Infrared synchrotron-radiation transmission measurements on YBa<SUB>2</SUB>Cu<SUB>3</SUB>O<SUB>7-&#948;</SUB>in the gap and supercurrent regions

We report infrared synchrotron-radiation transmission measurements on ~1-&#956;m-thick films of high-Tc YBa2Cu3O7-&#948; from 40 to 290 cm-1 (5 to 36 meV). The films were deposited onto MgO substrates and showed resistive superconducting transitions with an onset at 83 K. Spectra below Tc show superconducting behavior, including the characteristic 1/&#969; supercurrent dependence for frequencies below 100 cm-1. The entire spectrum can be fit when a mattis-bardeen-type conductivity is included identifying increasing absorption beginning at a photon energy of 23 meV. However, a substantial residual absorption persists to the lowest measured frequencies. In addition, the appearance of phonons, some attributable to c-axis vibrations, suggests we are sampling a mixture of both a-b-plane and c-axis propertie