title: Novel Electronic Behavior Driving NdNiO3 Metal-Insulator Transition.
abstract: We present evidence that the metal-insulator transition (MIT) in a tensile-strained NdNiO3 (NNO) film is facilitated by a redistribution of electronic density and that it neither requires Ni charge disproportionation nor a symmetry change [U. Staub et al., Phys. Rev. Lett. 88, 126402 (2002); R. Jaramillo et al., Nat. Phys. 10, 304 (2014)]. Given that epitaxial tensile strain in thin NNO films induces preferential occupancy of the e(g) d(x(2)-y(2)) orbital we propose that the larger transfer integral of this orbital state with the O 2p orbital state mediates a redistribution of electronic density from the Ni atom. A decrease in the Ni d(x(2)-y(2)) orbital occupation is directly observed by resonant inelastic x-ray scattering below the MIT temperature. Furthermore, an increase in the Nd charge occupancy is measured by x-ray absorption at the Nd L(3) edge. Both spin-orbit coupling and crystal field effects combine to break the degeneracy of the Nd 5d states, shifting the energy of the Nd e(g) d(x(2)-y(2)) orbit towards the Fermi level, allowing the A site to become an active acceptor during the MIT. This work identifies the relocation of electrons from the Ni 3d to the Nd 5d orbitals across the MIT. We propose that the insulating gap opens between the Ni 3d and O 2p states, resulting from Ni 3d electron localization. The transition seems to be neither a purely Mott-Hubbard transition nor a simple charge transfer.
authors.affiliation: Advanced Photon Source, Argonne National Laboratory, Argonne, Illinois 60439, USA.
authors.forename: M H
authors.initials: MH
authors.lastname: Upton
authors.affiliation: Advanced Photon Source, Argonne National Laboratory, Argonne, Illinois 60439, USA.
authors.forename: Yongseong
authors.initials: Y
authors.lastname: Choi
authors.affiliation: Department of Physics, University of Illinois at Chicago, Chicago, Illinois 60607, USA.|Materials Science Division, Argonne National Laboratory, Argonne, Illinois 60439, USA.
authors.forename: Hyowon
authors.initials: H
authors.lastname: Park
authors.affiliation: Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA.|Materials Science Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA.
authors.forename: Jian
authors.initials: J
authors.lastname: Liu
authors.affiliation: Department of Physics, University of Arkansas, Fayetteville, Arkansas 72701, USA.
authors.forename: D
authors.initials: D
authors.lastname: Meyers
authors.affiliation: Department of Physics, University of Arkansas, Fayetteville, Arkansas 72701, USA.
authors.forename: J
authors.initials: J
authors.lastname: Chakhalian
authors.affiliation: Department of Physics, University of Arkansas, Fayetteville, Arkansas 72701, USA.
authors.forename: S
authors.initials: S
authors.lastname: Middey
authors.affiliation: Advanced Photon Source, Argonne National Laboratory, Argonne, Illinois 60439, USA.
authors.forename: Jong-Woo
authors.initials: JW
authors.lastname: Kim
authors.affiliation: Advanced Photon Source, Argonne National Laboratory, Argonne, Illinois 60439, USA.
authors.forename: Philip J
authors.initials: PJ
authors.lastname: Ryan