Local electronic structure, O-T phase transition and oxygen content in Y0.8Ca0.2Ba2Cu3Oy (y = 6.84 - 6.32) high-Tc superconductors

Y0.8Ca0.2Ba2Cu3Oy samples with large range of oxygen content, have been systematically studied by means of positron annihilation technology, transport property measurements and X-ray diffraction. The positron lifetime parameters have a strong dependence on oxygen content y, and show evident changes during orthorhombic-tetragonal phase transition (y 6.50 ± 0.05). The local electron density ne and vacancy concentration Cv are evaluated as a function of oxygen content. The correlations between local electronic structure, O-T phase transition and superconductivity are discussed. ne mainly has an effect on high-Tc superconductivity, no direct evidence shows that it is related to the O-T phase transition; and Cv is mainly caused by oxygen vacancy. The small increase of Tc with decreasing y around y ∼ 6.80 is due to the charge carriers over-doping, which is confirmed by the change of ne in this work