Order-disorder-driven change in hole concentration and superconductivity in ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{6.5}$

The results of electronic-structure calculations for the compound ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{6.5}$ 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 ${\mathrm{CuO}}_2$ planes to the plane of the chains. Three models of crystal structure for ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{6.5}$ 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 ${\mathrm{CuO}}_2$ planes towards the plane of the chains in the latter two models, while in the first ∼0.16 electrons ${\mathrm{CuO}}_2$ planes are transferred. This shows that ordered chains are important for hole creation and superconductivity.