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

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.