Field evolution of magnons in α − RuCl 3 by high-resolution polarized terahertz spectroscopy

The Kitaev quantum spin liquid (KSL) is a theoretically predicted state of matter whose fractionalized quasiparticles are distinct from bosonic magnons, the fundamental excitation in ordered magnets. The layered honeycomb antiferromagnet $\alpha$-RuCl$_3$ is a KSL candidate material, as it can be driven to a magnetically disordered phase by application of an in-plane magnetic field, with $H_c \sim 7$ T. The spectrum of magnetic fluctuations at zero wave vector ($\bf{Q}$ = 0) in the antiferromagnetic state is of central importance to understanding the proximity of this system to a fractionalized spin liquid. Here we report a detailed characterization of this spectrum by high-resolution time-domain terahertz (THz) spectroscopy. We observe two sharp magnon resonances whose frequencies and amplitudes exhibit a discontinuity as a function of applied magnetic field, as well as two broader peaks at higher energy. Motivated by the existence of well-defined spin wave resonances, we compare these results to calculations based on linear spin wave theory (LSWT). Below the N\'eel temperature, we find that LSWT can account for these essential features of the spectra when a $C_3$-breaking distortion of the honeycomb lattice and the presence of structural domains are taken into account. We conclude that in the antiferromagnetic state the low energy $\bf{Q}$ = 0 spectrum in $\alpha$-RuCl$_{3}$ is dominated by well-defined magnons, even when approaching $H_c$, that are well-described by a semi-classical, non-interacting theory.