Anomaly in the heat capacity of fluid monolayers of 3He at low millikelvin temperatures

3He adsorbed on molecular hydrogen surfaces

Using a diffusion Monte Carlo technique, we calculated the phase diagram of 3He adsorbed on a first solid layer of a molecular hydrogen isotope (H2, HD, and D2) on top of graphite. The results are qualitatively similar in all cases: a two-dimensional gas spanning from the infinite dilution limit to a second-layer helium density of 0.048 ± 0.004 Å-2. That gas is in equilibrium with a 7/12 commensurate structure, more stable than any incommensurate triangular solid of similar density. These findings are in reasonably good agreement with available experimental data.

Observation of self-binding in monolayer 3He

We report clear experimental signatures of the theoretically unexpected gas-liquid transition in the first three monolayer systems 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≤T≤80 mK) below a critical liquid density (ρ(c0)). Surprisingly, the measured ρ(c0) values (0.6-0.9 nm(-2)) are nearly the same for all these layers in spite of their quite different environments. We conclude that the ground state of (3)He in strictly two dimensions is not a dilute quantum gas but a self-bound quantum liquid with the lowest density ever found.