# Spin-wave excitations evidencing the Kitaev interaction in single crystalline $\alpha$-RuCl$_3$

## TL;DR

This study provides experimental evidence of Kitaev interactions in $ ext{α-RuCl}_3$ through inelastic neutron scattering and theoretical modeling, confirming the material's potential to realize quantum spin liquid physics.

## Contribution

The paper combines neutron scattering experiments with first-principles calculations to quantify Kitaev and off-diagonal interactions in $ ext{α-RuCl}_3$, demonstrating real material realization.

## Key findings

- Observation of a ~2 meV spin-wave gap at the M point
- Kitaev interaction $K$ is significantly larger than Heisenberg exchange
- Effective model fits yield $K=-6.8$ meV and $ ext{Γ}=9.5$ meV

## Abstract

Kitaev interactions underlying a quantum spin liquid have been long sought, but experimental data from which their strengths can be determined directly is still lacking. Here, by carrying out inelastic neutron scattering measurements on high-quality single crystals of $\alpha$-RuCl$_3$, we observe spin-wave spectra with a gap of $\sim$2 meV around the M point of the two-dimensional Brillouin zone. We derive an effective-spin model in the strong-coupling limit based on energy bands obtained from first-principle calculations, and find that the anisotropic Kitaev interaction $K$ term and the isotropic antiferromagentic off-diagonal exchange interaction $\Gamma$ term are significantly larger than the Heisenberg exchange coupling $J$ term. Our experimental data can be well fit using an effective-spin model with $K=-6.8$ meV and $\Gamma=9.5$ meV. These results demonstrate explicitly that Kitaev physics is realized in real materials.