Low Energy Spectrum of Proximate Kitaev Spin Liquid $\alpha$ -RuCl$_{3}$ by Terahertz Spectroscopy

A Quantum Spin Liquid (QSL) is an ultra-quantum state of matter with no ordered ground state. Recently, a route to a QSL identified by Kitaev has received a great deal of attention. The compound $\alpha$ -RuCl$_{3}$, in which Ru atoms form a honeycomb lattice, has been shown to possess Kitaev exchange interactions, although a smaller Heisenberg interaction exists and leads to a zig-zag antiferromagnetic state below 7 K. Because of proximity to the exactly-solvable Kitaev spin-liquid model, this material is considered a potential host for Majorana-like modes. In this work, we use time-domain terahertz (THz) Spectroscopy to probe the low-energy excitations of $\alpha$ -RuCl$_{3}$. We observe the emergence of a sharp magnetic spin-wave absorption peak below the AFM ordering temperature at 7 K— on top of a broad continuum that persists up to room temperature. Additionally we report the polarization dependence of the THz absorption, which reveals optical birefringence, indicating the presence of large monoclinic domains.