Time-dependent oxygen ordering and its effect on ${\mathit{T}}_{\mathit{c}}$ in off-stoichiometric ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{\mathit{z}}$

The increase in ${\mathit{T}}_{\mathit{c}}$ recently observed by Veal et al. and Jorgensen et al. during room-temperature aging of quenched ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{\mathit{z}}$ samples is explained by studying the microscopic changes that take place in a simulated aging experiment. Monte Carlo simulation shows that the threefold coordinations of oxygen around Cu in the basal plane are converted into twofold and threefold coordinations during room-temperature aging. The hole doping induced by this conversion results in the increase in ${\mathit{T}}_{\mathit{c}}$. Both tetragonal-orthorhombic ordering and additional superstructure ordering are responsible for the observed changes in Cu coordinations. It is shown that aging of samples with high oxygen content will not significantly increase ${\mathit{T}}_{\mathit{c}}$.