Giant phonon anomalies in the proximate Kitaev quantum spin liquid α-RuCl3
Haoxiang Li, Tiantian Zhang, Ayman Said, G. Fabbris, D. G. Mazzone, Jiaqiang Yan, David Mandrus, Gábor B. Halász, Satoshi Okamoto, Shuichi Murakami, M. P. M. Dean, Ho Nyung Lee, H. Miao
Abstract
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 Z 2 gauge flux. 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 detect anomalous phonon effects in α-RuCl 3 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 Z 2 gauge flux energy scale. These phonon anomalies signify the coupling of phonon and Kitaev magnetic excitations in α-RuCl 3 and demonstrates a proof-of-principle method to detect anomalous excitations in topological quantum materials.