Observation of Self-binding in Monolayer $^3$He

TL;DR

This study provides experimental evidence of a gas-liquid transition in monolayer $^3$He on graphite, revealing that its ground state is a self-bound quantum liquid with remarkably low density, contrary to expectations of a dilute gas.

Contribution

First experimental observation of a self-binding quantum liquid in monolayer $^3$He, challenging previous assumptions of a dilute quantum gas in two dimensions.

Findings

Identified a gas-liquid transition in monolayer $^3$He.

Measured nearly identical critical densities across different monolayers.

Concluded the ground state is a self-bound quantum liquid at very low density.

Abstract

We report clear experimental signatures of the theoretically unexpected gas-liquid transition in the first three monolayers of $^3$He adsorbed on graphite. The transition is inferred from the linear density dependence of the $\gamma$-coefficient of the heat capacity measured in the degenerate region (2 $\le T \le$80 mK) below a critical liquid density ($\rho_{c0}$). Surprisingly, the measured $\rho_{c0}$ values (0.6$\sim$0.9 nm$^{-2}$) are nearly the same for all these monolayers in spite of their quite different environments. We conclude that the ground-state of $^3$He in strict two dimensions is not a dilute quantum gas but a self-bound quantum liquid with the lowest density ever found.