Weak Links and Proximity Effects in Microstructures of He-4

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*****NON-TECHNICAL ABSTRACT*****This low temperature physics project has the goal of understanding how a system undergoing a phase transformation can be influenced by a nearby system undergoing the same transformation. This is referred to as a "proximity" effect. The question is how close such systems have to be to influence each other. The details of the atomic interactions, which normally would be considered to answer such a question, are not relevant because near a phase transition there are long-range fluctuations that take place over lengths much larger than the distance between individual atoms. These fluctuations dominate the thermodynamic behavior of the system. This study focuses on liquid helium near its superfluid transition as a paradigm of what is expected to be a general behavior of many other systems. The experiments will study relatively large regions of liquid linked with small channels of various dimensions. These studies will be analogous to studies of arrays of superconductors separated by weak barriers. Students and postdoctoral researchers working on this project will be trained in silicon wafer processing, direct wafer bonding, a variety of diagnostic techniques, and high-resolution low temperature measurements. This prepares them for job placements in academia, industry and government laboratories.***** TECHNICAL ABSTRACT*****This low temperature physics research project will address issues associated with the coupling of an array of superfluid regions via links of various strength. This will be done by forming lithographically on a silicon wafer an array of boxes that are linked with nanometer channels. The heat capacity of this array and the superfluid fraction will be measured. The aspect ratios of the channels will be varied systematically to modify the coupling. The physics being addressed is similar to that of superconductors linked by nanobridges and of Bose condensates coupled through a weak potential. The proposed work differs from these in that the studies will be done in the region near the superfluid transition where fluctuations dominate the thermodynamic response. The work will lead to a better understanding of the concept of weak links and the role that fluctuations play in this coupling. The performance of this work will involve silicon lithography; direct wafer bonding; diagnostics using atomic force microscopy, electron microscopy and optical interferometry; and, ultimately high resolution low temperature measurements. Students and postdoctoral researchers will be trained in a variety of experimental techniques relevant to careers in academia, industry and government laboratories.