Effect of Cu alloying on metastable photoinduced absorption in ${\mathrm{Cu}}_{\mathit{x}}$(${\mathrm{As}}_{0.4}$${\mathrm{Se}}_{0.6}$${)}_{1\mathrm{-}\mathit{x}}$ and ${\mathrm{Cu}}_{\mathit{x}}$(${\mathrm{As}}_{0.4}$${\mathrm{S}}_{0.6}$${)}_{1\mathrm{-}\mathit{x}}$ glasses

We have investigated the metastable photoinduced absorption (midgap absorption and photodarkening) in ${\mathrm{Cu}}_{\mathit{x}}$(${\mathrm{As}}_{0.4}$ ${\mathrm{Se}}_{0.6}$ ${)}_{1\mathrm{-}\mathit{x}}$ and ${\mathrm{Cu}}_{\mathit{x}}$(${\mathrm{As}}_{0.4}$ ${\mathrm{S}}_{0.6}$ ${)}_{1\mathrm{-}\mathit{x}}$ glasses. The addition of copper reduces the fraction of the chalcogen atoms which are twofold coordinated. Essentially no photodarkening effect has been observed in the sulfur system for x≥0.01 or in the selenium system for x≥0.05. Two possible interpretations are suggested for this observation. (1) The photodarkening effect may occur but be masked by some electronic states created by the copper alloying. (2) The photodarkening effect may be due to correlated changes of the lone-pair p-electron wave functions on a scale greater than that of a nearest-neighbor distance. The observed coexistence in ${\mathrm{Cu}}_{\mathit{x}}$(${\mathrm{As}}_{0.4}$ ${\mathrm{Se}}_{0.6}$ ${)}_{1\mathrm{-}\mathit{x}}$ with large x (around 0.02%) of a dark and a photoinduced electron-spin-resonance signal suggests the coexistence of defects with positive and negative effective electron-electron correlation energies. Midgap absorption can be induced optically at low temperatures in ${\mathrm{Cu}}_{\mathit{x}}$(${\mathrm{As}}_{0.4}$ ${\mathrm{Se}}_{0.6}$ ${)}_{1\mathrm{-}\mathit{x}}$ with x≥0.05 and can be annealed away at room temperature.