Evidence for magnetic fractional excitations in a Kitaev quantum-spin-liquid candidate $\alpha$-RuCl$_3$

TL;DR

This study provides experimental and theoretical evidence for fractional magnetic excitations in $ ext{RuCl}_3$, supporting its proximity to the Kitaev quantum-spin-liquid phase and validating the $K$-$ ext{Gamma}$ model as an effective description.

Contribution

First polarized inelastic neutron scattering study on $ ext{RuCl}_3$ revealing persistent magnetic continuum and supporting the $K$-$ ext{Gamma}$ model for its quantum spin liquid behavior.

Findings

Magnetic continuum around the $ ext{Gamma}$ point is robust against temperature.

The $K$-$ ext{Gamma}$ model reproduces both spin-wave and continuum excitations.

Evidence for fractional excitations originating from the Kitaev QSL state.

Abstract

$\alpha$-RuCl$_3$ has been studied extensively because of its proximity to the Kitaev quantum-spin-liquid (QSL) phase and the possibility of approaching it by tuning the competing interactions. Here we present the first polarized inelastic neutron scattering study on $\alpha$-RuCl$_3$ single crystals to explore the scattering continuum around the $\Gamma$ point at the Brillouin zone center, which was hypothesized to be resulting from the Kitaev QSL state but without concrete evidence. With polarization analyses, we find that while the spin-wave excitations around the M point vanish above the transition temperature $T_{\rm N}$, the pure magnetic continuous excitations around the $\Gamma$ point are robust against temperature. Furthermore, by calculating the dynamical spin-spin correlation function using the cluster perturbation theory, we derive magnetic dispersion spectra based on the $K$-$\Gamma$ model, which involves with a ferromagnetic Kitaev interaction of -7.2 meV and an off-diagonal interaction of 5.6 meV. We find this model can reproduce not only the spin-wave excitation spectra around the M point, but also the non-spin-wave continuous magnetic excitations around the $\Gamma$ point. These results provide evidence for the existence of fractional excitations around the $\Gamma$ point originating from the Kitaev QSL state, and further support the validity of the $K$-$\Gamma$ model as the effective minimal spin model to describe $\alpha$-RuCl$_3$.