Supersolidity and superfluidity
Is it possible that a solid flows like a liquid, or even without friction like a superfluid? At first sight, the crystalline order in real space looks contradictory with the coherent motion which is responsible for superfluidity. However, several authors considered this possibility in 1969 - 1970, and experiments done by Kim and Chan in 2004 suggest that solid 4He is indeed a superfluid solid - a 'supersolid' - below about 100 mK. This article summarizes my present understanding of this paradoxical issue.