Evidence for a Self-Bound Liquid State and the Commensurate-Incommensurate Coexistence in 2D $^3$He on Graphite

TL;DR

This study provides evidence for a self-bound 2D Fermi liquid and a coexistence of commensurate and incommensurate phases in 2D $^3$He on graphite, revealing complex phase behavior at ultra-low temperatures.

Contribution

It reports the first observation of a self-bound 2D Fermi liquid and phase coexistence in 2D $^3$He on graphite, expanding understanding of quantum liquid states.

Findings

Evidence of a self-bound 2D Fermi liquid with ~1 nm$^{-2}$ density.

Identification of a first-order transition between 4/7 and incommensurate phases.

Observation of phase coexistence in the second layer at specific densities.

Abstract

We made heat-capacity measurements of two dimensional (2D) $^3$He adsorbed on graphite preplated with monolayer $^4$He in a wide temperature range (0.1 $\leq T \leq$ 80 mK) at densities higher than that for the 4/7 phase (= 6.8 nm$^{-2}$). In the density range of 6.8 $\leq \rho \leq$ 8.1 nm$^{-2}$, the 4/7 phase is stable against additional $^3$He 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 $\gamma$-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 $\leq \rho \leq$ 9.5 nm$^{-2}$.