Field evolution of magnons in α−RuCl3 by high-resolution polarized terahertz spectroscopy

The quantum spin liquid (QSL) -- a phase of matter with no ordered ground state -- is of fundamental interest as excitations in this phase may break the molds of bosons and fermions and develop more complicated properties. The honeycomb magnet $\ensuremath{\alpha}$-RuCl${}_{3}$ has emerged as a candidate for realization of a QSL. Although it orders at low temperature, an in-plane magnetic field of around 7 Tesla destroys the order; the nature of this transition is intensely studied. Here, the authors use polarized time-domain terahertz spectroscopy to study the spectrum of magnetic excitations in $\ensuremath{\alpha}$-RuCl${}_{3}$ in an applied field. Motivated by the unusual evolution of the spectra, the authors develop a linear spin wave theory to describe the data with strikingly good agreement. These results provide clarity for ongoing work on this compound, and inform future studies aiming to detect field-induced QSL states.