Today, we wonder whether superfluidity only concerns liquids and gases, or if some solids could be partially superfluids while being elastic like ordinary solids. It is possible, though paradoxical. Apparently, it is the case for solid helium-4 below 0.1 K. Indeed, E. Kim and M. Chan (Pennsylvania State University) discovered in 2004 that in a rotating container with solid helium in it, one percent of the helium mass seems not to be rotating along with the sides of the container, the same way usual superfluid helium does, but at a temperature twenty times lower.
This superfluidity in a solid is called “supersolidity”. It is being studied in about twenty laboratories around the world (for a review on the matter, cf. article “The enigma of supersolidity”, S. Balibar, Nature 464, 176, 2010), and this phenomenon still remains a mystery.
