Transient photoinduced conductivity in single crystals of YBa₂Cu₃O_6.3: ‘‘Photodoping’’ to the metallic state

The temperature dependences of the transient photoinduced conductivity in YBa₂Cu₃O_7-δ at different light intensities (I_L) and of the doping-induced conductivity at different δ are similar, indicative of ‘‘photodoping.’’ For δ≊0.7, signatures of the photoinduced transition to metallic behavior are observed at I_L>5×10¹⁵ photons/cm²; the deep resistivity minimum below 100 K, reminiscent of the onset of superconductivity in granular superconductors and in inhomogeneously doped samples, is interpreted in terms of phase separation and metallic-droplet formation. A longitudinal magnetic field (≤0.5 T) reduces both the resistivity minimum and the superlinear contribution to the transient photoconductance. For δ≊0.6, the lifetime of the photoexcited state is enhanced by nearly 3 orders of magnitude at high excitation levels, indicative of metastability.