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 LaNiO3 and the band insulator LaAlO3. 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.
@article{arxiv.1008.1518,
title = {Orbital control in strained ultra-thin LaNiO$_3$/LaAlO$_3$ superlattices},
author = {J. W. Freeland and Jian Liu and M. Kareev and B. Gray and J. W. Kim and P. Ryan and R. Pentcheva and J. Chakhalian},
journal= {arXiv preprint arXiv:1008.1518},
year = {2012}
}