title: Spin waves and revised crystal structure of honeycomb iridate Na2IrO3.
abstract: 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.
authors.affiliation: Clarendon Laboratory, University of Oxford, Parks Road, Oxford OX1 3PU, United Kingdom.
authors.forename: S K
authors.initials: SK
authors.lastname: Choi
authors.forename: R
authors.initials: R
authors.lastname: Coldea
authors.forename: A N
authors.initials: AN
authors.lastname: Kolmogorov
authors.forename: T
authors.initials: T
authors.lastname: Lancaster
authors.forename: I I
authors.initials: II
authors.lastname: Mazin
authors.forename: S J
authors.initials: SJ
authors.lastname: Blundell
authors.forename: P G
authors.initials: PG
authors.lastname: Radaelli
authors.forename: Yogesh
authors.initials: Y
authors.lastname: Singh
authors.forename: P
authors.initials: P
authors.lastname: Gegenwart
authors.forename: K R
authors.initials: KR
authors.lastname: Choi
authors.forename: S-W
authors.initials: SW
authors.lastname: Cheong
authors.forename: P J
authors.initials: PJ
authors.lastname: Baker
authors.forename: C
authors.initials: C
authors.lastname: Stock
authors.forename: J
authors.initials: J
authors.lastname: Taylor