Superfluidity in a Crystal?

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Abstract
The phenomenon of nonclassical rotational inertia (NCRI), in which a body does not rotate with its rotating container, has long been thought to be peculiar to Bose-Einstein condensates of certain liquids and gases, in particular liquidHe. In his Perspective, Leggett discusses the experimental observation by Kim and Chan of similar behavior in solidHe. The new results are most naturally interpreted as showing that the crystalline solid phase ofHe displays NCRI below about 230 mK, although the fraction of the system that behaves in this way is very small. This experiment presents a major challenge to our current theoretical understanding of solid helium.