Tunable Correlated-Electron Phases in (111) LaAlO3/SrTiO3 Band Insulator Heterostructures

Density functional theory calculations reveal the existence of different correlated-electron ground states in (111)-oriented n-type LaAlO3/SrTiO3 symmetric superlattices. They can be tuned by selecting the SrTiO3 thickness, and range from a trivial metal for thick SrTiO3 slabs to a Mott-type antiferromagnet in the ultrathin limit. An itinerant ferromagnet and a half-metal phase are also stable in the intermediate region. This remarkable property is a distinct characteristic of (111) perovskite heterostructures and originates from the combined effect of polar discontinuity at the interface, trigonal lattice symmetry, and quantum confinement. While the polar discontinuity promotes the filling of the empty d states of the SrTiO3 with one electron, the trigonal symmetry dictates that the wave function of the occupied bands spreads over the entire SrTiO3 slab. Thus, the electron density can be chosen by selecting the number of SrTiO3 layers. For high densities, symmetry breaking and on-site Coulomb interaction drive the occurrence of correlated-electron ground states. Our results show that low dimensionality can lead to unconventional behavior of oxide heterostructures formed by electronically fairly simple nonmagnetic band insulators, and can open perspectives for the use of LaAlO3/SrTiO3 superlattices grown along the [111] direction to explore quantum phase transitions.