Hard x-ray photoemission study on strain effect in LaNiO$_3$ thin films

Abstract: 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 LaNiO${3}$ thin films, strongly correlated perovskite TMO, grown on four substrates: LaAlO${3}$, (LaAlO${3}$)${0.3}$(SrAl${0.5}$Ta${0.5}$O${3}$)${0.7}$, SrTiO${3}$, and DyScO${3}$. 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 a presence of the La 5$p$ state near the Fermi level.