Implication for phase separation in an overdoped Y-Ca-Ba-Cu-O superconducting system

A series of polycrystalline, oxygenated Y 1-x Ca x Ba 2 Cu 3 O 7-δ (x=0; 0.025; 0.05; 0.1 and 0.2) samples is investigated by XRD and magnetic measurements. It is established that increasing Ca content leads to T c suppression, thus supporting the sample overdoping. The width of magnetization loops (ΔM(H) T=const ) is reduced. However ΔM(H) T=const is a measure of the intragranular critical current in a magnetic field. Its suppression may indicate a decrease in the pinning ability of the material and/or the quality of the superconducting condensate. The mAlN pinning centers in Y 1-x Ca x Ba 2 Cu 3 O 7-δ are the oxygen vacancies, but their amount increases with x. Therefore, ΔM(H) T=const reduction suggests that phase separation appeares and the number of Cooper pairs decreases when the overdoping is stronger. This assumption is also supported by an observed degradation of the Meissner volume fraction with increasing carrier concentration, which obviously excludes the role of the magnetic field.