Insulating state of ultrathin epitaxial LaNiO3 thin films detected by hard x-ray photoemission

Abstract

In order to understand the influence of strain and film thickness on the electronic structure of thin films of strongly correlated oxides, we have applied hard x-ray photoemission (HXPS) at 6 keV, soft x-ray photoemission (XPS) at 1.5 keV, and transmission electron microscopy to epitaxial LaNiO3 films deposited on two substrates: LaAlO3 (compressive strain) and (LaAlO3)(0.3)(Sr2AlTaO6)(0.7) (tensile strain). Using inelastic attenuation lengths in LaNiO3 determined from the HXPS data, we have decomposed valence-band spectra into layer-specific contributions. This decomposition is validated by comparing with the results of first-principles calculations using a hybrid functional. The resultant thin-film LaNiO3 densities of states exhibit significant differences in spectral weights for the thinnest LaNiO3 films. A gap opening consistent with a metal-to-insulator transition is observed for the thinnest 2.7 nm LaNiO3 film on an (LaAlO3)(0.3)(Sr2AlTaO6)(0.7) substrate, with a similar gap opening also being observed in complementary soft x-ray photoemission at 1.5 keV for a thinner 1.4 nm film on an LaAlO3 substrate. A metal-to-insulator transition in very thin nm-scale films of LaNiO3 is thus suggested as a general phenomenon.