Gradual localization of Ni 3d states in LaNiO₃ ultrathin films induced by dimensional crossover

In situ photoemission spectroscopy and x-ray absorption spectroscopy (XAS) have been performed on LaNiO₃ (LNO) ultrathin films grown on LaAlO₃ substrates to investigate the origin of the thickness-dependent metal-insulator transition (MIT). With decreasing film thickness, the progressive weakening of a quasiparticle peak at the Fermi level (E_F) occurs below 10 monolayer (ML), and the further depletion of spectral weight at E_F leads to pseudogap behavior at 3-6 ML. The pseudogap finally evolves into a full gap, indicating that the thickness-dependent MIT takes place at a critical film thickness of 2-3 ML. The observed spectral behavior is in line with the transport properties of LNO ultrathin films. The thickness dependence of the spectral intensity is compared with realistic multiorbital dynamical mean-field theory. The experimental spectral function was found to depend on the film thickness more strongly than the theoretical one for thinner systems, indicating that the thickness-dependent MIT in LNO is caused by the crossover from three to two dimensions, during which the spatial correlations are progressively enhanced. The XAS results suggest that a charge disproportionate state is strongly suppressed in LNO ultrathin films plausibly as a result of epitaxial strain from the substrates. These results strongly suggest that a novel insulating state is realized in LNO films at a thin limit.