  The nature of the metal insulator transition in thin films and superlattices
of LaNiO3 with only 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 in the 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.
