Low Frequency Optical Response in High-Temperature Superconductors

The mechanism of optical response in YBa _2Cu_3O_ {7-delta} (YBCO) thin films has been a debated issue for a long time. A large part of our research has concentrated on identifying the mechanisms of optical response observed in the granular YBCO films, particularly, the origin of the optical response below the transition temperature that has been termed nonbolometric. We performed our experiments with controlled samples where the contribution from the weak-links can be separated from other effects such as vortex unbinding. We restricted our studies to devices whose response times are in the order of milliseconds, typical of the response times of bolometers used in commercial detectors. We observed that the normalized optical response from the grain-boundary is much larger than that from the epitaxial thin film during the superconducting transition. Since most of the optical response anomalies have been reported at temperatures below the superconducting transition in granular films, we measured the dependence of the optical response of single grain-boundaries on the temperature and the bias current at temperatures below the superconducting transition. The bias current and the temperature dependence of the optical response of single grain boundary weak-links in our experiments were qualitatively similar to that observed in granular YBCO films. It has been speculated that these responses were dominated by weak-links but we demonstrated it experimentally for the first time. We observed that the optical response peaks at certain temperatures depending on the bias current and reduces to zero at lower temperatures. We modeled the temperature and the bias current dependence of the optical response observed below the transition temperature in terms of radiation induced thermal modulation of the critical currents of Josephson junctions. This shift in the peak response temperatures with bias currents is significant indicating that the bicrystal grain-boundaries offer the potential for fabricating temperature tunable radiation detectors. Our optical response measurements with Nd _{1.85}Ce_{0.15 }CuO_{4-rm y} (NCCO) thin films revealed possible presence of photoconductivity in this electron-doped superconductor. Though NCCO may not be useful for fabricating practical bolometers, our studies do support the emerging idea that unlike other high-T_{c} superconductors, NCCO may have a complex band structure where both electrons and holes may participate in the transport phenomenon.