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Abstract

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 antibonding e_g states. In the monoclinic insulating state, bond-length disproportionation splits the manifold of e_g bands, corresponding to a modulation of the effective on-site energy. We show that, when subject to a local Coulomb repulsion U and Hund's coupling J, the resulting bond-disproportionated state is a paramagnetic insulator for a wide range of interaction parameters. Furthermore, we find that when U−3J is small or negative, a spontaneous instability to bond disproportionation takes place for large enough J. 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.