Comparative study of transport properties of Bi₂Sr₂Ca₂Cu₃ O_10+δ and Bi₂Sr₂CaCu₂O_8+δsingle crystals

The doping dependence of the in-plane resistivity ρ_a(T) and the out-of-plane resistivity ρ_c(T) have been systematically measured for the triple-layered system, Bi₂Sr₂Ca₂Cu₃ O_10+δ (Bi-2223). In comparison with the bilayered system, Bi₂Sr₂CaCu₂O_8+δ (Bi-2212), we found in Bi-2223 that the superconducting transition temperature T_c and pseudogap formation temperature T*, below which ρ_c shows a typical upturn, do not change from their optimum values in the overdoped region, even though doping actually proceeds. This result suggests that inequivalent hole doping occurs between the outer and inner planes. In the overdoped region, the carriers are mostly doped in the outer plane and the inner plane remains at an optimum doping level.