Kitaev-type interactions between neighboring magnetic moments emerge in the honeycomb material $\alpha -\mathrm{Ru}{\mathrm{Cl}}_3$. It is debated, however, whether these Kitaev interactions are ferromagnetic or antiferromagnetic. With electron energy loss spectroscopy (EELS) we study the lowest excitation across the Mott-Hubbard gap, which involves a $d^4$ triplet in the final state and therefore is sensitive to nearest-neighbor spin-spin correlations. At low temperatures the spectral weight of these triplets is strongly enhanced, in accordance with optical data. We show that the magnetic correlation function that determines this EELS spectral weight is directly related to a Kitaev-type spin-spin correlator and that the temperature dependence agrees very well with the results of a microscopic magnetic Hamiltonian for $\alpha -\mathrm{Ru}{\mathrm{Cl}}_3$ with ferromagnetic Kitaev coupling.

• Received 8 September 2017
• Accepted 26 August 2020

DOI:https://doi.org/10.1103/PhysRevMaterials.4.094408