Fractional Excitation Induced Giant Phonon Anomalies in the Proximate Kitaev Quantum Spin Liquid a-RuCl3

The Kitaev quantum spin liquid epitomizes an entangled topological state, for which two flavors of fractionalized low-energy excitations are predicted: the itinerant Majorana fermion and the Z₂ gauge flux. Detection of these excitations remains challenging, because of their fractional quantum numbers and non-locality. It was proposed recently that fingerprints of fractional excitations are encoded in the phonon spectra of Kitaev quantum spin liquids through a novel fractional-excitation-phonon coupling. Here, we uncover this effect in a-RuCl₃ using inelastic X-ray scattering with meV resolution. At high temperature, we discover interlaced optical phonons intercepting a transverse acoustic phonon between 3 and 7 meV. Upon decreasing temperature, the optical phonons display a large intensity enhancement near the Kitaev energy, J_K~8 meV, that coincides with a giant acoustic phonon softening near the Z₂ gauge flux energy scale. This fractional excitation induced giant phonon anomalies uncover the key ingredient of the quantum thermal Hall effect in a-RuCl₃ and demonstrates a proof-of-principle method to detect fractional excitations in topological quantum materials.