Enhancement of persistent photoconductivity in insulating high-Tcthin films

Persistent photoconductivity, characterized by a long-term increase in the conductivity due to illumination of oxygen-deficient high-temperature superconductors, is greatly enhanced in R${\mathrm{Ba}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{\mathit{x}}$ (R=Y or Gd) films as x decreases to 6.1. These results are in qualitative agreement with a model similar to that used to explain the existence of DX centers in GaAs. However, they cast doubts over several models commonly used to explain the mechanism behind this phenomenon and that of photoinduced superconductivity.