We report a high-resolution neutron diffraction study on the orbitally-degenerate spin-1/2 hexagonal antiferromagnet AgNiO2. A structural transition to a tripled unit cell with expanded and contracted NiO6 octahedra indicates root(3) x root(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 Jahn-Teller distortions. A novel magnetic ground state is observed at base temperatures with the electron-rich S = 1 Ni sites arranged in alternating ferromagnetic rows on a triangular lattice, surrounded by a honeycomb network of non-magnetic and metallic Ni ions. We also report first-principles band-structure calculations that explain microscopically the origin of these phenomena.
@article{arxiv.0705.0668,
title = {Orbital degeneracy removed by charge order in triangular antiferromagnet AgNiO2},
author = {E. Wawrzynska and R. Coldea and E. M. Wheeler and I. I. Mazin and M. D. Johannes and T. Sorgel and M. Jansen and R. M. Ibberson and P. G. Radaelli},
journal= {arXiv preprint arXiv:0705.0668},
year = {2009}
}
Comments
4 pages including 4 eps figures, improved Fig 4, to appear in Physical Review Letters