# Unconventional spin dynamics in the honeycomb-lattice material $\alpha$-RuCl$_3$: high-field ESR studies

## TL;DR

This study uses high-field ESR to explore the complex spin dynamics in $ ext{RuCl}_3$, revealing unconventional multiparticle excitations and a field-induced energy gap consistent with theoretical predictions.

## Contribution

It provides new experimental evidence of multiparticle spin excitations and detailed high-field ESR spectra in $ ext{RuCl}_3$, advancing understanding of Kitaev physics.

## Key findings

- Detection of two antiferromagnetic resonance modes in the ordered phase
- Observation of a rich excitation spectrum in the quantum paramagnetic phase
- Consistency of ESR data with the field-induced energy gap from thermodynamic measurements

## Abstract

We present high-field electron spin resonance (ESR) studies of the honeycomb-lattice material $\alpha$-RuCl$_3$, a prime candidate to exhibit Kitaev physics. Two modes of antiferromagnetic resonance were detected in the zigzag ordered phase, with magnetic field applied in the $ab$ plane. A very rich excitation spectrum was observed in the field-induced quantum paramagnetic phase. The obtained data are compared with results of recent numerical calculations, strongly suggesting a very unconventional multiparticle character of the spin dynamics in $\alpha$-RuCl$_3$. The frequency-field diagram of the lowest-energy ESR mode is found consistent with the behavior of the field-induced energy gap, revealed by thermodynamic measurements.