Validation of density-functional versus density-functional+ U approachesfor oxide ultrathin films

摘要

A comparison between available experimental information and the predictions of density-functionaland density-functional+ U approaches is presented for oxide ultrathin films grown on single-crystalmetal surfaces. Prototypical examples of monolayer phases of an ionic oxide (ZnO), a late transitionmetal oxide (NiO), and an early transition metal oxide (TiO_2) are considered. The aim is to validatethe theoretical approaches, focusing on the prediction of structural features and the reproduction ofscanning tunneling microscopy images, rationalized in terms of the local density of states of thesystems. It is found that it is possible to reasonably estimate the optimal lattice constant of ultrathinsupported films and that the inclusion of the Hubbard U term appreciably improves the accuracy oftheoretical predictions, especially in the case of nonpolar ultrathin phases of a transition metal oxide.Moreover, the optimal value of U for the oxide layer at the interface with the metal support is foundto differ from that appropriate for the bulk oxide, as a consequence of the intermixing of oxide andsupport electronic states and screening effects.