Observation of chiral solitons in the quantum spin liquid phase of LiCuVO${}_4$

Abstract

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 the first experimental evidence of a vector-chiral quantum spin liquid state in the $S=1/2$ spin chain compound LiCuVO${}_4$ from measurements of the complex permittivity ${\epsilon }^{\ast }$ in the GHz range. In zero magnetic field our results show short-lived thermally activated chiral fluctuations above the multiferroic phase transition at $T_{\mathrm{N}}=2.4$ K with divergent life-times when approaching $T_{\mathrm{N}}$. In ${\epsilon }^{\ast }$ this fluctuation dynamics are seen as the slowing down of a relaxation with a critical dynamical exponent ${\nu }_{\xi }z\approx 1.3$ in agreement with mean-field predictions. When using a magnetic field to suppress $T_N$ towards 0 K the influence of quantum fluctuations is increased until they condense into the chrial spin liquid phase below 400 mK. Within this phase we measure a nearly-gapless chiral soliton excitation with a tiny energy gap of $E_{\mathrm{SE}}\approx 14.1$ $\mu$eV.

15 pages, 5 figures, supplementary material is included on the last page