Phys. Rev. Lett. 113, 055301 (2014)

One of the first things that springs to mind when thinking about superfluids — apart from a liquid creeping up the sides of its container — is a laboratory stuffed with equipment. Indeed, keeping a superfluid at temperatures close to absolute zero is a task that demands formidable experimental effort. And that's not even the whole story. In addition to maintaining low temperatures, one also has to keep thermally driven phase fluctuations to a minimum, as they are primarily responsible for the breakdown of the long-range order that supports the superfluid state. Or are they?

Johan Carlström and Egor Babaev have now shown that it is possible to find cases in which these thermal fluctuations actually drive the system into a superfluid state. And for this, it seems, we have entropy to thank. The pair predicted that in certain systems, an energy landscape featuring a local minimum surrounded by a flat slope can make phase fluctuations around the minimum energetically cheap, allowing the existence of entropically stabilized states. Instead of restoring symmetry, the fluctuations permit a superfluid that wouldn't otherwise exist.