Effect of annealing on the superconductivity of ${\mathrm{Bi}}_2$${\mathrm{Sr}}_2$${\mathrm{CaCu}}_2$${\mathrm{O}}_{\mathit{y}}$ single crystals

High-quality as-grown ${\mathrm{Bi}}_2$ ${\mathrm{Sr}}_2$ ${\mathrm{CaCu}}_2$ ${\mathrm{O}}_{\mathit{y}}$ single crystals with superconducting transition temperatures ${\mathit{T}}_{\mathit{c}}$ of 70–83 K have been annealed in air, flowing oxygen, and nitrogen at 400–750 °C. ac susceptibility measurements show that at 450–550 °C ${\mathit{T}}_{\mathit{c}}$ is atmosphere and time independently enhanced to 85–86 K and a very narrow diamagnetic transition Δ${\mathit{T}}_{\mathit{c}}$ (10–90 % of the complete transition) of 0.5–1 K is obtained, and at 550–750 °C when treated in nitrogen or oxygen ${\mathit{T}}_{\mathit{c}}$ increases or decreases and Δ${\mathit{T}}_{\mathit{c}}$ broadens with annealing temperature compared with crystals annealed at 450–550 °C. The results are difficult to explain simply by the consideration of oxygen diffusion. Combined with the phase decomposition induced in the annealed crystals, it is suggested that the structural modification may play an important role in the variation of superconductivity of ${\mathrm{Bi}}_2$ ${\mathrm{Sr}}_2$ ${\mathrm{CaCu}}_2$ ${\mathrm{O}}_{\mathit{y}}$ superconductors.