Helium Puddles Near Absolute Zero

Helium has always been a bit of an outsider on the Periodic Table. Its two isotopes, helium-3 and helium-4, remain liquid down to absolute zero and have the lowest liquefaction temperature among the gases. These features can be traced to the high zero-point energy relative to their attractive potential. In two dimensions, helium4 will form a liquid and become superfluid at temperatures that depend on the density. Contrary to this, the state of helium-3 as a two-dimensional gas is not known. Many theories predict that in two-dimensions helium-3 is the only material that will stay gaseous in the ground state. However, a minority view is that a liquid phase in 2D helium-3 might indeed exist [1], and certain measurements seemed to support this idea [2]. 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) [3]. 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. Understanding the nature of this transition will lead to a better understanding of the interactions that lead to the formation of this liquid phase—and perhaps other phases—much like understanding the interactions in ordinary gases leads to an understanding of the existence of their critical point. Two-dimensional helium-3 cannot be realized experimentally unless one has a suitable substrate upon which to adsorb the atoms. Ideally, this substrate will let the helium-3 move freely in two dimensions—either spreading out over the surface as a gas or aggregating into a self-bound liquid. But invariably the substrate will play some role in the helium-3 behavior. For example, a crystalline substrate can cause the helium-3 to form a twodimensional solid or registered phases. For this reason, a number of experiments on 2D helium-3 have used the free surface of bulk liquid helium4 as a substrate [see Fig. 1(a)]. One might consider this FIG. 1: These illustrations show helium-3 (yellow) adsorbed on different substrates: (a) on the surface of bulk helium4 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. 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)