Is $\kappa$-(BEDT-TTF)2Cu2(CN)3 a spin liquid or a gapped valence-bond state?
In plain words
This organic salt was the first proposed triangular-lattice spin liquid. Later experiments suggest a spin gap opens near 6 K, which would mean the spins pair into singlets instead.
Precise statement
In kappa-(BEDT-TTF)2Cu2(CN)3 at ambient pressure, determine whether the anomaly near $6\,\mathrm{K}$ is a transition to a gapped valence-bond-solid state with spin gap $\Delta_s$, or a crossover within a gapless spin liquid. An answer gives $\Delta_s$ (or an upper bound) and the symmetry of the low-temperature state.
What would settle it
Low-temperature ESR, NMR and x-ray diffraction searches for lattice dimerization below 6 K on the same crystals, together with thermal conductivity below 0.1 K.
Status in the literature
Miksch et al. (Science 2021) reported a gapped magnetic ground state from ESR; a structural confirmation of the singlet pattern is still lacking.