Superfluidity of Grain Boundaries and Supersolid Behavior

When two communicating vessels are filled to a different height with liquid, the two levels equilibrate because the liquid can flow. We have looked for such equilibration with solid ⁴He. For crystals with no grain boundaries, we see no flow of mass, whereas for crystals containing several grain boundaries, wedetect a mass flow. Our results suggest that the transport of mass is due to the superfluidity of grain boundaries.

http://www.sciencemag.org/cgi/content/full/313/5790/1098