title: Direct observation of charge order in triangular metallic AgNiO2 by single-crystal resonant X-ray scattering.
abstract: We report resonant x-ray scattering measurements on a single crystal of the orbitally degenerate triangular metallic antiferromagnet 2H-AgNiO2 to probe the spontaneous transition to a triple-cell superstructure at temperatures below T(S)=365  K. We observe a strong resonant enhancement of the supercell reflections through the Ni K edge. The empirically extracted K-edge shift between the crystallographically distinct Ni sites of 2.5(3) eV is much larger than the value expected from the shift in final states, and implies a core-level shift of ∼1  eV, thus providing direct evidence for the onset of spontaneous honeycomb charge order in the triangular Ni layers. We also provide band-structure calculations that explain quantitatively the observed edge shifts in terms of changes in the Ni electronic energy levels due to charge order and hybridization with the surrounding oxygens.
authors.affiliation: Clarendon Laboratory, University of Oxford, Parks Road, Oxford OX1 3PU, United Kingdom.
authors.forename: G L
authors.initials: GL
authors.lastname: Pascut
authors.forename: R
authors.initials: R
authors.lastname: Coldea
authors.forename: P G
authors.initials: PG
authors.lastname: Radaelli
authors.forename: A
authors.initials: A
authors.lastname: Bombardi
authors.forename: G
authors.initials: G
authors.lastname: Beutier
authors.forename: I I
authors.initials: II
authors.lastname: Mazin
authors.forename: M D
authors.initials: MD
authors.lastname: Johannes
authors.forename: M
authors.initials: M
authors.lastname: Jansen