Time-dependent superconducting behavior of oxygen-deficient ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{\mathit{x}}$: Possible annealing of oxygen vacancies at 300 K

Oxygen stoichiometries in single crystals of ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{\mathit{x}}$ are controlled by quenching from an ${\mathrm{O}}_2$-${\mathrm{N}}_2$ atmosphere at 520 °C to liquid nitrogen. After quenching, the superconducting behavior and lattice parameters are found to be time dependent. ${\mathit{T}}_{\mathit{c}}$ increases as much as 15 K in a period of several days. Apparently, annealing, with increasing oxygen vacancy order, occurs at room temperature.