OPTICAL-RESPONSE OF AN ULTRATHIN-FILM AND A LARGE-ANGLE GRAIN-BOUNDARY IN SUPERCONDUCTING YBA2CU3O7-DELTA

We have investigated the optical response of epitaxial ultrathin (3 nm ) YBa2Cu3O7-delta film on NdGaO3 substrate and thin (50 nm) film of YB a2Cu3O7-delta on a bicrystal SrTiO3 substrate. While the current-volta ge characteristics of the ultrathin film indicate a Kosterlitz-Thoules s transition (associated with vortex-antivortex unbinding) at a temper ature T(KT), we do not observe any optical response at or below T(KT). An enhanced optical response with nonbolometric characteristics was o bserved from the grain-boundary as compared to the bolometric optical response of the epitaxial film at the resistive transition. These resu lts indicate that the grain-boundary weak links play a significant rol e in the enhanced optical response near T(c).