Unconventional spin dynamics in the honeycomb-lattice material α-RuCl₃: High-field electron spin resonance studies

We present high-field electron spin resonance (ESR) studies of the honeycomb-lattice material α-RuCl₃, 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 the results of recent numerical calculations, strongly suggesting a very unconventional multiparticle character of the spin dynamics in α-RuCl₃. 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.