TRANSIENT PHOTOINDUCED CONDUCTIVITY IN SINGLE-CRYSTALS OF YBA2CU3O7-DELTA - PHOTODOPING TO THE METALLIC STATE

The temperature dependences of the transient photoinduced conductivity in YBa2Cu3O7-delta at different light intensities (I(L)) and of the doping-induced conductivity at different-delta are similar, indicative of "photodoping." For delta almost-equal-to 0.7, signatures of the photoinduced transition to metallic behavior are observed at I(L) > 5 x 10(15) photons/cm2; the deep resistivity minimum below 100 K, reminiscent of the onset of super-conductivity in granular superconductors and in inhomogeneously doped samples, is interpreted in terms of phase separation and metallic-droplet formation. For delta almost-equal-to 0.6, the lifetime of the photoexcited state is enhanced by nearly 3 orders of magnitude at high excitation levels, indicative of metastability.