Lowest density liquid in nature

— Self-binding of helium-3 in two dimensions —

Original paper: gObservation of Self-Binding in Monolayer 3Heh, D. Sato, K. Naruse, T.Matsui, and Hiroshi Fukuyama, Physical Review Letters 109, 235306 (2012).

Quantum matters consisting of light particles can stay liquid or gaseous without solidifying even at absolute zero. They are called quantum liquids or quantum gases. The helium-3 system confined in a two-dimensional space has long been thought of as the only material which stays gaseous at the ground state. Here, the group of Prof. Hiroshi Fukuyama of the Graduate School of Science, The University of Tokyo and Dr. Daisuke Sato of the Japan Society for the Promotion of Science has discovered that the system liquefies with the lowest density ever found by measuring heat capacities of monolayers of helium-3 adsorbed on a graphite surface down to ultra-low temperatures around two thousandths of a Kelvin. This finding contradicts the conventional wisdom and presses theorists to reconsider this problem or to reconstruct new theories. Liquid helium occupies a unique position in many other quantum fluids in nature such as electrons in solids, nuclei, neutron stars, etc, since the quantum gas-liquid phase transition can be experimentally examined in detail only in this system. The present work has brought us a new control parameter of the transition, which is dimensionality, and will deepen our understanding of nature.

Fig. 1

Fig. 1. (a) Measured heat capacities of helium-3 confined in two dimensions at a graphite surface. The numbers are areal densities in nm-2. (b) Density variation of the T-linear coefficient of heat capacity. The dashed lines are behaviors for a degenerate Fermi gas spreading over the whole surface. The inset is a schematic side view of the system.

Fig. 2

Fig. 2. Schematic view of self-bound puddles of helium-3 (yellow) confined in a two dimensional space at a graphite surface near absolute zero. These puddles are the lowest density liquid in nature. Blue dots are a solid monolayer of helium-4 which preplates a graphite surface.

Further reading: gHelium Puddles Near Absolute Zeroh, F. G. Gasparini, Viewpoint in Physics 5, 136 (2012) ;

gA liquid ground state for 2D helium-3? h, A. G. Smart, Physics Update in Physics Today (online).

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