Superconducting cuprates of the series Bi₂Ca_1-xLn_xSr₂Cu₂O_{8+ δ} (Ln = rare earth or Y)

Abstract

A systematic investigation of the properties of the series Bi₂Ca_1-xLn_xSr₂Cu₂O_{8+ δ} (Ln-rare earth or Y) of superconducting cuprates has been carried out by employing a variety of techniques. In these cuprates, the a and b parameters of the unit cell increase with x while the c parameter decreases. These cuprates show interesting superlattice modulation with both 4b and 8b periodicity depending on the rare earth when x=1.0. This observation suggests that the superlattice modulation has nothing to do with superconductivity, but may be determined by the oxygen stoichiometry. The oxygen excess, delta , increases linearly with x, but the hole concentration shows a maximum around x = 0.25 or delta approximately = 0.225+or-0.025. Interestingly, T_c also shows a maximum around x = 0.25. The authors have examined the nature of copper by XANES and Cu core level spectra. The relative intensity of the satellite in the Cu(2p) spectrum shows a minimum at x = 0.25. The Raman band around 630 cm^-1 due to the oxygen atoms in Bi-O layers decreases with increase in x, as does the a parameter.