Infrared synchrotron-radiation transmission measurements on YBa2Cu3O7- in the gap and supercurrent regions

We report infrared synchrotron-radiation transmission measurements on 1-m-thick films of high-Tc YBa2Cu3O7- 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/ 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-bplane and c-axis properties.