Local electronic structure, O-T phase transition and oxygen content inY(0.8)Ca(0.2)Ba(2)CU(3)O(y) (y=6.84-6.32) high-T-c superconductors

摘要

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 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,, is mainly caused by oxygen vacancy. The small increase of T-c with decreasing y around y similar to 6.80 is due to the charge carriers over-doping, which is confirmed by the change of n(e) in this work. (C) 2004 Elsevier B.V. All rights reserved.