Effective Renormalization Behavior of Ag Films Backed by Superconducting Pb

Abstract
When the superconducting half of a superconductor-normal-metal film sandwich is a strong-coupling superconductor, phonon structure due to the superconductor (Pb)—but observed by tunneling into the normal metal (Ag)—provides a direct probe of the effective renormalization interaction in the normal metal. Unexpectedly, even fairly thick Ag films (≈ 350 Å) backed by Pb exhibit effective renormalizations approaching that of Pb, both in overall strength and energy dependence.