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 $γ$-coefficient of the heat capacity measured in the degenerate region (2 $\le T \le$80 mK) below a critical liquid density ($ρ_{c0}$). Surprisingly, the measured $ρ_{c0}$ 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.

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