Studies of Supersolid Phenomena in Helium and Hydrogen

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*****NON-TECHNICAL ABSTRACT*****One of the most exciting recent discoveries in condensed matter physics has been the possible observation of the so-called "supersolid" state at temperatures only a few tenths of a degree above absolute zero. This supersolid state has the remarkable property that a fraction of a solid made of helium can actually pass through itself as a frictionless flow provided the temperature is low enough. This property of the supersolid is counterintuitive and is at odds with our instinctive concept of a solid. The supersolid is another example of a quantum mechanical state known as a superfluid and takes its place beside the previously known superfluids; superfluid 4He and superfluid 3He, superconducting electrons, and the superfluids observed in low-density trapped atomic gases. The possible discovery of the supersolid state is a very recent finding and it is desirable to have an independent confirmation of this phenomena. The first goal of this individual investigator project is to provide such confirmation. The next step will be to study the effects of sample geometry and crystal annealing on the properties of the supersolid. An annealing experiment is particularly interesting, since recent theories suggest that a degree of disorder is required in the solid before the supersolid state can appear. If this is true, then possibly the supersolid behavior might be removed by annealing process, which reduces the amount of disorder in the helium crystalline solid. In addition to advancing the frontier of knowledge of quantum mechanical states of matter, this work will provide training for one graduate student.***** TECHNICAL ABSTRACT*****The recent discovery of a potential "supersolid" phase in solid 4He at temperatures below 200 mK by Kim and Chan provides the realization of a prediction made more than a quarter of a century ago. The initial suggestion for a supersolid state in crystalline 4He is based on the possibility that Bose-Einstein condensation might occur in solid helium at sufficiently low temperatures. This individual investigator award supports a project aimed at confirming the existence of the supersolid state and investigating its nature. In a variation on the original experiment, torsional oscillator technique in a cubic geometry will be used. This ensures the solid is geometrically locked to the torsional oscillator and the possibility of slip at the boundary is eliminated. Current theoretical understanding of the supersolid state suggests that the supersolid phenomena will not occur in an ideal 4He hcp crystal but some degree of disorder in the solid will be required. Thus an important study will be examining the effects of crystal annealing on any observed supersolid signal. This project will involve one graduate student who should profit greatly from working in a fast evolving area at the current forefront of condensed matter research.