Oxygen-Diffusion and Structural Modification in Air-Annealed Superconducting Bi₂Sr₂CaCu₂O_y Single Crystals

Bi₂Sr₂CaCu₂O_y single crystals were air-annealed at 300, 400, 600 and 750°C for 20h in consequence. Measurements of ac susceptibility and x-ray diffraction showed that at annealing temperature below 400°C, the change of T_c was dominated by oxygen-diffusion involved in pervoskite unit and was strongly sample-dependent. This diffusion from pervoskite unit occurred earlier than that from Bi-O layers. At annealing temperature over 400°C, with the intercalation of excess oxygen into Bi-O layers, the hole concentration in the crystal was redistributed so that Bi-O reservoir fully determined superconductivity. From the changes of non-uniform strain in annealing process, it was found that structural modification was also closely correlated to superconductivity.