C-AXIS REFLECTANCE AND THE SUPERCONDUCTING ORDER-PARAMETER OF YBA2CU3O7-DELTA

We have performed reflectivity measurements with (E) over right arrow parallel to c on superconducting YBa2Cu3O7-delta Single crystals in th e energy range from 5 meV to 600 meV. The temperature was varied betwe en 10 K and 300 K. At room temperature the energy range was extended t o 6 eV to improve the basis for a Kramers-Kronig analysis. At low temp erature each sample shows a reflectivity edge at an energy of about kT (c). With decreasing temperature we observe a rapid decrease of the el ectronic scattering rate below T-c. Using a simple two-fluid-model the analysis of the data shows evidence for a non BCS-like superconductin g order parameter.