Abstract
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 metalinsulator transition (MIT). With decreasing film thickness, the progressive weakening of a quasiparticle peak at the Fermi level (EF ) occurs below 10 monolayer (ML), and the further depletion of spectral weight at EF 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.
