Fourth Sound Propagation in Supersolid Helium 4

****非技术摘要****刚性固体表现为可以无摩擦流动的超流体，这似乎非常奇怪。然而，这样的“超固体”在理论上已经被预测存在。之前所有寻找这种固体的尝试都失败了。最近，用一个装有加压固体氦-4的特殊圆柱摆进行的实验表明，固体明显地从容器壁部分分离。一个令人兴奋但有争议的解释是，这些实验为寻找已久的超固体提供了证据。如果这种解释得到证实，它就提出了一种关于物理固体的全新观点。因此，为超固体提供一个独立的和结论性的证据是至关重要的。当且仅当材料成为超流体时，一种被称为“第四声”的独特波传播才能在材料中传播。这个项目将在超低温下寻找固体氦-4中的这种波传播。如果这种波被发现，氦-4的超固体性将得到证实。它将成为测量这种有趣材料性质的新探针，并开辟固态物理学的新领域。参与该项目的本科生、研究生和博士后将体验到基础研究的兴奋。所有参与者都将接触到仪器仪表、数据采集和分析方面的现代技术。这次研究经历将是为将来成为美国科学工作者做准备的一个很好的训练基地****技术摘要****最近在200 mK以下的量子固体He-4上进行的扭振实验表明，该固体与其移动的容器壁部分解耦。对这些实验的一个令人兴奋但有争议的解释是，固体He-4表现得像超流体一样，同时保留了其晶体性质。本项目将利用超导量子干涉器件技术，在低温固体He-4中寻找新的第四次声波传播。第四，当且仅当材料成为超流体时，声音才能在材料中传播。第四次声传播的成功观测将为固体氦-4的超流动性提供决定性和重要的证据。它将提供一个独立的探针来测量超流体分数、热学和力学性能。研究生和博士后将参与现代凝聚态物理的真正前沿研究。本科生将作为荣誉和独立项目参与研究。他们都将接触到仪器仪表、数据采集和分析方面的现代技术。

****NON-TECHNICAL ABSTRACT****It might seem extremely odd that a rigid solid behaves as a superfluid, which can flow without friction. Nevertheless, such a "supersolid" has been theoretically predicted to exist. All prior attempts to find such a solid have failed. Recently, experiments with a special cylindrical pendulum containing pressurized solid helium-4 showed that the solid apparently partially separates from its container wall. An exciting but controversial interpretation is that the experiments provide evidence for the long-sought supersolid. If the interpretation is confirmed, it presents an entirely new view of the physical solid state. It is thus of paramount importance to furnish an independent and conclusive proof for supersolidity. A unique wave propagation known as "fourth sound" can propagate in a material if and only if it becomes a superfluid. This project will search for just such wave propagation in solid helium-4 at ultra low temperatures. If the wave is found, the supersolidity in helium-4 will be confirmed. It will then become a new probe for measuring the properties of this interesting material and open up a new field of solid state physics. The undergraduate and graduate students as well as postdoctoral fellows participating in this project will experience the excitement of fundamental research. 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.**** TECHNICAL ABSTRACT****Recent torsional oscillator experiments on the quantum solid He-4 below 200 mK indicate that the solid partially decouples from its moving container wall. An exciting but controversial interpretation of these experiments is that the solid He-4 behaves as a superfluid while retaining its crystalline properties. In this project, new fourth sound wave propagation will be searched for in the low temperature solid He-4 using the techniques of superconducting quantum interference device. Fourth sound can propagate in a material if and only if the material becomes a superfluid. A successful observation of fourth sound propagation will supply the definitive and important proof of superfluidity of solid He-4. It will provide an independent probe for measuring the superfluid fraction, thermal and mechanical properties. 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.