Mechanism of T_c Increase in Na-Doped Bi₂Sr₂CaCu₂O_8+y

Samples of Bi_2.2Sr_1.8Ca_1.05Cu_2.15-xNa_xO_8+y (x=0, 0.7) were prepared by melt-processing technique. The maximum transition temperature increased up to 100 K, and T_c0 achieved 92.5K. In order to study the mechanism of T_c increasing, samples were annealed in a flowing oxygen, atmosphere. The reversibility of T_c was observed in undoped sample, while the T_c of Na-doped sample had no significant changes. We believe that the T_c is correlated with a redistribution of holes between the BiO layers and the CuO planes. Doping sodium changes the hole density of CuO planes into an “optimium” value so as to increase T_c.