Thickness dependent structural and electronic properties of CuO grown on SrTiO₃(100): a hybrid density functional theory study

We discuss the structural and electronic properties of tetragonal CuO grown on SrTiO₃(100) by means of hybrid density functional theory. Our analysis explains the anomalously large Cu-O vertical distance observed in the experiments (≈2.7 Å) in terms of a peculiar frustration between two competing local Cu-O environments characterized by different in-plane and out-of-plane bond lengths and Cu electronic populations. The proper inclusion of substrate effects is crucial to understanding the tetragonal expansion and to reproduce correctly the measured valence band spectrum for a CuO thickness of 3-3.5 unit cells, in agreement with the experimentally estimated thickness.