Metal-insulator transition through a semi-Dirac point in oxide nanostructures: VO₂ (001) layers confined within TiO₂

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Multilayer (TiO₂)ₘ/(VO₂)ₙ nanostructures (d¹ - d⁰ interfaces with no polar discontinuity) show a metal-insulator transition with respect to the VO₂ layer thickness in first principles calculations. For n ≥ 5 layers, the system becomes metallic, while being insulating for n = 1 and 2. The metal-insulator transition occurs through a semi-Dirac point phase for n = 3 and 4, in which the Fermi surface is point-like and the electrons behave as massless along the zone diagonal in k-space and as massive fermions along the perpendicular direction. We provide an analysis of the evolution of the electronic structure through this unprecedented insulator-to-metal transition, and identify it as resulting from quantum confinement producing a non-intuitive orbital ordering on the V d¹ ions, rather than being a specific oxide interface effect. Spin-orbit coupling does not destroy the semi-Dirac point for the calculated ground state, where the spins are aligned along the rutile c-axis, but it does open a substantial gap if the spins lie in the basal plane.