Optical magnons with dominant bond-directional exchange interactions in a honeycomb lattice iridate $\alpha$-Li$_{2}$IrO$_{3}$

TL;DR

This study reveals optical magnons in the honeycomb iridate $ ext{Li}_2 ext{IrO}_3$, showing dominant bond-directional interactions consistent with Kitaev physics, through resonant inelastic x-ray scattering and spin-wave modeling.

Contribution

It demonstrates the presence of optical magnons and dominant Kitaev interactions in $ ext{Li}_2 ext{IrO}_3$, highlighting bond anisotropy effects distinct from similar compounds.

Findings

Optical magnons observed with momentum dependence.

Dominant bond-directional Kitaev interaction of ~20 meV.

Diffuse magnetic scattering indicating short-range correlations.

Abstract

We have used resonant inelastic x-ray scattering to reveal optical magnons in a honeycomb lattice iridate $\alpha$-Li$_{2}$IrO$_{3}$. 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, ($\textit{h}, \textit{k}$) = ($\pm$0.32, 0), and lowered toward (0, 0) and ($\pm$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 $\textit{T}_{N}$. In contrast to Na$_{2}$IrO$_{3}$, this diffuse scattering lacks the $C_3$ rotational symmetry of the honeycomb lattice, suggesting that the bond anisotropy is far from negligible in $\alpha$-Li$_{2}$IrO$_{3}$.