Phase Diagram of Helium Monolayers

Abstract

In the past few years there has been increased interest in the physics of thin films, stimulated largely by the technical achievements and promise in solid state electronics and in superconductivity. Another important stimulus comes from surface science research, which has brought new and highly detailed insights into the properties of the top few atomic layers of solids. The field of adsorption can be considered as a part of “thin films,” with a subdivision for physisorption and a further subdivision for adsorbed helium films. But there has been very little communication between the helium film subfield and the other parts of the parent field, even though there appear to be some extremely important areas of overlap and much can be learned from each. The reason for this lack of communication is that those who work on helium films have been primarily interested in the films as special cases of the bulk phases. To some extent they are: Certainly there is a range of film thickness in which a film behaves mainly as a thin slice of the bulk. But when they become thin enough this approximation no longer holds: A film begins to take on such different character that one must treat the layer as a new regime. This change does not occur suddenly; furthermore, the thickness at which it occurs depends on what properties are being considered. But where it does occur, there one must take account of specific properties of the substrate. In the limit of a single monolayer the substrate properties can become supremely important; and one must be at least as concerned with the characteristics of the substrate and the surface-helium interactions as with the helium-helium interactions. We speak as new converts to this view, which was forced on us by work done in the past couple of years.

Research supported by the National Science Foundation.