Spin waves and revised crystal structure of honeycomb iridate Na2IrO3
  We report inelastic neutron scattering measurements on Na2IrO3, a candidate for the Kitaev spin model on the honeycomb lattice. We observe spin-wave excitations below 5 meV with a dispersion that can be accounted for by including substantial further-neighbor exchanges that stabilize zig-zag magnetic order. The onset of long-range magnetic order below 15.3 K is confirmed via the observation of oscillations in zero-field muon-spin rotation experiments. Combining single-crystal diffraction and density functional calculations we propose a revised crystal structure model with significant departures from the ideal 90 deg Ir-O-Ir bonds required for dominant Kitaev exchange. 
S. K. Choi, R. Coldea, A. N. Kolmogorov, T. Lancaster, I. I. Mazin, S.   J. Blundell, P. G. Radaelli, Yogesh Singh, P. Gegen...
cond-mat.str-el