SUPERCONDUCTIVITY IN PR-DOPED AND CE-DOPED BI2CASR2CU2OY SYSTEMS

We report here a systematic study of Pr and Ce doping (at the Ca site) in Bi2CaSr2Cu2Oy. For both Pr and Ce substitutions T(c) decreases with the dopant concentration. Results are interpreted in terms of the hole filling due to aliovalent substitution in the system. At a higher doping level a metal-insulator transition is observed. Near the transition the localization length (alpha-1) is calculated and the possibility of having local pairs in the insulating phase is invoked. Around the transition region a dimensional changeover from two to three dimensions is observed in the variable-range hopping (VRH) regime and its consequence on superconductivity is discussed. The lattice parameters have been estimated through x-ray diffractometry and the possibility of Pr and Ce existing in more than the 3 + oxidation state in the system is inferred. Results are also compared with the existing data on other rare-earth dopings (at the Ca site) in the same system.