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Lowest density liquid in nature

Self-binding of helium-3 in two dimensions


Graduate School of Science / Faculty of Science

Schematic view of self-bound puddles of helium-3 (yellow) confined in a two dimensional space on 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 the graphite surface. © Fukuyama lab

Quantum matter consisting of light particles can stay liquid or gaseous without solidifying even at absolute zero. Such matter is termed a quantum liquid or quantum gas. 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 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.

Press release (Japanese)


D. Sato, K. Naruse, T. Matsui, and Hiroshi Fukuyama,
“Observation of Self-Binding in Monolayer 3He”,
Physical Review Letters (2012): 235306-1-4, doi: 10.1103/PhysRevLett.109.235306.
Article link


Graduate School of Science

Department of Physics

Fukuyama laboratory

Viewpoint in Physics

Physics Update in Physics Today

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