EFFECT OF ANNEALING ON THE SUPERCONDUCTIVITY OF BI2SR2CACU2OY SINGLE-CRYSTALS

High-quality as-grown Bi2Sr2CaCu2Oy single crystals with superconducting transition temperatures T-c of 70-83 K have been annealed in air, flowing oxygen, and nitrogen at 400-750 degrees C. ac susceptibility measurements show that at 450-550 degrees C T-c is atmosphere and time independently enhanced to 85-86 K and a very narrow diamagnetic transition Delta T-c (10-90 % of the complete transition) of 0.5-1 K is obtained, and at 550-750 degrees C when treated in nitrogen or oxygen T-c increases or decreases and Delta T-c broadens with annealing temperature compared with crystals annealed at 450-550 degrees 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 Bi2Sr2CaCu2Oy superconductors.