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 α-RuCl3, a system proximate to the Kitaev honeycomb model, as a function of temperature and magnetic field. An extended magnetic continuum develops below the structural phase transition at Ts2=62K. With the onset of a long-range magnetic order at TN=6.5K, spectral weight is transferred to a well-defined magnetic excitation at ℏω1=2.48meV, which is accompanied by a higher-energy band at ℏω2=6.48meV. Both excitations soften in magnetic field, signaling a quantum phase transition at Bc=7T where we find a broad continuum dominating the dynamical response. Above Bc, the long-range order is suppressed, and on top of the continuum, various 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.