Microdrops of superfluid.

Liquid helium cooled to temperatures lower than 2.12 kelvins can become a superfluid, flowing without friction. Now, researchers have shown experimentally that superfluidity can occur in a cluster consisting of as few as 60 helium-4 atoms. J. Peter Toennies and his colleagues at the Max Planck Institute in Gottingen, Germany, report their findings in the March 27 Science.

The scientists studied the rotation of a single molecule of oxygen carbon sulfide (OCS) inside a tiny droplet of helium-3 at a temperature at which helium-4 is a superfluid but helium-3 is not. When helium-4 atoms are injected into the helium-3 droplet, they collect in the droplet's center around the molecule. The presence of sufficient helium-4 allows the molecule to rotate nearly without friction--an effect detectable in the spectrum of infrared light absorbed by the OCS molecule. The molecule does not rotate freely in pure helium-3.

"These experiments indicate that superfluidity sets in gradually with the addition of about 60 atoms," the researchers say. In other words, only about two layers of helium-4 atoms surrounding the molecule are needed for the effect to show. Scientists had found earlier that it takes at least two layers of helium-4 atoms in a thin film on a solid surface to obtain superfluidity.