Magnetic excitations in the Kitaev material Na${}_2$IrO${}_3$ studied by neutron scattering

Abstract

Inelastic neutron scattering experiments with a large set of comounted Na${}_2$IrO${}_3$ crystals reveal the low-energy magnon dispersion in this candidate material for Kitaev physics. The magnon gap amounts to 1.7(1) meV and can be interpreted similarly to the sister compound $\alpha$-RuCl${}_3$ to stem from the zone boundaries in the antiferromagnetic zigzag structure. The neutron experiments find no evidence for low-energy excitations with ferromagnetic character, which contrasts to the findings in $\alpha$-RuCl${}_3$. Our results are consistent with a recently proposed microscopic model that involves an antiferromagnetic Heisenberg nearest-neighbor exchange in Na${}_2$IrO${}_3$ in contrast to the ferromagnetic one considered for $\alpha$-RuCl${}_3$. Although the magnetic response shows the signatures of bond-directional anisotropy in both materials the different relative signs of Kitaev and Heisenberg interaction result in different deviations from the initial Kitaev model. Low-energy ferromagnetic fluctuations cannot be considered as a fingerprint of ferromagnetic Kitaev interaction.

11 pages, 6 figures

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