The Supersolid State of Matter (Minipgm)

Coordinators: David Ceperley, Moses H.W. Chan

Andreev, Lifshitz, Chester, and Leggett suggested in about 1970 that a quantum crystal such as bulk 4He under pressure might show both crystallinity and superfluid behavior. Experiments within the last two years have found indications for such a supersolid phase. Similar experimental results have also been observed for helium absorbed in Vycor glass and in porous gold with supersolid fraction that ranges from 1 to 2 percent (depending on pressure) and also in solid para-hydrogen. The phenomenon seems to be rather robust with respect to crystal formation, pressure, type of quantum solid,...

However, the theoretical explanation of superflow in a crystal is far from clear. Early theoretical work has focussed on the possibility of bose condensation of vacancies at low temperature, however such vacancies have not been clearly seen experimentally and computer simulations do not find stable vacancies. One approach has been to posit wavefunctions having supersolid behavior, but it is not clear whether they can describe other properties of helium. Most microscopic calculations do not show superfluid response, but they are limited to equilibrium samples of small crystals. It is possible that defects such as grain boundaries, dislocations or impurities are an essential part of the mechanism leading to the supersolid signature observed.

The KITP workshop will focus on examining various proposed mechanisms for supersolidity and experimental approaches for probing the observed supersolid response.