| We report inelastic neutron scattering measurements on Na2IrO3, a candidate for the Kitaev spin model on the honeycomb lattice. We observe spin-wave excitations with a dispersion that can be accounted for by including substantial further-neighbor exchanges that stabilize zig-zag magnetic order. Combining single-crystal diffraction and density functional calculations we propose a revised crystal structure model with significant departures from the ideal 90 degrees Ir-O-Ir bonds required for dominant Kitaev exchange. |
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