FAR-INFRARED SPECTROSCOPY OF THE HIGH-TC SUPERCONDUCTOR YBA2CU3O7-DELTA

We report measurements of the far-infrared reflectivity of polycrystalline samples of YBa//2Cu//3O//7// minus // delta which have their superconducting transition temperatures in the 90 K range. The measurements performed between 300 K and 10 K reveal temperature-dependent structures attributed to optical phonons in the 150 cm** minus **1 - 350 cm** minus **1 range, and excitation of the electrons across the energy gap. Complicated structures make unambiguous fitting with the Mattis-Bardeen theory difficult, but it seems that the energy gap is around 3. 5 KT//c for T VM LS TH T//c. Additionally, our results indicate the possibility of a second gap at an energy corresponding to 5. 4 kT//c.