Phonon Contribution to the Far-Infrared Absorptivity of Superconducting and Normal Lead

Abstract

Low-temperature measurements of the absorptivity of superconducting and normal Pb in the phonon frequency region 15-200 ${\mathrm{cm}}^{\ensuremath{-}1}$ show an onset of absorption when the photon energy is large enough to excite appreciable numbers of acoustic phonons. The onset begins aroung the transverse phonon frequency $\ensuremath{\Omega}\ensuremath{\simeq}35$ ${\mathrm{cm}}^{\ensuremath{-}1}$ in the normal metal and around $\ensuremath{\Omega}+2\ensuremath{\Delta}\ensuremath{\simeq}55$ ${\mathrm{cm}}^{\ensuremath{-}1}$ in the superconductor. Most features of the data can be explained by a "Golden Rule" calculation of the phonon-generation process proposed by Holstein.