Electronic structure and properties of vacancy-ordered YBa₂Cu₃O_6.5. Chain versus plane Fermi surfaces in YBa₂Cu₃O_7-δ

Results of local density calculations of the electronic structure for vacancy-ordered YBa₂Cu₃O_6.5 using the highly precise full-potential linearized augmented-plane-wave (FLAPW) method are presented. For a model structure of the vacancy-ordering in YBa₂Cu₃O_6.5 with a "full-and empty-chain" configuration in the Cu(1) chain plane, detailed analyses of energy bands, density of states, Fermi surface, and electron-phonon interaction parameters are made and compared with results for YBa₂Cu₃O₇ and YBa₂Cu₃O₆. The 2D dpσ Cu(2) plane bands crossing EF are found to be insensitive to the oxygen stoichiometry or ordering in the Cu(1) chain; on the other hand, the formation of the ordered CuO chains in the Cu(1) plane is crucial in the determination of the chain electronic structure near EF. Thus, it is shown that both the ordering of oxygens (i.e., formation of chains) in the Cu(1) plane as well as the oxygen content (i.e., δ) are important in understanding the band-filling of the 2D dpσ bands and the observed metallic behavior in YBa₂Cu₃O_7-δ, where the CuO chains play the roles not only as a charge reservoir but also as a. metallic chain. We also present the calculated FS in the "vacancy-ordered" YBa₂Cu₃O_6.5, which agrees with a recent photoemission experiment.