Phase separation of photogenerated carriers and photoinduced superconductivity in high-T_c materials

The temperature dependences of the transient photoinduced conductivity at different light levels in single crystals of YBa₂Cu₃O_7-δ and ultrathin (∼40-Å) epitaxial films of DyBa₂Cu₃O_7-δ with fixed δ are qualitatively and quantitatively similar to those of the doping-induced conductivity in YBa₂Cu₃O_7-δ and DyBa₂Cu₃O_7-δ with different oxygen content δ, indicative of ‘‘photodoping’’ over a wide range of resistivities. Signatures of the photoinduced transition to metallic behavior are observed at light intensities greater than 10¹⁵ photons/cm². At these high levels of photoexcitation, the thermal activation energy approaches zero and a minimum appears in the temperature dependence of the photoresistivity below 100 K. The resistivity minimum, reminiscent of the onset of superconductivity in inhomogeneous samples and in granular superconductors, is interpreted in terms of a phase separation of the photogenerated carriers and metallic-droplet formation subsequent to photoexcitation. A modest longitudinal magnetic field (≤0.5 T) reduces both the resistivity minimum and the superlinear contribution to the transient photoconductance. For oxygen levels close to the metal-insulator transition (δ≊0.6), the lifetime of the photoexcited state is enhanced by nearly three orders of magnitude at high excitation levels, indicative of metastability.