Effective renormalization behavior of Ag films backed by superconducting Pb

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 (approx. = 350 A) backed by Pb exhibit effective renormalizations approaching that of Pb, both in overall strength and energy dependence