Reversible photostructural change in oxygen-deficient ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{\mathit{y}}$

Recently, several kinds of photoinduced effects on superconducting properties of ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{\mathit{y}}$ at low temperatures have been reported. Particularly, in the sample at the boundary of a superconducting-semiconducting transition, a large decrease in the resistivity after laser irradiation has been observed at low temperatures. The decreased resistivity was recovered by room-temperature annealing. To clarify the mechanism of these effects, we have investigated the hysteretic behavior of resistivity near room temperature and the photoirradiation effect on the x-ray-diffraction pattern at low temperatures. We explain this effect as a photostructural change involving oxygen rearrangement in the Cu-O basal planes induced by irradiation at low temperatures. This is similar to the mechanism whereby a recombination-enhanced defect reaction is observed in ionic crystals and semiconductors.