(Submitted on 19 Jun 2017)
We report on terahertz spectroscopy of quantum spin dynamics inα -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 atTs2=62 K. With the onset of a long-range magnetic order atTN=6.5 K, spectral weight is transferred to a well-defined magnetic excitation atℏω1=2.48 meV, which is accompanied by a higher-energy band atℏω2=6.48 meV. Both excitations soften in magnetic field, signaling a quantum phase transition atBc=7 T where we find a broad continuum dominating the dynamical response. AboveBc , 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 the field-induced quantum spin liquid.
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