Experiments in our laboratory center on the behavior of the quantum liquids, He4 and He3, at low temperatures. The quantum refers to the fact that the zero point motion plays a crucial role in the phase behavior of these liquids. This is responsible for the fact that the liquid phase persists to absolute zero. The experiments which we have done most recently all involve the behavior of these systems in a lower dimension. That is, in a situation where the helium is constrained in some way so that, for instance, it behaves as a two-dimensional thermodynamic system rather than three dimensional. This is the case for films consisting of one or two atomic layers.
Liquid He4 undergoes a phase transition into a superfluid state at 2.18K. This is analogous in many ways to the superconducting transition in the case of electrons. This transition in He4 is of considerable interest from the point of view of critical phenomena. The transition has been studied most extensively in order to verify theoretical predictions of critical behavior. An important aspect of this, which is still ongoing, is the behavior at the transition when the helium is not in the thermodynamic limit. That is, when a spatial dimension is imposed which limits the critical fluctuations which are responsible for the thermodynamic singularities.
We have recently developed a technique whereby we are able to space two silicon wafers at a uniform separation which can be as small as 1000 Angstroms over the full area of the wafers. This involves lithography and a bonding process in a microclean chamber. The bonded wafers allow us to confine the helium in a geometry which leads to a behavior which changes from 3-dimensional to 2-dimensional as the transition is approached. We have measured the superfluid fraction in this geometry by using a torsional oscillator technique and, we are currently measuring the heat capacity using an AC technique. Future work will involve geometries where, near the transition, the behavior will change to 1-dimension and 0-dimension.
Another area in which we are currently interested is the helium-fullerite system. One issue involved here is the possibility of intercalating helium atoms inside a crystal made of fullerene molecules. These molecules are spheroids made of carbon atoms. The most popular is C60, this has 60 carbon atoms arranged in a molecule which resembles a soccer ball. Another issue of interest, apart from intercalation, is the nature of the monolayer which adsorbs on crystals of these molecules. The interaction potential for helium and the C60 crystal has been calculated recently. It appears that some interesting new phases of helium could exist in the near-monolayer adsorption region.