Fourth Sound Propagation in Supersolid Helium 4

* 非技术摘要 * 刚性固体表现为超流体，可以在没有摩擦的情况下流动，这似乎非常奇怪。 尽管如此，这种“超固体”在理论上已经被预测存在。 以前所有寻找这种固体的尝试都失败了。 最近，一个特殊的圆柱形摆装有加压固体氦-4的实验表明，固体显然部分从容器壁分离。 一个令人兴奋但有争议的解释是，这些实验为长期寻找的超固体提供了证据。 如果这种解释得到证实，它将提出一种全新的物理固态观点。 因此，为超实性提供一个独立的和决定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“第四声音”的独特波传播才能在材料中传播。 该项目将在超低温下在固体氦-4中寻找这种波的传播。 如果发现了这种波，氦-4的超固态性将得到证实。 它将成为测量这种有趣材料性质的新探针，并开辟固体物理学的新领域。 本科生和研究生以及博士后研究员参加这个项目将体验基础研究的兴奋。所有参与者将接触到仪器，数据采集和分析的现代技术。 研究经验将是一个很好的训练场，为成为美国科学工作力量的一部分做准备 *最近在低于200 mK的量子固体He-4上进行的扭转振荡器实验表明，该固体部分地从其移动的容器壁中分离。 对这些实验的一个令人兴奋但有争议的解释是，固体He-4在保持其晶体性质的同时表现出超流体的行为。本项目将利用超导量子干涉装置技术，在低温固态He-4中探索新的四阶声波传播。 第四，当且仅当材料成为超流体时，声音才能在材料中传播。 第四声传播的成功观测将为He-4固体的超流性提供决定性的重要证据。 它将提供一个独立的探针测量超流分数，热和机械性能。 研究生和博士后研究员将参与现代凝聚态物理学的真正前沿研究。 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，数据采集和分析的现代技术。

****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.