Ionic relaxation contribution to the electronic reconstruction at the n-type LaAlO3/SrTiO3 interface

Density-functional theory calculations reveal that the compensation mechanism at the isolated n-type interface in LaAlO3/SrTiO3 superlattices involves both ionic and electronic degrees of freedom. Strong polar distortions screen the local electric field and reduce the band discontinuity across the interface. We find that the electronic reconstruction depends sensitively on whether structural optimization is performed within GGA (conventional exchange and correlation effects) or GGA+U (which includes strong intra-atomic interactions). For a structural optimization within GGA+U the excess charge is confined to the interface TiO2 layer with a charge-ordered, orbitally polarized arrangement of Ti3+ and Ti4+. While the charge-ordered phase represents the ground state, optimization within GGA leads to more pronounced lattice polarization, suppression of charge order (with remaining d(xy)-orbital occupation in the interface layer), and a delocalization of the excess charge extending over a few SrTiO3 layers.