Tunneling investigation of Zn--Pb proximity sandwiches

We have measured the tunneling characteristics (I, dV/dI, and d2V/dI2 vs. voltage) for a series of junctions of the form Al--oxide--Zn--Pb for Zn thicknesses ranging from 400 to 4000 A (T>Tatsub c,Znat). From the first and second derivative characteristics we observe two electron interference effects (Tomasch and McMillan interference) plus structure due to the interaction of the superconducting electrons and the Zn phonons. For junctions with Zn layer thicknesses < or approx. =800 A these contributions can be separated by application of a magnetic field less than the critical field Hatsub cat of the sandwich. For thicker sandwiches a breakdown occurs before Hatsub cat is reached. The dominant zero-field interference effect is seen to be the Tomasch oscillation. The electron interference terms provide a direct measurement of the Fermi velocity in Zn, while interactions with the Zn phonon provide a measurement of the electronphonon coupling constant. For Zn we find vatsub fat<0.73 +- 0.1 x 108 cm/sec and lambda (dimensionless electron--phonon coupling constant) <1.38 +- 0.1. Measurement of the dependence of the breakdown field on the Zn layer thickness indicate that proximity effect Zn exhibits a zero-field penetration depth of 750 A