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

TL;DR

This study uses hard x-ray photoemission spectroscopy to examine how substrate-induced strain affects the electronic states in LaNiO₃ thin films, revealing covalent bonding effects and La 5p states near the Fermi level.

Contribution

It provides new insights into the strain-dependent electronic structure of LaNiO₃ thin films using advanced spectroscopy and theoretical analysis.

Findings

Madelung potential analysis shows point-charge model validity for La ions

Covalent bonding causes breakdown of point-charge model for Ni and O

La 5p states are present near the Fermi level as revealed by DFT

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.