Charge localization or itineracy at LaAlO3/SrTiO3 interfaces:: Hole polarons, oxygen vacancies, and mobile electrons

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 LaAlO3 and SrTiO3. 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 Al3+ ions in the AlO2 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 Ti4++Ti3+ layer is obtained with ferromagnetic alignment of the spins resulting from occupied d(xy) orbitals at checkerboard arranged Ti3+ 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.