Local electronic structure, O–T phase transition and oxygen content in Y 0.8Ca 0.2Ba 2Cu 3O y ( y = 6.84 − 6.32) high- Tc superconductors

Abstract

Y 0.8Ca 0.2Ba 2Cu 3O y 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 n e and vacancy concentration C v are evaluated as a function of oxygen content. The correlations between local electronic structure, O–T phase transition and superconductivity are discussed. n e mainly has an effect on high- T c superconductivity, no direct evidence shows that it is related to the O–T phase transition; and C v is mainly caused by oxygen vacancy. The small increase of T c with decreasing y around y ∼ 6.80 is due to the charge carriers over-doping, which is confirmed by the change of n e in this work.