Optical magnons with dominant bond-directional exchange interactions in the honeycomb lattice iridate α−Li2IrO3

We have used resonant inelastic x-ray scattering to reveal optical magnons in a honeycomb lattice iridate alpha-Li2IrO3. The spectrum in the energy region 20-25 meV exhibits momentum dependence, of which energy is highest at the location of the magnetic Bragg peak, (h,k)=(±0.32,0), and lowered toward (0,0) and (±1,0). We compare our data with a linear spin-wave theory based on a generic nearest-neighbor spin model. We find that a dominant bond-directional Kitaev interaction of order 20 meV is required to explain the energy scale observed in our study. The observed excitations are understood as stemming from optical magnon modes whose intensity is modulated by a structure factor, resulting in the apparent momentum dependence. We also observed diffuse magnetic scattering arising from the short-range magnetic correlation well above TN. In contrast to Na2IrO3, this diffuse scattering lacks the C3 rotational symmetry of the honeycomb lattice, suggesting that the bond anisotropy is far from negligible in alpha-Li2IrO3.