LAUSR

The strain effect from a substrate is an important experimental route to control electronic and magnetic properties in transition-metal oxide (TMO) thin films. Using hard x-ray photoemission spectroscopy, we investigate the strain dependence of the valence states in LaNiO3 thin films, strongly correlated perovskite TMO, grown on four substrates: LaAlO3, (LaAlO3)0.3(SrAl0.5Ta0.5O3)0.7, SrTiO3, and DyScO3. A Madelung potential analysis of core-level spectra suggests that the point-charge description is valid for the La ions, while it breaks down for Ni and O ions due to a strong covalent bonding between the two. A clear x-ray photon-energy dependence of the valence spectra is analyzed by the density functional theory, which points to the presence of the La 5p state near the Fermi level.