Orbital control in strained ultra-thin LaNiO₃/LaAlO₃ superlattices

J. W. Freeland^1a, Jian Liu², M. Kareev², B. Gray², J. W. Kim¹, P. Ryan¹, R. Pentcheva³ and J. Chakhalian²

¹ 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

^a freeland@anl.gov

Received: 27 July 2011
Accepted: 11 October 2011

Abstract

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.