STUDY OF PHOTORESPONSE OF HIGH-T-C Y-BA-CU-O SUPERCONDUCTING ULTRATHIN FILMS USING A PICOSECOND LASER-PULSE TRAIN

We have systematically studied the photoresponse of Y-Ba-Cu-O (YBCO) u ltrathin films to a train of picosecond laser pulses. The onset and de cay of nonequilibrium superconductivity due to different levels of opt ical excitation can be observed within the single pulse train. Thermal and nonthermal responses can be clearly identified. In addition, the superconducting film resistance was found to increase progressively wi th laser power above the onset threshold. The YBCO thin films were pos sibly excited into a nonequilibrium intermediate (resistive) state fro m the superconducting state. The laser intensity dependence indicated a nonthermal origin for these voltage pulses.