Dimensionality and Weak-links Effects at the Superfluid Transition of 4He
Francis Gasparini Principal Investigator
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This low-temperature physics research project will investigate the superfluid transition of confined 4He as a paradigm of critical behavior. The work is intended to test theories of finite-size scaling at a second order phase transition. Here, the thermodynamic behavior should be scaled by the correlation length of the unconfined system. Excepting helium, there are very few experimental studies of this problem. Scaling has come into question from earlier experiments with helium confined in a planar geometry. It has been observed that on the superfluid side the expected scaling fails. This is also supported by theoretical calculations of the superfluid fraction. Other theoretical models also point out possible lack of scaling. The role of the lower dimension is also important and will be pursued. Films, representing 2-dimensional crossover, might behave quite differently from confinements where additional dimensions are made small. This research, because of the universal character of second order transitions, may have implications for critical behavior of other systems near a second order phase transition. The technique used in defining the confinement involves silicon lithography, and direct wafer bonding to achieve unprecedented uniformity in confinement at the nanometer level over cm2 areas. This is coupled with experimental techniques that allow study of very small samples with high accuracy. Both graduate and undergraduates, are involved in this work, and receive training in an unusually broad range of fabrication and experimental methods. This prepares them for careers in industry, academia, or government.<br/><br/>This low-temperature physics project involves studies of the behavior of a confined fluid near a phase transition. Usually materials do not change their properties with size because the largest relevant length scale is of the order of the interatomic spacing. However, near a phase transition an additional length scale emerges, the correlation length, which can grow as function of temperature to become quite large. In this case, samples which have dimensions comparable to the correlation length behave quite differently. This study focuses on liquid helium at its superfluid transition as a paradigm of what is expected to be a general behavior of many other systems. This is important in the current pursuit of nanotechnology, especially for magnetic systems. Substantial technological development is needed to do this work. A process has been developed which involves lithography of silicon wafers, and direct wafer bonding to construct exceptionally uniform chambers in which the liquid is confined. Techniques have also been developed for measuring very small samples. The present research focuses on the role of dimensionality. The program will investigate the effects of making 1, 2, or all 3 dimensions small. Students working in this area, both graduate and undergraduates are trained in cryogenic techniques, silicon lithography, wafer bonding, and high-resolution, precision measurements using computer controls. This prepares them for careers in academe, industry, and government or for additional study for advanced degrees.