Quantum Frustration in the “Spin Liquid” Phase of Two-DimensionalH3e

We have measured the ultralow temperature and low field magnetic susceptibility of the $\frac{4}{7}$ phase of two-dimensional ${}^3\mathrm{He}$ adsorbed on graphite preplated by one layer of ${}^4\mathrm{He}$. The experiments are performed by progressively adding ${}^4\mathrm{He}$ to the system, thus suppressing in a controlled way the ${}^3\mathrm{He}$ atoms trapped in substrate heterogeneities. This procedure enables us to determine the intrinsic properties of this spin $\frac{1}{2}$ model magnet in the zero field limit. The results show quantitatively that the system is strongly frustrated by multiple spin exchange interactions. A characteristic gapped spin liquid behavior is observed at ultralow temperature.