Absolute values of the far-infrared (FIR) absorption of Pb, ${\mathrm{Pb}}_{0.9}$${\mathrm{Bi}}_{0.1}$, ${\mathrm{Pb}}_{0.8}$${\mathrm{Bi}}_{0.2}$, and ${\mathrm{Au}}_{0.9}$${\mathrm{Cu}}_{0.1}$ have been obtained at 4.2 K using a calorimetric technique. Measurements were made at discrete frequencies over the range 20–150 ${\mathrm{cm}}^{\mathrm{-}1}$ (0.6–4.5 THz or 500–70 μm) using an optically pumped FIR laser as the source of radiation, and with a typical absolute error in the absorptivity of approximately 0.1%. The absorption in pure Pb and Pb-Bi alloys increases sharply at the superconducting energy gap and shows structure at frequencies that can be associated with peaks in the phonon density of states, and which is ascribed to the Holstein photon-phonon-electron process. However, it appears from both the magnitude and the enhanced structure of the absorption in the alloys that the process is enhanced in the presence of impurity scattering, suggesting that existing theories are incomplete.

• Received 10 July 1989

DOI:https://doi.org/10.1103/PhysRevB.41.4056