Observation of chiral solitons in LiCuVO4

Quantum spin liquids represent a magnetic ground state arising in the presence of strong quantum fluctuations that preclude ordering down to zero temperature and leave clear fingerprints in the excitation spectra. While theory bears a variety of possible quantum spin liquid phases their experimental realization is still scarce. Here, we report experimental evidence for chiral solitons in the S = 1/2 spin chain compound LiCuVO4 from measurements of the complex permittivity ε* in the GHz range. In zero magnetic field our results show short-lived thermally activated chiral fluctuations above the multiferroic phase transition at TN = 2.4 K. In ε* these fluctuations are seen as the slowing down of a relaxation with a critical dynamical exponent νξz ≈ 1.3 in agreement with mean-field predictions. When using a magnetic field to suppress TN towards 0 K the influence of quantum fluctuations increases until the thermally activated fluctuations vanish and only an excitation can be observed in the dielectric response in close proximity to the phase transition below 400 mK. From direct measurements we find this excitation’s energy gap as ESE ≈ 14.1 μeV, which is in agreement with a nearly gapless chiral soliton that has been proposed for LiCuVO4 based on quantum spin liquid theory.