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We explore the spin dynamics in the frustrated honeycomb magnets
Na2IrO3 [1] and Li2IrO3, candidates to display novel magnetic states
stabilized by the strong spin-orbit coupling at the 5d Ir
ions. Theory predicts composite spin-orbital J=1/2 moments at the Ir
ions coupled by strongly-anisotropic and bond-directional exchanges,
so-called Kitaev honeycomb model, which has in its phase diagram
novel magnetically-ordered and spin liquid phases. Inelastic neutron
scattering to probe the spin dynamics is very challenging due to the
large absorption cross-section of neutrons by the Ir nuclei. Using
an optimised experimental setup we have successfully observed
strongly dispersive spin-wave excitations of the Ir moments in both
compounds and results are compared with predictions for a
Kitaev-Heisenberg model as well as a Heisenberg model with further
neighbour couplings.
[1] S.K. Choi, R. Coldea, A.N. Kolmogorov et al, Phys. Rev. Lett. 108, 127204 (2012). |
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