![]() ![]() This may be the lowest density liquid in nature, say authors Daisuke Sato and colleagues from the University of Tokyo in Japan, reporting in Physical Review Letters. New experiments looking at the specific heat of helium- 3 adsorbed on a graphite surface provide additional evidence for helium-3 existing as puddles at temperatures below about 80 millikelvin ( mK). However, a minority view is that a liquid phase in 2D helium- 3 might indeed exist, and certain measurements seemed to support this idea. Many theories predict that in two-dimensions helium- 3 is the only material that will stay gaseous in the ground state. ![]() Contrary to this, the state of helium- 3 as a two-dimensional gas is not known. In two dimensions, helium-4 will form a liquid and become superfluid at temperatures that depend on the density. These features can be traced to the high zero-point energy relative to their attractive potential. Its two isotopes, helium- 3 and helium- 4, remain liquid down to absolute zero and have the lowest liquefaction temperature among the gases. Helium has always been a bit of an outsider on the Periodic Table. Recent experimental evidence with graphite and helium- 4 on graphite indicate the formation of a 2D liquid phase in the adsorbed helium- 3 near absolute zero-depicted in (d). APS/ Alan Stonebraker Figure 1: These illustrations show helium- 3 (yellow) adsorbed on different substrates: (a) on the surface of bulk helium- 4 represented in blue (b) on the surface of graphite represented in gray and (c) on a solid layer of helium- 4 above a graphite surface. ![]()
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