Non-Equilibrium Response In High Temperature Superconductors

The mechanism of optical response in high temperature superconducting thin films remains to be uncovered after a long period of time debating in the research society. A large part of our research has been concentrated on how to identify the mechanism of optical and microwave responses observed in granular YBCO films. In a particularly, we try to specify the origin of the response below the transition temperature of the samples, which has been thought of an nonbolometric process. The samples used in our experiments are carefully fabricated so that the contribution from the weak-links to the response can be separated from other effects such as vortex unbinding, etc.We observed that the response from the granular films is much larger than that from the epitaxial thin film during the superconducting transition. Since most of the response anomalies have been reported at temperatures below the superconducting transition in granular films, we measured the dependence of the response of the granular films on the bias current, temperature, and magnetic fields at temperatures below the superconducting transition. It has been speculated that these responses were dominated by weak links Josephson junction.We observed that the response peak at certain temperatures depending on the bias current reduced to zero at low temperatures in the absence of magnetic fields, while in the presence of magnetic field the response does not vanish even at very low temperatures. We modeled the temperature, bias current, and magnetic field dependence of theresponse by assuming a network of weak links embedded in the sample. Our analysis suggested that the radiation induced thermal modulation of critical current of the Josephson junctions covered partly account for the observation. Beside, the shift in temperatures; where response peaks appeared as bias current varies is significant which indicates that HTSC thin films posses the potentiality for fabricating temperature tunable radiation detectors.Measurements with YBCO granular Josephson junctions revealed possible presence of nonbolometric components, which is then useful for fabricating practical bolometers or switches. Finally our studies do support the emerging idea that unlike other high-Tc superconductors thin films, YBCO granular films may have a complex microwave-coupling interaction.