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Freeland, J. W.; Liu, J.; Kareev, M.; Gray, B.; Kim, J.W.; Ryan, P.; Pentcheva, Rossitza; Chakhalian, J. (2011): Orbital control in strained ultra-thin LaNiO3/LaAlO3 superlattices. In: EPL, Vol. 96, No. 5
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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 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.