1,414 research outputs found

    Emblem Types in Gabriel Rollenhagen`s Nucleus Emblematum

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    Orbital Polarization in Strained LaNiO3_{3}: Structural Distortions and Correlation Effects

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    Transition-metal heterostructures offer the fascinating possibility of controlling orbital degrees of freedom via strain. Here, we investigate theoretically the degree of orbital polarization that can be induced by epitaxial strain in LaNiO3_3 films. Using combined electronic structure and dynamical mean-field theory methods we take into account both structural distortions and electron correlations and discuss their relative influence. We confirm that Hund's rule coupling tends to decrease the polarization and point out that this applies to both the d8L‾d^8\underline{L} and d7d^7 local configurations of the Ni ions. Our calculations are in good agreement with recent experiments, which revealed sizable orbital polarization under tensile strain. We discuss why full orbital polarization is hard to achieve in this specific system and emphasize the general limitations that must be overcome to achieve this goal.Comment: 13 pages, 13 figure

    Low-energy description of the metal-insulator transition in the rare-earth nickelates

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    We propose a simple theoretical description of the metal-insulator transition of rare-earth nickelates. The theory involves only two orbitals per nickel site, corresponding to the low-energy anti-bonding ege_g states. In the monoclinic insulating state, bond-length disproportionation splits the manifold of ege_g bands, corresponding to a modulation of the effective on-site energy. We show that, when subject to a local Coulomb repulsion UU and Hund's coupling JJ, the resulting bond-disproportionated state is a paramagnetic insulator for a wide range of interaction parameters. Furthermore, we find that when U−3JU-3J is small or negative, a spontaneous instability to bond disproportionation takes place for large enough JJ. This minimal theory emphasizes that a small or negative charge-transfer energy, a large Hund's coupling, and a strong coupling to bond-disproportionation are the key factors underlying the transition. Experimental consequences of this theoretical picture are discussed.Comment: 17 pages, 10 figures; published version in the updat
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