Abstract
Through a combination of experimental measurements and theoretical modeling, we describe a strongly orbital-polarized insulating ground state in an (LaTiO_{3})_{2}/(LaCoO_{3})_{2} oxide heterostructure. X-ray absorption spectra and abinitio calculations show that an electron is transferred from the titanate to the cobaltate layers. The charge transfer, accompanied by a large octahedral distortion, induces a substantial orbital polarization in the cobaltate layer of a size unattainable via epitaxial strain alone. The asymmetry between in-plane and out-of-plane orbital occupancies in the high-spin cobaltate layer is predicted by theory and observed through x-ray linear dichroism experiments. Manipulating orbital configurations using interfacial coupling within heterostructures promises exciting ground-state engineering for realizing new emergent electronic phases in metal oxide superlattices.
Highlights
Transition-metal oxides (TMOs) provide a rich array of physical phenomena and phases stemming from strong electron-electron interactions between valence d electrons [1,2,3,4,5,6,7]
Tensile strained LCO thin films are stabilized in the high-spin state with ferromagnetic ordering developing due to CoO6 octahedral distortions and increased Co—O—Co bond angles [22,23,24,25,26]
Orbital polarization of high-spin 3d7 Co2þ can be achieved by inducing epitaxial strain in CoO, but the degree of the polarization is limited by the small magnitude of strain induced by the substrate [33]
Summary
Our theoretical simulation suggests that the antiferromagnetic Co spin configuration emerges as a result of the superexchange interaction between electrons occupying half-filled orbitals in two confined CoO2 planes. Judicious design of LCO-based heterostructures allows one to control the spin and orbital configuration of cobalt and manipulate the macroscopic properties through charge transfer, interfacial coupling, and dimensional confinement. We use UTi 1⁄4 3 eV to fit the optical gap of LaTiO3 (0.2 eV), in line with previous work [38], and UCo 1⁄4 2.5 eV to describe the electronic and magnetic properties of bulk LaCoO3ðCo3þÞ and CoOðCo2þÞ. The energy gap of LCO within GGA þ U using this UCo is 0.6 eV, consistent with the experiment, and the experimental nonmagnetic insulating phase of bulk LCO is the theoretical ground state for UCo ≤ 2.5 eV. We have considered a wide range, 0 ≤ UCo ≤ 5 eV, and confirmed that our results are qualitatively unchanged for UCo ≥ 2 eV
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