LOCAL ELECTRONIC DENSITY AND VACANCY PROPERTIES FOR YBA(2)CU(3-X)M(X)OY (M = AL, SN) CERAMIC MATERIALS

In this paper, we report the positron lifetime data in high-T-c cuprat e perovskite YBa2Cu3-xMxOy (M = Al, Sn) systems with different dopant concentration x. A significant substitution dependence of the positron lifetime parameters was observed, and the local electron density n(c) and defect concentration C-v were also evaluated as a function of sub stitution content x. The results show that in the region of known stru ctural transition from orthorhombic to tetragonal phase, n(c) and C-v present an large anomalous change. Here, n(e), is decreased and C-v in creases. These can be interpreted by electronic weak localization in C u-O chains and would affect on the charge transfer in superconducting state. The mechanisms of positron annihilation were also discussed. (C ) 1998 Elsevier Science B.V. All rights reserved.