ELECTRONIC-STRUCTURE AND PROPERTIES OF VACANCY-ORDERED YBA2CU3O6.5 - CHAIN VERSUS PLANE FERMI SURFACES IN YBA2CU3O7-DELTA

Results of local density calculations of the electronic structure for vacancy-ordered YBa2Cu3O6.5 using the highly precise full-potential li nearized augmented-plane-wave (FLAPW) method are presented. For a mode l structure of the vacancy-ordering in YBa2Cu3O6.5 with a ''full- and empty-chain'' configuration in the Cu(1) chain plane, detailed analyse s of energy bands, density of states, Fermi surface, and electron-phon on interaction parameters are made and compared with results for YBa2C u3O7 and YBa2Cu3O6. The 2D dpsigma Cu(2) plane bands crossing E(F) are found to be insensitive to the oxygen stoichiometry or ordering in th e Cu(1) chain; on the other hand, the formation of the ordered Cu-O ch ains in the Cu(1) plane is crucial in the determination of the chain e lectronic structure near E(F). Thus, it is shown that both the orderin g of oxygens (i.e., formation of chains) in the Cu(1) plane as well as the oxygen content (i.e., delta) are important in understanding the b and-filling of the 2D dpsigma bands and the observed metallic behavior in YBa2Cu3O7-delta, where the Cu-O chains play the roles not only as a charge reservoir but also as a metallic chain. We also present the c alculated FS in the ''vacancy-ordered'' YBa2Cu3O6.5, which agrees with a recent photoemission experiment.