While correlated electron behavior is to be expected at oxide interfaces (IFs) involving Mott insulators, we show how strong correlations in the oxygen $2p$ states may be necessary to account for observed insulating behavior at charged (001)-IFs between the band insulators $\mathrm{La}\mathrm{Al}{\mathrm{O}}_3$ and $\mathrm{Sr}\mathrm{Ti}{\mathrm{O}}_3$. Using correlated band theory applied to the O $2p$ states, an insulating $p$-type IF is obtained only when a disproportionated, charge-, orbital-, and spin-ordered O $P_{\pi }$ magnetic hole is formed, centered between ${\mathrm{Al}}^{3+}$ ions in the $\mathrm{Al}{\mathrm{O}}_2$ layer at the IF. As an alternative explanation, charge compensation by oxygen vacancies that accommodate the holes as charge-conjugate F centers is modeled. For the $n$-type IF, a charge disproportionated ${\mathrm{Ti}}^{4+}+{\mathrm{Ti}}^{3+}$ layer is obtained with ferromagnetic alignment of the spins resulting from occupied $d_{xy}$ orbitals at checkerboard arranged ${\mathrm{Ti}}^{3+}$ sites. Electron hopping on a 50% occupied Ti sublattice (a quarter-filled band) and/or lattice relaxations are discussed as origin of the measured conductivity.

• Received 3 February 2006

DOI:https://doi.org/10.1103/PhysRevB.74.035112