One of the most exciting recent discoveries in condensed matter physics has been the 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 helium solid can actually pass through itself as a frictionless flow provide 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 may be another example of a superfluid system to take its place beside the previously know superfluids; superfluid 4He and superfluid 3He, superconducting electrons, and the superfluids observed in low density trapped atomic gases. One of the most important questions that must be addressed is whether the supersolid phenomena thus far observed is actual evidence for a true superfluid state in solid helium. This research project will address this question on two fronts. The approach asks the question, does the superfluid obey superfluid hydrodynamics? The second approach will examine the nature of the phase transition to the supersolid state by an ultra sensitive heat capacity measurement utilizing state-of-the-art superconducting technology. These projects will involve several undergraduate students and will give them a head start in scientific research.
The recent discovery of a "Supersolid" phase in solid 4He at temperatures below 200 mK by the Eunsong Kim and Moses 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. It is crucial to establish whether or not the supersolid state represents a new type of superfluid. This individual investigator award supports an attempt to clarify this issue using several approaches. First, the hydrodynamics of the supersolid will be examined in non-circular geometries where the predicted behavior based on ordinary superfluid hydrodynamics is clear. Second, the question of an actual phase transition to the supersolid state will be addressed through measurement of the possible heat capacity anomaly associated with the transition utilizing ultra sensitive SQUID technology. This project is expected to involve two undergraduate students who should profit greatly from working in a fast evolving area at the current forefront of condensed matter research.
PI: John D. Reppy Awardee: Cornell University Award Number: 0965698 Award Expires: 09/30/2012 Thirty-five years elapsed between the predictions of a supersolid state of solid 4He, by Chester (1968), Andreev and Lfschitz (1969) and Leggett (1970), and the surprising discovery of experimental evidence for its existence. In 2004 Eunsoeng Kim and Moses Chan reported on a series of torsional oscillator measurements with solid 4He contained in porous Vycor glass. In these measurements they observed an unexpected decrease in the period of the torsional oscillator at temperatures below 250 mK. They interpreted this decrease in period as the result of a superfluid-like decoupling of a fraction of the solid helium moment of inertia from the oscillator. This discovery generated a flurry of experimental and theoretical interest that persists to this day. This initial excitement has recently been tempered by the realization that many of the early supersolid observations were contaminated by effects arising from an anomaly in the elastic properties of solid 4He that also appear in the same temperature range as the supersolid phenomenon reported by Kim and Chan. In an attempt to separate dynamic elastic effects from a true supersolid signal, we are employing torsional oscillators with two eigen frequencies and have achieved some unexpected results that calls into question the original interpretation of Kim and Chan’s original experiment. In particular we have found in a repeat of Kim and Chan’s original Vycor experiment that the observed period shift signals at the two different frequencies can be entirely accounted for the dynamic elastic effects and that there is no evidence of a supersolid signal. We are now continuing our two frequency measurements and examining the case of bulk solid samples where putative supersolid signals have been reported by a number of experimental groups. A definative demonstration of the existence the the supersolid state has proved remarkably elusive, but we believe that the two-frequency experiments may be able to settle the question of the existance or non-existance of the important new state of matter.
