Nature of the metal-insulator transition in few-unit-cell-thick LaNiO₃ films

The nature of the metal-insulator transition in thin films and superlattices of LaNiO₃ only a few unit cells in thickness remains elusive despite tremendous effort. Quantum confinement and epitaxial strain have been evoked as the mechanisms, although other factors such as growth-induced disorder, cation non-stoichiometry, oxygen vacancies, and substrate-film interface quality may also affect the observable properties of ultrathin films. Here we report results obtained for near-ideal LaNiO₃ films with different thicknesses and terminations grown by atomic layer-by-layer laser molecular beam epitaxy on LaAlO₃ substrates. We find that the room-temperature metallic behavior persists until the film thickness is reduced to an unprecedentedly small 1.5 unit cells (NiO₂ termination). Electronic structure measurements using X-ray absorption spectroscopy and first-principles calculation suggest that oxygen vacancies existing in the films also contribute to the metal-insulator transition.