Is second-layer 3He on graphite a gapless spin liquid
In plain words
A layer of helium-3 atoms adsorbed on graphite forms a triangular solid in which nuclear spins interact through atoms exchanging places in rings. Down to the lowest temperatures reached, these spins show no order, suggesting a quantum spin liquid.
Precise statement
At the $4/7$ commensurate density of the 2D solid 3He layer on preplated graphite, multiple-spin ring exchange on a triangular lattice produces no magnetic order: magnetization measured by direct nuclear demagnetization rises smoothly down to about $1e-5\,\mathrm{K}$, which bounds any spin gap below about $1e-5\,\mathrm{K}$ (Masutomi et al. 2004). Determine the ground state of the triangular ring-exchange model at the measured $J2, J4, J5, J6$ ratios and test experimentally whether the state is a gapless spin liquid, gapped, or ordered. An answer is a classification supported by both computation and data below $1e-5\,\mathrm{K}$.
What would settle it
Heat capacity and susceptibility below 1e-5 K together with a controlled numerical solution of the ring-exchange model at the measured couplings.