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-RuCl3, 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 K. With the onset of a long-range magnetic order at T-N = 6.5 K, spectral weight is transferred to a well-defined magnetic excitation at.. 1 = 2.48 meV, which is accompanied by a higher-energy band at h omega(2) = 6.48 meV. Both excitations soften in a magnetic field, signaling a quantum phase transition close to B-c = 7 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.