Correlation-Driven Charge Order at the Interface between a Mott and a Band Insulator

To study digital Mott insulator LaTiO₃ and band insulator SrTiO₃ interfaces, we apply correlated band theory within the local density approximation including a Hubbard U to (n, m) multilayers, 1≤n, m≤9 using unit cells with larger lateral periodicity. If the on-site repulsion on Ti is big enough to model the Mott insulating behavior of undistorted LaTiO₃, the charge imbalance at the interface is found in all cases to be accommodated by disproportionation (Ti⁴⁺+Ti³⁺), charge ordering, and Ti³⁺ d_xy-orbital ordering, with antiferromagnetic exchange coupling between the spins in the interface layer. Lattice relaxations lead to conducting behavior by shifting (slightly but importantly) the lower Hubbard band, but the charge and orbital order is robust against relaxation.