1/8 problems in the La-, Bi- and Y-based cuprates and new anomalies in the overdoped region of the La-based cuprate

Recent experimental work on the 1/8 problem by our group is reviewed not only in the La-based cuprate but also in the Bi- and Y-based ones. In the partially Zn-substituted Bi₂Sr₂Ca_1-xY_x(Cu _1-yZn_y)₂O_8+δ, we have found anomalous suppression of superconductivity at x = 0.30 = 0.35, where the hole concentration per Cu p ∼ 1/8. Moreover, it has been revealed from the μSR measurements that the Cu-spin fluctuations exhibit slowing-down behavior at low temperatures in these samples. These results are suggestive of the stripe correlations of holes and spins tending to be pinned by Zn at p ∼ 1/8 in the Bi-based cuprate as well as in the La-based cuprate. In Y_1-xCa_xBa₂Cu₃O_7-δ, the 60 K plateau of T_c has been found not to be correlated with the oxygen content but to be interpreted as being due to the suppression of superconductivity at p ∼ 1/8. Accordingly, we conclude that the 1/8 problem is common to all high-T_c cuprates with the CuO₂ plane. Moreover, we have found transport and T_c anomalies around x = 0.22 in La_2-xSr_xCu_1-yM_yO₄ (M = Zn, Ga), suggesting a possibility that an order of holes and/or spins is formed or fluctuates in these samples in the overdoped region.