Experiments in Quantum Solid He-4

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Technical AbstractExperimental evidence has shown that the quantum solid He-4 partially decouples mechanically from its moving container wall. An exciting yet controversial interpretation of these experiments is that the quantum solid He-4 behaves as a superfluid while retaining its crystalline properties. It is generally agreed that the presence of defects such as dislocation lines and grain boundaries is somehow involved. In this project, experiments will be carried out to measure both ultrasonic propagation (for probing the defects) and torsional oscillation (for probing mechanical decoupling) will be simultaneously measured in the identical sample of solid He-4 in order to elucidate the relationship between these two effects. Fourth sound, which propagates in a material only if the material becomes a superfluid, will be searched. A successful observation of fourth sound propagation will supply a definitive evidence for superfluidity of solid He-4. Graduate students and postdoctoral fellows will participate in a truly frontier research of modern condensed matter physics. Undergraduate students will be involved in the research as honors and independent projects. They will all be exposed to the modern technologies in instrumentation, data acquisition and analyses.Non-Technical AbstractUnderstanding how quantum mechanics affects the physical properties of solids is one of the most important goals of solid state physics field. Measured by the degree of delocalization or "fuzziness" of atoms, solid helium at very low temperatures is the substance known in nature most affected by quantum mechanics. It is then crucial in fundamental physics and ultimately in utilization of materials to acquire thorough knowledge of the properties of solid helium. The internal structure such as defects and crystalline faults in solid helium will be measured by non-destructive high frequency ultrasound propagation techniques. This ultrasound probe will be complemented by the measurement of the mechanical response of the same solid to a moving body attached to its surface. Comparison of these two measurements is designed to reveal the origins of quantum mechanical behavior of solid helium and ultimately other solids of technological importance. The exciting fundamental research in the project will be experienced by participating undergraduate and graduate students as well as postdoctoral fellows. All participants will be exposed to the modern technologies in instrumentation, data acquisition and analyses. The research experience will be an excellent training ground in preparation for becoming a part of scientific work force in the U.S.