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

TL;DR

This paper reports the discovery of giant phonon anomalies in $ ext{RuCl}_3$ linked to fractional excitations predicted in the Kitaev quantum spin liquid, providing a new method to detect these excitations via phonon spectra.

Contribution

It demonstrates the first observation of fractional-excitation-phonon coupling effects in $ ext{RuCl}_3$ using inelastic X-ray scattering, revealing phonon anomalies associated with fractional excitations.

Findings

Optical phonons show large intensity enhancement near the Kitaev energy.

Giant acoustic phonon softening occurs near the Z2 gauge flux energy.

Phonon anomalies are linked to fractional excitations in the quantum spin liquid.

Abstract

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.