Researchers uncovered giant phonon anomalies that are a signature of fractional excitations in a quantum spin liquid, α-RuCl3. This experimental work for the first time validates phonons as a sensitive probe of hidden fractional and non-local excitations, which may advance quantum information science.
In correlated quantum materials, the nature of electronic interactions and their ground state topology is intimately linked to the geometry of the underlying lattice. The low-energy excitations arising from pure electronic degrees of freedom inevitably interact with the crystal lattice, leaving behind their fingerprints in the phonon spectrum. Hitherto, the interactions of phonons with “conventional” quasiparticles of either Bose–Einstein or Fermi–Dirac statistics, such as magnons, phasons and amplitudons in density waves and Bogoliubons in superconductors, have been explored extensively. In contrast, the coupling between phonons and fractional excitations, including spinons in one-dimensional magnets and Majorana fermions and Z2 gauge fluxes in the Kitaev quantum spin liquids, have remained elusive. Here we report the experimental signature of the fractional-excitation-phonon coupling in α-RuCl3 by uncovering two-types of phonon anomalies at zero magnetic field: a 35% enhancement of the phonon spectral weight near the Kitaev energy JK and a giant phonon softening of ~15% below 2 meV.
