Spin waves and revised crystal structure of honeycomb iridate Na ₂IrO ₃

We report inelastic neutron scattering measurements on Na ₂IrO ₃, a candidate for the Kitaev spin model on the honeycomb lattice. We observe spin-wave excitations below 5meV with a dispersion that can be accounted for by including substantial further-neighbor exchanges that stabilize zigzag magnetic order. The onset of long-range magnetic order below T _N=15.3K 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°Ir-O-Ir bonds required for dominant Kitaev exchange.