title: Charge disproportionation without charge transfer in the rare-earth-element nickelates as a possible mechanism for the metal-insulator transition.
abstract: We study a model for the metal-insulator (M-I) transition in the rare-earth-element nickelates RNiO3, based upon a negative charge transfer energy and coupling to a rocksaltlike lattice distortion of the NiO6 octahedra. Using exact diagonalization and the Hartree-Fock approximation we demonstrate that electrons couple strongly to these distortions. For small distortions the system is metallic, with a ground state of predominantly d8L character, where L_ denotes a ligand hole. For sufficiently large distortions (δdNi-O∼0.05-0.10  Å), however, a gap opens at the Fermi energy as the system enters a periodically distorted state alternating along the three crystallographic axes, with (d8L_2)S=0(d8)S=1 character, where S is the total spin. Thus the M-I transition may be viewed as being driven by an internal volume "collapse" where the NiO6 octahedra with two ligand holes shrink around their central Ni, while the remaining octahedra expand accordingly, resulting in the (1/2, 1/2, 1/2) superstructure observed in x-ray diffraction in the insulating phase. This insulating state is an example of charge ordering achieved without any actual movement of the charge.
authors.affiliation: Department of Physics and Astronomy, University of British Columbia, Vancouver, British Columbia, Canada V6T 1Z1 and Quantum Matter Institute, University of British Columbia, Vancouver, British Columbia, Canada V6T 1Z4.
authors.forename: Steve
authors.initials: S
authors.lastname: Johnston
authors.affiliation: Department of Physics and Astronomy, University of British Columbia, Vancouver, British Columbia, Canada V6T 1Z1.
authors.forename: Anamitra
authors.initials: A
authors.lastname: Mukherjee
authors.affiliation: Department of Physics and Astronomy, University of British Columbia, Vancouver, British Columbia, Canada V6T 1Z1 and Quantum Matter Institute, University of British Columbia, Vancouver, British Columbia, Canada V6T 1Z4.
authors.forename: Ilya
authors.initials: I
authors.lastname: Elfimov
authors.affiliation: Department of Physics and Astronomy, University of British Columbia, Vancouver, British Columbia, Canada V6T 1Z1 and Quantum Matter Institute, University of British Columbia, Vancouver, British Columbia, Canada V6T 1Z4.
authors.forename: Mona
authors.initials: M
authors.lastname: Berciu
authors.affiliation: Department of Physics and Astronomy, University of British Columbia, Vancouver, British Columbia, Canada V6T 1Z1 and Quantum Matter Institute, University of British Columbia, Vancouver, British Columbia, Canada V6T 1Z4 and Department of Chemistry, University of British Columbia, Vancouver, British Columbia, Canada V6T 1Z1.
authors.forename: George A
authors.initials: GA
authors.lastname: Sawatzky