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 LiCuVO 4 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 T N = 2.4 K. In ε * these fluctuations are seen as the slowing down of a relaxation with a critical dynamical exponent ν ξ z ≈ 1.3 in agreement with mean-field predictions. When using a magnetic field to suppress T N 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 E SE ≈ 14.1 μeV, which is in agreement with a nearly gapless chiral soliton that has been proposed for LiCuVO 4 based on quantum spin liquid theory. Quantum spin liquids describe a system where any type of long-range or local ordering is absent even at absolute zero but despite prominent theoretical studies definitive experimental evidence of such a system is difficult to obtain. Here, by analysing the complex permittivity of LiCuVO 4 the authors find evidence of chiral solitons suggesting the presence of a quantum spin liquid state.