Effect of Oxygen-Diffusion Possibly Involved in Perovskite Unit on Superconductivity of Various Oxygen-Doped Bi₂Sr₂CaCu₂0_8+y Single Crystals

For a highly oriented Bi₂Sr₂CaCu₂0_8+y single crystal with a lower as-grown T_c of 72K, on air-annealing at 250°C for 2h, its T_c remained unchanged, while increased to 83 K after annealing for 25 h. For another crystal with a higher as-grown T_c of 86.7K and annealed under the same condition, its T_c was unchanged except for a slight improvement of ΔT_c and was independent of the annealing time. X-ray diffraction showed that there were no changes of c-axis lattice parameter for the annealed crystals. If the above two crystals, after annealing at 250°C for 25 h, were further annealed at 600°C for 20 h, then T_c was increased to the same value of almost 88K and the c-axis lattice parameter showed a dramatic contraction. We suggested that the improvement of superconductivity of the sample annealed at much lower annealing temperature (250°C ) are associated with the oxygen diffusion involved in the pervoskite unit and this diffusion from the perovskite unit occurs earlier than that from Bi-O layers.