Time-dependent Meissner fraction as a function of aging in an oxygen-deficientYBa2Cu3O7−δsingle crystal

Meissner- and shielding-effect measurements were made on an oxygen-deficient single crystal of ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$ as a function of aging time at room temperature following a quench from 225 \ifmmode^\circ\else\textdegree\fi{}C. It is seen that the Meissner fraction systematically increases from 11.1% to 13.2% as ${\mathit{T}}_{\mathit{c}}$ changes from 11.2 to 25.0 K. All aging was done within the helium exchange gas environment of the sample chamber. The results point to oxygen vacancy ordering rather than oxygen absorption as the mechanism for both the ${\mathit{T}}_{\mathit{c}}$ and Meissner-fraction increase.