Superconductivity in Pr- and Ce-doped Bi₂CaSr₂Cu₂O_y systems

Abstract

We report here a systematic study of Pr and Ce doping (at the Ca site) in Bi₂CaSr₂Cu₂O_y. 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 (α⁻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.