Low temperature optical response of a single grain boundary in superconducting YBa₂Cu₃O_7-δ thin film

We have investigated the optical response from a single grain boundary in a thin film (∼300 nm) of superconducting YBa₂Cu₃O _7-δ fabricated on bicrystals SrTiO₃ substrate. We have studied the temperature, bias current, and magnetic field dependence of the optical response in the temperature range of 15 K≤T≤35 K. We find that the optical response from the single grain boundary in this temperature range can be satisfactorily explained in terms of the optical response of a Josephson weak link. The optical response of the grain boundary is qualitatively similar to that of granular films seen in the earlier studies, indicating that the latter may be dominated by the optical response of grain-boundary weak links.