Enhancement of the superconducting critical temperature in Bi₂Sr₂CaCu₂O_8+δ by controlling disorder outside CuO₂ planes

We investigate the effect of disorder at various lattice sites outside the CuO₂ plane on the superconducting critical temperature T_c of the cuprate superconductor Bi₂Sr₂CaCu₂O_8+δ (Bi2212). The most effective disorder turns out to be that at the Sr site in the neighboring blocks, which contain apical-oxygen atoms. The Sr-site disorder reduces T_c and also produces a residual component (ρ₀) in the in-plane resistivity, the magnitude of which is found to be proportional to the magnitude of T_c reduction. We demonstrate that both the T_c-degradation rate and ρ₀ decrease as the number of the CuO₂ planes increases in the unit cell, that is, as the number of the neighboring SrO blocks decreases. In real crystals of Bi-based cuprates, main source of disorder is Bi atoms randomly occupying the Sr sites. We have succeeded in reducing the Bi content at Sr sites as much as possible and achieved T_c=98 K, a high-T_c record in Bi2212.