Effect of carrier concentration on the superconducting property of Bi₂Sr₂Ca_1-xY_xCu₂O_y

The effect of the hole concentration on the superconducting property of the double CuO layer system Bi₂Sr₂Ca_1-xY_xCu₂O_y (0 ≦ × ≦ 0.5) was studied. The compounds were synthesized by the polymerized complex method. The onset temperature of the superconducting transition T_c (onset) was carefully measured in terms of complex susceptibility. T_c (onset) is enhanced as x increases up to x ∼ 0.1 and then decreases as x further increases. Wet chemical analyses were carried out to determine the average charges of Bi and Cu ions. In the Bi₂Sr₂CaCu₂Cu_y compound, relatively high concentration of Bi⁵⁺ ions was detected. To interpret the results, we propose that mixed valence Bi⁵⁺/Bi³⁺ acts as a charge reservoir for the CuO₂ planes.