Thickness-dependent electronic structure in ultrathin LaNiO3 films under tensile strain

We investigated electronic-structure changes of tensile-strained ultrathin LaNiO3 (LNO) films from ten to one unit cells (UCs) using angle-resolved photoemission spectroscopy (ARPES). We found that there is a critical thickness t(c) between four and three UCs below which Ni e(g) electrons are confined in two-dimensional space. Furthermore, the Fermi surfaces (FSs) of LNO films below t(c) consist of two orthogonal pairs of one-dimensional (1D) straight parallel lines. Such a feature is not accidental as observed in constant-energy surfaces at all binding energies, which is not explained by first-principles calculations or the dynamical mean-field theory. The ARPES spectra also show anomalous spectral behaviors, such as no quasiparticle peak at the Fermi momentum but fast band dispersion comparable to the bare-band one, which is typical in a 1D system. As its possible origin, we propose 1D FS nesting, which also accounts for FS superstructures observed in ARPES.