Studies with Liquid Helium Films

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9729805 Hallock This proposal will support a component of the experimental low temperature physics program at the University of Massachusetts/Amherst. The objectives are: (1) to study new aspects of the He-3-He-4 mixture film system, (2) to study in more detail capillary condensation and avalanche phenomena with helium in porous materials, (3) to conduct studies with helium films on patterned substrates in one and two dimensions, with a focus on two dimensional localization, and (4) to bring to a conclusion our study of helium in the context of C60 substrates. Experimental methods include quartz crystal resonance, third sound, and heat capacity studies of the thermal properties of He- 3/He-4 mixture films; and high resolution capacitance measurements of capillary condensation and avalanche events in porous materials. The work will lead to an improved understanding of helium on hydrogen, including "two transitions" in the case of mixture films; a test of the appropriateness of recent theoretical work concerning avalanche events in magnetic systems for the description of similar events seen in hysteretic capillary condensation, and an improved understanding of a method for determining pore distributions and draining phenomena in porous materials. In addition to contributing to progress in understanding the behavior of helium films themselves, the work will have relevance to the wider community in the areas of Fermi systems, avalanche phenomena, capillary condensation, and non- linear phenomena. The project provides excellent training for graduate students in a number of important techniques that are used in technical fields as well as fundamental research projects. %%% Helium is a remarkable substance with special liquid properties when cooled near the absolute zero of temperature. This proposed grant will study various problems in condensed m atter physics using the unique properties of liquid helium, particularly liquid helium films, as the vehicle to facilitate the study. In some cases the helium itself will be the subject of the study, in others, helium will serve as a convenient fluid to explore the phenomena of interest. In particular, the grant will allow study He-4 and He-3/He-4 mixture films and thereby enhance our understanding of phase transitions on two dimensional surfaces and on weak binding substrates such as hydrogen. Capillary condensation phenomena and avalanche events in porous materials will be studied to gain a deeper understanding of how porous materials drain, an understanding that cannot be reached using ordinary liquids. Finally, studies of helium on C60 substrates will be concluded. The project provides excellent training for graduate students in a number of important techniques that are used in technical fields as well as fundamental research projects. ***