What is Minimal Model of 3 He Adsorbed on Graphite ? – Importance of Density Fluctuations in 4 / 7 Registered Solid –

We show theoretically that the second layer of He adsorbed on graphite and solidified at 4/7 of the first-layer density is close to the fluid-solid boundary with substantial density fluctuations on the third layer. The solid shows a translational symmetry breaking as in charge-ordered insulators of electronic systems. We construct a minimal model beyond the multiple-exchange Heisenberg model. An unexpectedly large magnetic field required for the measured saturation of magnetization is well explained by the density fluctuations. The emergence of quantum spin liquid is understood from the same mechanism as in the Hubbard model and in κ-(ET)2Cu2(CN)3 near the Mott transitions.