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.
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.
