The basic understanding of superfluidity has
challenged theoretical physicists ever since its discovery in
helium 60 years ago. One fundamental question has been the
minimum number of atoms needed for superfluidity. The answer
to this question was recently provided by spectroscopic
experiments performed by the physicist Andrej Vilesov and the
Ph.D. student Slava Grebenev, both natives of Russia, working
together with Peter Toennies, scientific member and director
at the Max-Planck-Institute for Fluid Dynamics in Göttingen.
They discovered as reported in the March 27 issue of Science
(Vol. 279) that a cluster having just 60 atoms of a
helium-four isotope is already a superfluid.
When first confronted with this number Yuri Kagan, the well
known director of the Theory Division of the Kurchatov
Institute in Moscow said "Impossible!". As a former
student of Lev Landau, the famous nestor of Russian
theoretical physics who received the Nobel Prize in Physics
in 1962 for his pioneering theoretical work on the
superfluidity of helium, Kagan argued that superfluidity is
by definition a macroscopic phenomenon. The name superfluid
derives from the fact that below 2.2K liquid helium can flow
through narrow channels without resistance, a property which
has close analogies to superconductivity. Another remarkable
phenomenon is that superfluid helium is able to defy gravity
and creep up and over the walls of a beaker and thereby
escape from the container. These and many other
manifestations discovered already in 1938 are indeed
macroscopic in nature and explain Kagan´s initial reaction.
Since experimental tests based on the above macroscopic
phenomena with so few atoms are impossible the Göttingen
physicists had to devise new ways to explore superfluidity on
a microscopic scale.
In a long series of experiments which go back more than ten
years they were able to develop methods to produce and
characterize molecular beams of small liquid helium droplets
consisting of few thousands of atoms inside a high vacuum
apparatus. Already in 1990 they found that these droplets
were able to capture single foreign molecules or larger
well-defined numbers of molecules, depending on the
experimental conditions.The molecules then migrate towards
the center of the droplets. In 1995 as a result of a bold
venturesome experiment they were able to measure sharp
spectral lines of embedded SF6 molecules which
indicated quite unexpectedly that the molecules rotate freely
inside the helium droplets. These and other spectroscopic
studies provided evidence that the helium-four droplets have
temperatures of only 0.38K. In the recent Science article
they report on a new experiment which strongly suggests that
the ability of molecules to rotate freely is a result of the
superfluidity of the droplets. To demonstrate this they
compared the spectrum in a superfluid with that in a normal
fluid. For the latter they used droplets consisting of the
rare isotope helium-three, which being a Fermion system, only
becomes a superfluid at much lower temperatures of 3
millidegrees K and therefore at the helium-three droplet
temperatures of 0.10K behaves as an ordinary classical fluid.
For these experiments they chose the simple linear molecule
OCS (oxygen carbon sulfide). Inside helium-four droplets a
very sharp rotational line spectrum was observed whereas in
helium-three only a single broad peak was found indicating
that, as expected for an ordinary fluid, the rotational
motion is strongly impaired by collisions. This experiment
provided the key evidence that indeed the phenomenon of free
molecular rotations in liquid helium is a new microscopic
manifestation of superfluidity which they call
"molecular superfluidity".
To determine the critical number of helium-four atoms for
molecular superfluidity they then added to the pure
helium-three droplets well defined numbers of helium-four
atoms. From other experiments and theory they could precisely
determine the actual number of atoms picked up. Moreover they
had evidence that the helium-four atoms aggregate around the
OCS molecule. On the addition of about 60 helium-four atoms
the sharp spectral lines reappeared, indicating the return to
free rotational motion. 60 atoms are in fact just sufficient
to build up a cage consisting of a double layer of
helium-four atoms which surround the molecule. The Figure
shows the results of a snap shot of a molecular dynamics
simulation of such a microscopic cluster inside a
helium-three droplet.
Journal
Science