****NON-TECHNICAL ABSTRACT**** This project is focused on the use of liquid helium to study fundamental aspects of helium as well as using helium's unusual properties to explore important questions in the broader area of Condensed Matter Physics. Very thin films of helium become a friction-free superfluid at temperatures lower than about 456 degrees below zero Fahrenheit, and support waves, which act much like tiny tidal waves. Localization, a phenomenon in which waves are confined by a disordered environment, is a phenomenon of widespread importance. This importance reaches from the fundamental (the behavior of atoms in optical traps) to the applied, (the behavior of light in the technological areas of signal processing and the field of photonics). This project seeks experimental evidence for a change in the fundamental localization behavior of waves on a thin film of helium in a non-orderly environment when a direct flow of the fluid that supports the waves is applied. In another direction, earlier work has shown that when a thin film of the two kinds of helium atoms (the rare helium-3 and the more common helium-4) is atop a smooth solid hydrogen surface at low temperature there is an unexpected decoupling of atoms from the surface in addition to the usual superfluid decoupling. This suggests the exciting possibility that there may be a new type of low temperature transition that takes place in such films, different from the usual superfluid transition. The students, from graduate school down to high school, involved in these studies will gain experience in fundamental physics and cutting-edge technology. They will be poised to contribute to scientific research and development in industrial, national laboratory, and academic settings.
This project is focused on the use of liquid helium to study fundamental aspects of helium as well as using helium's unusual properties to explore important questions in the broader area of Condensed Matter Physics. The former studies will probe the unusual and unexplained de-coupling of helium atoms (in addition to the Kosterlitz-Thouless decoupling) from a weak binding substrate, specifically solid hydrogen, when a mixture of 3He and 4He is present. This decoupling suggests the exciting possibility that there may be a new type of low temperature transition that takes place in such films, different from the usual superfluid transition. A second focus will be the investigation of helium films in the presence of disorder. Third sound waves on thin films of superfluid 4He will be used to study classical wave localization on randomly patterned surfaces. It is predicted that the imposition of a flow field in the helium film will change the localization behavior. The results of this study have a potential to impact the study of the physics of cold atoms in optical potentials and the localization of light and signal processing in the field of photonics. The students, from high school through graduate school, involved in this research will gain experience in fundamental physics and cutting-edge technology. They will be poised to contribute to scientific research and development in industrial, national laboratory, and academic settings.
The research carried out involved several studies of the behavior of materials at temperatures well below room temperature. At these low temperatures the behavior can be quite different from expectations based on ordinary room temperature experience. In one case the work involved a study of the behavior of the ordinary gas helium under the extraordinary conditions of extremely low temperatures (below – 456 degrees Fahrenheit) and at times unusually high pressures (25 times atmospheric pressure). At these temperatures and at low pressure helium is a liquid and the work investigated the unusual properties of this liquid in the form of very thin films. These properties include behavior as a two-dimensional system (rather than the ordinary three dimensional). One of these two-dimensional properties is a transition from "ordinary" behavior to "superfluid" behavior and this transition was studied on structured surfaces. The work also investigated the structured surfaces themselves on which these films reside. At higher pressures (above about 25 atmospheres) and these low temperatures the helium becomes a rather strange solid. The work showed that under some circumstances of low temperature and elevated pressure it was actually possible to pass helium atoms through the solid helium – which is, to say the least, extremely unusual behavior. There are theoretical models that suggest just how this might be possible and the work carried out under this grant appears to be explainable by one of these models. This aspect of the work is controversial and the field of solid helium research very active. Graduate students participated in this work with helium and have gone on to more advanced work or the cryogenic industry after completion of their Ph.D. To construct apparatus at low temperatures, at times special construction epoxies are used. An undergraduate associated with the work carried out a detailed study of the temperature dependence of the diffusion (leakage) of helium gas through several such epoxies from room temperature down to -320 degrees Fahrenheit, at which temperature the diffusion has stopped. That student has gone on to graduate work in Physics. Presentations of all of this work were made at national and international conferences and the work published in the scientific literature. And, presentations of this work were made to freshman physics majors to provide a window for them into the remarkable world of extremely low temperatures.
