Swirls of superfluid flow.

Swirls of super fluid flow

Chilling liquid helium to temperatures below 2.172 kelvins has a curious effect. The helium changes into a superfluid, a state in which the liquid flows without friction. The picture gets somewhat more complicated when liquid helium sits as a thin film, only a few atomic layers thick, atop a solid hydrogen surface. In this case, the critical temperature at which the liquid becomes a superfluid depends on the film thickness and characteristics of the underlying surface.

Two years ago, a team led by Jack M. Mochel of the University of Illinois at Urbana-Champaign discovered that sound waves passing through a frigid helium film behave differently than they do in either the normal liquid or superfluid. This unexpected finding suggested that in a thin film over a certain temperature range, liquid helium apparently goes into an intermediate phase unlike the normal or superfluid state.

One way to model the behavior of liquid helium in a thin film is to think of it as a collection of tiny whirlpools. In the normal liquid, these vortices move freely; in the superfluid, they are bound together tightly in pairs. Physicist Shou-Cheng Zhang of Stanford University now proposes that the intermediate state can be interpreted as a particular pattern of vortices in the liquid.

Zhang suggests that the intermediate stage is dominated by an array of clockwise and counterclockwise eddies that settle into a relatively stable, orderly pattern - much like the regular arrangement of negatively and positively charged ions in a crystal. Interactions between this lattice of whirlpools and sound waves produce the distinctive signal detected by Mochel and his coworkers, he says.

Zhang's theory has already passed one experimental test. It correctly predicted that adding atoms of a lighter helium isotope - helium-3 - to a helium-4 film would widen the temperature range over which the intermediate phase is observed.