb phonon structure through electron interference effects and also reveal structure at the Ag phonon frequencies. Application of a magnetic field less than the critical field of the sandwhich reduces the Ag phonon structure. Introduction of impurities reduces the interference effects with little effect on the Ag phonon structure. We are thus able to separate the two contributions to the tunnelling characteristics. Compa- rison with the theory of McMillan and Rowell then allows determination of VF. Comparison of the theoretical and experimental curves for the interference term shows a frequency dependent mean free path from phonon emission and allows an independent measure of the electron-phonon coupling in