1/8 PROBLEMS IN THE La-, Bi- AND Y-BASED CUPRATES AND NEW ANOMALIES IN THE OVERDOPED REGION OF THE La-BASED CUPRATE

Abstract

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 Bi 2 Sr 2 Ca 1-x Y x( Cu 1-y Zn y)2 O 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-nbased cuprate as well as in the La-based cuprate. In Y 1-x Ca x Ba 2 Cu 3 O 7-δ, the 60 K plateau of Tc 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-Tc cuprates with the CuO 2 plane. Moreover, we have found transport and Tc anomalies around x=0.22 in La 2-x Sr x Cu 1-y M y O 4 ( 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.