Spin Waves and Revised Crystal Structure of Honeycomb Iridate Na 2 IrO 3

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 zigzag magnetic order. The onset of long-range magnetic order below T(N)=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° Ir-O-Ir bonds required for dominant Kitaev exchange.

• UD
  U.S. Department of Energy
• EA
  Engineering and Physical Sciences Research Council

  Awards: EP/G003092/2, EP/G003092/1, EP/J003557/1