SUBSTITUTIONAL AND RELATED STUDIES IN CUPRATE SUPERCONDUCTORS

A short account of the substitutional studies in various high T-c cuprates carried out in the authors' laboratory are presented here. General considerations about the iso- and alio-valent substitutions at various cationic sites have been enunciated. Phase diagram of the La-Sr-Cu-O system has been studied as a function of Sr content. Further localization effect with Zn incorporation in place of Cu leads to significant results about the intrinsic competition between localization and superconductivity. Substitution of rare-earths at Ca site in Bi-2122 synthesized with varying heat treatment also substantiate these results. Though Cu-site replacements have been seen to affect superconductivity deleteriously, substitution of Pr, Ce and Tb in rare-earth based cuprates present complex situation. The ensuing localization effects have been examined in conjunction with the variable range hopping processes in the low temperature regime where a semiconducting like behaviour is observed. A brief resume about the thermal properties measurement techniques like specific heat and thermopower is also presented. Pressure induced effects on the superconducting transition of Y- and Bi- based cuprates have been examined to look for a possible T-c enhancement or a phase transformation. More recent techniques of scanning tunneling microscopy and spectroscopy (STM/STS) have been employed to obtain atomically resolved images of high T-c cuprates. The extra oxygen intercalated in the Bi-O redox layers is seen to be detrimental to the Tc of Bi-2122. The STS observations confirm that intercalation makes the Bi-O layers more insulating which has the adverse effect on T-c. These studies have paved way in enhancing the T-c of Bi-based superconductors.