OXYGEN ORDERING AND PLANE-TO-CHAIN ELECTRON-TRANSFER IN YBA2CU3O7-DELTA

A microscopic model is described which is able to reproduce the observed dependence of T(c) on oxygen stoichiometry for the 123 superconductor YBa2Cu3O7-delta. The approach provides a way of estimating the electron hole concentration at the planar copper sites and makes use of the expected location of the d orbital energy levels of copper atoms in various coordination geometries. There are two important features. The first is that the presence of isolated oxygen vacancies in a CuO3 chain leads to the generation of excess holes in the x2 - y2 band of the planar copper atoms. Such an effect leads to the 90 K plateau and that part of the 60 K plateau for delta > 0.5. The second is the ineffectiveness of short chains in plane-to-chain charge transfer. This is responsible for the 60 K plateau for delta < 0.5, and gives insight into the time-dependent behavior of T(c) for rapidly quenched samples.