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

TL;DR

This study provides the first experimental evidence of a chiral quantum spin liquid phase in LiCuVO$_4$, revealing short-lived chiral fluctuations and nearly-gapless chiral soliton excitations in the material.

Contribution

It reports the observation of a vector-chiral quantum spin liquid state in LiCuVO$_4$, including measurements of chiral fluctuations and soliton excitations, advancing understanding of quantum spin liquids.

Findings

Chiral fluctuations above the multiferroic transition with divergent lifetimes.

Critical slowing down of relaxation dynamics near the transition temperature.

Detection of nearly-gapless chiral soliton excitations below 400 mK.

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 $\varepsilon^*$ 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 $\varepsilon^*$ 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 $E_\mathrm{SE}\approx14.1$ $\mu$eV.