Evidence for a Self-bound Liquid State and the Commensurate–Incommensurate Coexistence in 2D ³He on Graphite

Abstract

We made heat-capacity measurements of two dimensional (2D) ³He adsorbed on graphite preplated with monolayer ⁴He in a wide temperature range (0.1≤T≤80 mK) at densities higher than that for the 4/7 phase (=6.8 nm⁻²). In the density range of 6.8≤ρ≤8.1 nm⁻², the 4/7 phase is stable against additional ³He atoms up to 20% and they are promoted into the third layer. We found evidence that such promoted atoms form a self-bound 2D Fermi liquid with an approximate density of 1 nm⁻² from the measured density dependence of the γ-coefficient of heat capacity. We also show evidence for the first-order transition between the commensurate 4/7 phase and the ferromagnetic incommensurate phase in the second layer in the density range of 8.1≤ρ≤9.5 nm⁻².