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

摘要

We present high-field electron spin resonance (ESR) studies of thehoneycomb-lattice material $\alpha$-RuCl$_3$, a prime candidate to exhibitKitaev physics. Two modes of antiferromagnetic resonance were detected in thezigzag ordered phase, with magnetic field applied in the $ab$ plane. A veryrich excitation spectrum was observed in the field-induced quantum paramagneticphase. The obtained data are compared with results of recent numericalcalculations, strongly suggesting a very unconventional multiparticle characterof the spin dynamics in $\alpha$-RuCl$_3$. The frequency-field diagram of thelowest-energy ESR mode is found consistent with the behavior of thefield-induced energy gap, revealed by thermodynamic measurements.