Spin waves and revised crystal structure of honeycomb iridate Na2IrO3

TL;DR

This study investigates the magnetic excitations and crystal structure of Na2IrO3, revealing spin-wave behavior, magnetic ordering, and a revised structure that challenges its suitability for the Kitaev model.

Contribution

The paper provides the first inelastic neutron scattering data on Na2IrO3 and proposes a revised crystal structure with implications for its magnetic interactions.

Findings

Spin-wave excitations observed below 5 meV.

Long-range magnetic order sets in below 15.3 K.

Revised crystal structure shows deviations from ideal Kitaev conditions.

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 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.