Prediction of thickness limits of ideal polar ultrathin films

Competition between electronic and atomic reconstruction is a constantly recurring theme in transition-metal oxides. We use density functional theory calculations to study this competition for a model system consisting of a thin film of the polar, infinite-layer structure ACuO₂ (A=Ca, Sr, Ba) grown on a nonpolar, perovskite SrTiO₃ substrate. A transition from the bulk planar structure to a chain-type thin film accompanied by substantial changes to the electronic structure is predicted for a SrCuO₂ film fewer than five unit cells thick. An analytical model explains why atomic reconstruction becomes more favorable than electronic reconstruction as the film becomes thinner, and suggests that similar considerations should be valid for other polar films.