Orbital control and altered covalency in strained ultra-thin LaNiO3/LaAlO3 superlattices

In pursuit of rational control of orbital polarization, we present a combined experimental and theoretical study of single unit cell LaNiO$_3$/LaAlO$_3$ superlattices. Polarized x-ray absorption spectra show a distinct asymmetry in the orbital response under tensile vs. compressive strain. A splitting of orbital energies $\sim$100 meV with octahedral distortions is found for the case of compressive strain which is much smaller than the $3d$ bandwidth. In sharp contrast, for tensile strain, no splitting is found although a strong orbital polarization is still present. Density functional theory calculations of the electronic properties reveal that the asymmetry results from a combination of strain effects and altered covalency in the bonding across the interfacial apical oxygen to the Al site, leading to the opening of a pseudogap in the heterostructure for tensile strain.