Almost 40 years ago, two Russian physicists predicted the existence of a new state of matter called a supersolid. They reasoned that at very low temperatures, the rules of quantum mechanics would allow a solid to move with zero resistance and that this would allow one solid to move through another like magician walking through a wall.
Like many quantum mechanical phenomenon, such behavior is entirely counterintuitive: how can the atoms that give a solid its rigidity also move with zero resistance?
But In the absence of any experimental evidence to back up this claim, supersolids were more or less forgotten. That changed in 2004 when Moses Chan and pals at Pennsylvania State University said they had stumbled across the first evidence of supersolidity in helium cooled to within a whisker of absolute zero.
Since then, interest in supersolidity has skyrocketed. But our ideas about supersolids are as confused as ever because of a number of puzzling results. Supersolids, if that is indeed what Chan has seen, are more complicated and mysterious than we ever imagined.
Today Davide Galli and Luciano Reatto from the Universita degli Studi di Milano in Italy review the field, doing a sterling job of drawing together the disparate ideas and the puzzling experimental evidence.
Ref: arxiv.org/abs/0811.3598: Solid 4He and the Supersolid Phase: From Theoretical Speculation to the Discovery of a New State of Matter? A Review of the Past and Present Status of Research