PHOTORESPONSE OF Y-BA-CU-O THIN-FILMS ON THE MU-S TIME SCALE-EVIDENCEFOR LOCALIZED STATES NEAR E-F

WE report a new feature in the photoresponse signal of current-biased YBCO microstructures on very long, mu s time scales. Negative voltage transient spreading to beyond 30 mu s after excitation by much shorter laser pulses has been observed, and detailed and careful analysis sho w conclusively that this cannot be explained by any kind of direct bol ometric mechanism. We therefore propose a nonbolometric kinetic induct ance mechanism as the origin of this response, where the relaxation of the photoexcited quasiparticles through localized states near E-F tak es place. This mechanism is entirely consistent with our recent observ ations using femtosecond time-resolved spectroscopy.