Temperature-dependent optical response of high- <i>T</i> c YBa2Cu3O7− <i>δ</i> (YBCO) thin films

We report temperature-dependent optical response measurements of YBa2Cu3O7−δ thin films in the visible spectral range under cryogenic conditions. We observe a sharp increase in transmittance near the superconducting transition temperature (Tc), followed by a saturation of transmittance within a few kelvins below Tc. This increase in transmittance is accompanied by a corresponding decrease in reflectance as the temperature drops below Tc; both quantities track the superconducting phase transition. Changes in transmittance are found to be wavelength dependent, with the maximum variation occurring at 633 nm and minimal at 450 nm. These observations establish a correlation between the variation in optical response and the superconducting phase transition in the visible regime. The results of our experiment highlight the potential for using non-contact optical measurements in the visible range to determine Tc. The effect can be explained using the two-fluid model, which can account for the observed temperature and wavelength dependence of the transmittance of the superconducting thin films.