Magnetic Excitations and Continuum of a Possibly Field-Induced Quantum Spin Liquid in α−RuCl3

We report on terahertz spectroscopy of quantum spin dynamics in $\alpha \text{−}{\mathrm{RuCl}}_3$, a system proximate to the Kitaev honeycomb model, as a function of temperature and magnetic field. We follow the evolution of an extended magnetic continuum below the structural phase transition at $T_{s2}=62\mathrm{K}$. With the onset of a long-range magnetic order at $T_N=6.5\mathrm{K}$, spectral weight is transferred to a well-defined magnetic excitation at $\hslash {\omega }_1=2.48\mathrm{meV}$, which is accompanied by a higher-energy band at $\hslash {\omega }_2=6.48\mathrm{meV}$. Both excitations soften in a magnetic field, signaling a quantum phase transition close to $B_c=7\mathrm{T}$, where a broad continuum dominates the dynamical response. Above $B_c$, the long-range order is suppressed, and on top of the continuum, emergent magnetic excitations evolve. These excitations follow clear selection rules and exhibit distinct field dependencies, characterizing the dynamical properties of a possibly field-induced quantum spin liquid.