Transient photoinduced conductivity in single crystals of ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{6.3}$: ‘‘Photodoping’’ to the metallic state

The temperature dependences of the transient photoinduced conductivity in ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ at different light intensities (${\mathit{I}}_{\mathit{L}}$) 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 ${\mathit{I}}_{\mathit{L}}$>5×${10}^{15}$ photons/${\mathrm{cm}}^2$; 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.