Phonon structure in the tunneling characteristics of tin-lead sandwiches

A series of tunneling experiments have been performed on bilayers of tin and lead. Tunneling into the tin side of the proximity effect sandwich the phonon structure of tin is readily observed in d2V/dI2 vs V at 4.2 K where tin is a normal metal. The structure is very similar to that observed in tin alone below its transition temperature. In addition there appears some structure from the lead phonons which is strongly dependent on the thickness of the tin film. (author)