ORDER-DISORDER-DRIVEN CHANGE IN HOLE CONCENTRATION AND SUPERCONDUCTIVITY IN YBA2CU3O6.5

The results of electronic-structure calculations for the compound YBa2Cu3O6.5 are presented in order to understand the role that the ordering of oxygen atoms in the plane of the chains plays in the charge transfer from the CuO2 planes to the plane of the chains. Three models of crystal structure for YBa2Cu3O6.5 have been considered for this purpose: (a) an alternate-chain model where each alternate chain is fully intact and the adjacent one fully empty, (b) an identical-chain model where all chains are identical but broken, and (c) a tetragonal crystal structure. We find that practically no charge transfer occurs from the CuO2 planes towards the plane of the chains in the latter two models, while in the first approximately 0.16 electrons/CuO2 Plane are transferred. This shows that ordered chains are important for hole creation and superconductivity.