high-field ESR studies*

We performed comprehensive multi-frequency high-field electron spin resonance (ESR) studies of α-RuCl₃, an anisotropic spin system with a honeycomb structure, a top candidate for exhibiting Kitaev's spin-liquid physics. Two modes of antiferromagnetic resonance were detected in the zigzag-ordered phase. Magnetic field applied in the ab plane suppresses the zigzag order, resulting in a significant change of the excitation spectrum. Four ESR modes were observed in the field-induced quantum paramagnetic state above H_c ~ 7 T, suggesting an unconventioanl multiparticle nature of magnetic excitations in α-RuCl₃. The obtained data are compared with results of recent numerical calculations, revealing good qualitative agreement.