Far-infrared measurements of the phonon density of states of superconducting lead

Abstract

Far-infrared measurements of the derivative of the reflectivity of lead and lead alloys in the superconducting state are presented. The derivative of the reflectivity contains detailed information about the phonon spectrum including peaks not observed by tunneling spectroscopy. The peaks are, in general, in good agreement with neutron measurements and include evidence of phonon lifetime effects at a phonon frequency of $2\Delta$