We report clear experimental signatures of the theoretically unexpected gas-liquid transition in the first three monolayer systems of 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 () below a critical liquid density (). Surprisingly, the measured values () are nearly the same for all these layers in spite of their quite different environments. We conclude that the ground state of in strictly two dimensions is not a dilute quantum gas but a self-bound quantum liquid with the lowest density ever found.
- Received 3 August 2012
- Publisher error corrected 18 December 2012
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