Enhancement of the superconducting critical temperature in Bi2Sr2CaCu2O8+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 Bi2Sr2CaCu2O8+delta (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 (rho(0)) 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 rho(0) 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.