Atomic and electronic structures of YBa₂Cu₃O₇ [001], [010] tilt and twist grain boundaries

We study the electronic structures of YBa₂Cu₃O₇ (YBCO) [010]([001]) tilt and twist grain boundaries using first-principles density functional theory. The Σ3(010)/[010](90°) twist grain boundary has the lowest grain-boundary energy. Across this interface, the (001)-CuO₂ layers on one side link almost perfectly to the (100)-Cu₃O₅ layers on the other side. To explain why the super-fluid remains almost unchanged when flowing across this kind of interface (Eom et al 1991 Nature 353 544), the (100)-Cu₃O₅ layers should have conductivity as good as that of the (001)-CuO₂ layers. The negative grain-boundary energy between such thin YBCO [010] films means that the special one-dimensional super-lattice structure along the [010] direction is more stable than a perfect YBCO lattice. A special Σ5[001] twist grain boundary with low grain-boundary energy probably has a small barrier for super-current transport because it has the same chemical composition as that in crystal and there is minimal damage to the basic crystal structure. The Σ5[001] tilt grain boundaries and the other twist grain boundaries generally have high grain-boundary energies—more than 1.0 (J m⁻²). (paper)