Far-infrared transmission of superconductingKxC60films

Far-infrared transmission measurements have been performed on normal and superconducting ${\mathrm{K}}_{\mathit{x}}$ ${\mathrm{C}}_{60}$ films on Si substrates. The films are prepared by thermal sublimation followed by in situ doping in a vacuum-tight cell. The observed peak in the ratio of the superconducting to normal transmittance near 50 ${\mathrm{cm}}^{\mathrm{-}1}$ is consistent with a distribution of superconducting gaps in the range (2–5)${\mathit{kT}}_{\mathit{c}}$.