Persistent photoconductivity in ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{6+\mathit{x}}$ films as a method of photodoping toward metallic and superconducting phases

Persistent photoconductivity (PPC) and metastable photoinduced superconductivity, recently discovered in semiconducting ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{6+\mathit{x}}$, have been investigated over the oxygen content of 0<x<1. Under exposure of semiconducting ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{6+\mathit{x}}$ films (x≃0.4) to visible light their resistance is found to decrease drastically. After interrupting the irradiation the resistance persists at this reduced level provided that the temperature is kept below 270 K. When the illumination dose is increased the semiconductorlike behavior of the films progressively changes to that of a metal and a superconductor. Prolonged irradiation leads to complete loss of resistivity below 5 K and simultaneous growth of a diamagnetic moment in the films, revealing pronounced enhancement of their superconducting properties. A slow relaxation of the PPC state was found only by warming the sample near to room temperature where it could be described by a thermally activated process with an energy barrier of ∼1 eV. The observed phenomena are attributed to photoexcitation of extra mobile holes into ${\mathrm{CuO}}_2$ planes, allowing a metastable superconducting phase to be initiated. We also discuss feasible microscopic mechanisms of PPC in oxygen-deficient Y-Ba-Cu-O films. It is suggested that the photoinduced superconductivity may have applications in fabrication of in situ optically tunable weak-link devices.