Influence of quantum confinement and strain on orbital polarization of four-layerLaNiO3superlattices: A DFT+DMFT study

Atomically precise superlattices involving transition-metal oxides provide a unique opportunity to engineer correlated electron physics using strain (modulated by choice of substrate) and quantum confinement (controlled by layer thickness). Here we use the combination of density-functional theory and dynamical mean-field theory (DFT+DMFT) to study $\mathrm{Ni}\phantom{\rule{0.16em}{0ex}}{E}_g\phantom{\rule{0.16em}{0ex}}d$-orbital polarization in strained ${\mathrm{LaNiO}}₃\text{/}{\mathrm{LaAlO}}₃$ superlattices consisting of four layers of nominally metallic ${\mathrm{NiO}}₂$ and four layers of insulating ${\mathrm{AlO}}₂$ separated by LaO layers. The layer-resolved orbital polarization is calculated as a function of strain and analyzed in terms of structural, quantum confinement, and correlation effects. The effect of strain is determined from the dependence of the results on the Ni-O bond-length ratio and the octahedral rotation angles, quantum confinement is studied by comparison to bulk calculations with similar degrees of strain, and correlation effects are inferred by varying interaction parameters within our DFT+DMFT calculations. The calculated dependence of orbital polarization on strain in superlattices is qualitatively consistent with recent x-ray-absorption spectroscopy and resonant reflectometry data. However, interesting differences of detail are found between theory and experiment. Under tensile strain, the two inequivalent Ni ions display orbital polarization similar to that calculated for strained bulk ${\mathrm{LaNiO}}₃$ and observed in experiment. Compressive strain produces a larger dependence of orbital polarization on Ni position, and even the inner Ni layer exhibits orbital polarization different from that calculated for strained bulk ${\mathrm{LaNiO}}₃$.