(Edmonton) - The study of phase transitions of matter is in transition, suggests Massimo Boninsegni, a University of Alberta theoretical condensed matter physicist.
Boninsegni co-authored, with Nikolay Prokof'ev of the University of Massachusetts, "Supersolids: What and where are they?", a paper recently accepted for publication by the Review of Modern Physics, and reviewed in this week's issue of Nature. Boninsegni and Prokof'ev put a different perspective on recent experiments providing some evidence for the existence of the supersolid phase of matter in solid helium.
"Our current belief is that a true supersolid transition, if it occurs at all in solid helium (and at this point I am truly skeptical) is of a very different character than what originally envisioned in the seminal works of the late 60s and early 70s," says Boninsegni.
The phase transitions most commonly observed occur when water molecules freeze into a solid (ice), melt into a liquid (water) and evaporate into a gas (vapour). These phases have very different macroscopic properties and behaviours. For example, water cannot flow through ice.
However, liquid helium at extremely low temperatures (below 2.2 Kelvin, or -270.8 Celsius, or -455.5 Farenheit) display a fourth phase, named superfluid, a liquid capable of flowing without dissipation through very narrow pores. Recent experiment have shown that solid helium may display a similar phase, referred to as supersolid, possessing some of the qualities of both a superfluid and a solid. Superfluid helium could conceivably flow through supersolid helium, because of quantum mechanical effects consisting of the exchange of indistinguishable atoms.
"The evidence observed in solid helium may not point to a true supersolid, but rather to a spurious effect occurring in solid helium, involving extended crystal defects such as dislocations," Boninsegni says.
Boninsegni says that a true supersolid could be observed in different systems. "It would likely occur in artificially fashioned systems, such as confined assemblies of cold atoms. The trick here is, that by means of optical techniques, one may modify the interaction that takes place between such atoms, obtaining interactions that do not naturally occur, nor are they really realizable in ordinary condensed matter."
Besides learning more about nature, understanding the supersolid phase of matter could lead to practical applications.
"A supersolid should have the property of allowing flow of its own particles, but blocking any foreign impurity," says Boninsegni. "Thus, one could imagine making membranes permeable to just one type of material, namely that which constitutes the membrane itself."