Weak Coupling Effects in helium-4

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TECHNICALFor a system near a continuous phase transition, the behavior of many thermodynamic properties arises from the growth of the correlation length. This length diverges in the thermodynamic limit and is responsible for singularities, for instance, in the heat capacity and susceptibility. This behavior stems from the ability of fluctuations in the average thermodynamic properties to propagate and be felt over progressively larger distances. The research to be pursued as a result of this award will address a particular aspect of this behavior: How do two adjoining, but weakly coupled helium samples behave near the transition from the normal state to the superfluid state? This will be studied in arrays of helium confined in boxes or channels and connected through a very thin film. This geometry is achieved on silicon wafers through lithography and the use of direct wafer bonding. Preliminary results for this work have been very surprising, indicating that the coupling extends over much larger distances than expected. In some ways this indicates a similarity between superfluid helium and observations with high temperature superconductors. This project will support two graduate students working towards their doctorate. The students will develop expertise in silicon processing, vacuum techniques, thin film deposition and an array of cryogenic techniques and data analysis. NON-TECHNICAL ABSTRACTAt a phase transition a system will change its characteristics in such a way that new properties become manifest. The melting of ice is an example of this whereby ice transforms into a liquid and the latter has the new ability to easily "flow". At some types of transitions the changes are more dramatic, and they are often accompanied with very unusual behavior. Liquid helium has a transition at low temperatures whereby it transforms from a normal liquid into a superfluid. The latter is able to flow without friction in an analogous way that a superconductor can carry an electrical current without resistance. The proposed research will focus on one aspect of this behavior: How do two regions of liquid helium which are "weakly" connected influence each other as they undergo this transition? One calls this a proximity effect. This is also manifest in superconductors and in magnetic systems, and can be quite general. The proposed work will provide some unique answers to this question in the sense that both the properties of the helium will be studied and those of the weak link. This project will support two graduate students working towards their doctorate. The students will develop expertise in silicon processing, vacuum techniques, thin film deposition and an array of cryogenic techniques and data analysis. It is expected that these results will be presented at conferences and published in archival journals as part of the students' doctoral training.