Orbital control in strained ultra-thin LaNiO3/LaAlO3 superlattices
Advanced Photon Source, Argonne National Laboratory - Argonne, IL 60439, USA
2 Department of Physics, University of Arkansas - Fayetteville, AR 72701, USA
3 Department of Earth and Environmental Sciences and Center of Nanoscience (CENS), University of Munich Theresienstr. 41, 80333 Munich, Germany, EU
Accepted: 11 October 2011
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 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.
PACS: 73.20.-r – Electron states at surfaces and interfaces / 78.70.Dm – X-ray absorption spectra
© EPLA, 2011