Abstract:

Andreev, Lifshitz, Chester, and Leggett suggested in about 1970 that a quantum crystal such as bulk helium-4 under pressure might show both crystallinity and superfluid behavior. Experiments by Kim and Chan within the last two years have found indications for such a supersolid phase. The theoretical explanation of superflow in a crystal assumed vacancies, however, they have not been seen experimentally and computer simulations do not find stable vacancies. We and others have done Path Integral Monte Carlo calculations of exchange frequencies, superfluid density and the condensate fraction. They do not show superfluid response, but they are limited to equilibrium samples of small crystals. Hence there is a possible discrepancy between the best computer simulations and the interpretation of experiments. Recent experimental results support the view that defects such as grain boundaries, dislocations or impurities are an essential part of the mechanism leading to the observations.