Orbital control in strained ultra-thin LaNiO$_3$/LaAlO$_3$ superlattices

TL;DR

This study combines experimental and theoretical approaches to understand how strain influences orbital polarization and electronic structure in ultra-thin LaNiO$_3$/LaAlO$_3$ superlattices, revealing strain-dependent orbital splitting and charge disproportionation.

Contribution

It provides new insights into strain-induced orbital and electronic modifications in LaNiO$_3$/LaAlO$_3$ superlattices through combined experimental and theoretical analysis.

Findings

Compressive strain causes orbital energy splitting consistent with octahedral distortions.

Tensile strain results in strong orbital polarization without energy splitting.

Strain and confinement induce octahedral rotations, distortions, and charge disproportionation.

Abstract

In pursuit of rational control of orbital polarization, we present a combined experimental and theoretical study of single unit cell superlattices of the correlated metal LaNiO$_3$ and the band insulator LaAlO$_3$. Polarized x-ray absorption spectra show a distinct asymmetry in the orbital response under strain. A splitting of orbital energies consistent with octahedral distortions is found for the case of compressive strain. In sharp contrast, for tensile strain, no splitting is found although a strong orbital polarization is present. Density functional theory calculations including a Hubbard U term reveal that this asymmetry is a result of the interplay of strain and confinement induces octahedral rotations and distortions and altered covalency in the bonding across the interfacial Ni-O-Al apical oxygen, leading to a charge disporportionation at the Ni sites for tensile strain.