Giant Phonon Anomalies in the Proximate Kitaev Quantum Spin Liquid \(\alpha\)-RuCl\(_3\)

• Published in: arXiv.org 2021-06
• Main Authors: H Li, Zhang, T T, Said, A, Fabbris, G, Mazzone, D G, Yan, J Q, Mandrus, D, Halasz, G B, Okamoto, S, Murakami, S, Dean, M P M, Lee, H N, Miao, H
• Format: Article
• Language: English
• Subjects: Alpha rays; Anomalies; Excitation; Fermions; Flavor (particle physics); High temperature; Inelastic scattering; Phonons; Quantum Hall effect; Quantum numbers; Ruthenium trichloride; Spin liquid; Topology; X-ray scattering
• ISSN: 2331-8422
• DOI: 10.48550/arxiv.2011.07036

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 \(\alpha\)-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, JK~8 meV, that coincides with a giant acoustic phonon softening near the Z2 gauge flux energy scale. This fractional excitation induced phonon anomalies uncover the key ingredient of the quantum thermal Hall effect in \(\alpha\)-RuCl3 and demonstrates a proof-of-principle method to detect fractional excitations in topological quantum materials.