The role of defects in persistent photoconductivity in YBa₂Cu₃O_6+x

In the persistent photoconductivity (PPC) phenomena, illumination of a material with light leads to a long-lived photoconductive state. During illumination of oxygen deficient YBCO near the insulator-metal transition, the illuminated material may even become superconducting. Accompanying the large relative changes in resistance are transport and structural changes as well as changes in the photoluminescence and infrared quenching spectrum. Infrared quenching data show that the magnitude of IR quenching saturates quickly as a function of visible photon dose, long before the PPC effect saturates. Our experimental and theoretical results have suggested that persistent photoconductivity (PPC) in YBCO may be explained by a model that incorporates both electrons trapped at defects (oxygen vacancies) and resulting structural rearrangement in the basal plane. A simple cellular automata model based on the idea that trapped electrons in the basal plane lead to structural rearrangements when illuminated with light predicts the correct time dependence of the photoexcitation process.