Orbital control in strained ultra-thin LaNiO 3 /LaAlO 3 superlattices
EPL (Europhysics Letters) 96(5): 57004
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 that induces octahedral rotations and distortions and altered covalency in the bonding across the interfacial Ni-O-Al apical oxygen, leading to a charge disproportionation at the Ni sites for tensile strain.