title: Orbital degeneracy removed by charge order in triangular antiferromagnet AgNiO2.
abstract: We report a high-resolution neutron diffraction study on the orbitally degenerate spin-1/2 hexagonal metallic antiferromagnet AgNiO2. A structural transition to a tripled unit cell with expanded and contracted NiO6 octahedra indicates sqrt[3]xsqrt[3] charge order on the Ni triangular lattice. This suggests charge order as a possible mechanism of lifting the orbital degeneracy in the presence of charge fluctuations, as an alternative to the more usual Jahn-Teller distortions. A novel magnetic ground state is observed at low temperatures with the electron-rich S=1 Ni sites arranged in alternating ferromagnetic rows on a triangular lattice, surrounded by a honeycomb network of nonmagnetic and metallic Ni ions. We also report first-principles band-structure calculations that explain microscopically the origin of these phenomena.
authors.affiliation: H.H. Wills Physics Laboratory, University of Bristol, Tyndall Avenue, Bristol BS8 1TL, United Kingdom.
authors.forename: E
authors.initials: E
authors.lastname: Wawrzyńska
authors.forename: R
authors.initials: R
authors.lastname: Coldea
authors.forename: E M
authors.initials: EM
authors.lastname: Wheeler
authors.forename: I I
authors.initials: II
authors.lastname: Mazin
authors.forename: M D
authors.initials: MD
authors.lastname: Johannes
authors.forename: T
authors.initials: T
authors.lastname: Sörgel
authors.forename: M
authors.initials: M
authors.lastname: Jansen
authors.forename: R M
authors.initials: RM
authors.lastname: Ibberson
authors.forename: P G
authors.initials: PG
authors.lastname: Radaelli