Helium II will "creep" along surfaces in order to find its own level - after a short while, the levels in the two containers will equalize. The
Rollin film also covers the interior of the larger container; if it were not sealed, the helium II would creep out and escape.
'Superfluidity' is a
phase of matter or description of
heat capacity in which "unusual" effects are observed when
liquids, typically of
helium-4 or
hydrogen, overcome
friction by surface interaction when at a stage, known as "
lambda point", at which the liquid's
viscosity becomes
zero. Also known as a major facet in the study of
quantum hydrodynamics, it was discovered by
Pyotr Leonidovich Kapitsa,
John F. Allen, and
Don Misener in
1937 and has been described through
phenomenological and microscopic theories.
Background
Although the phenomenologies of the superfluid states of helium-4 and
helium-3 are very similar, the microscopic details of the transitions are very different. Helium-4
atoms are
bosons, and their superfluidity can be understood in terms of the
Bose statistics that they obey. Specifically, the superfluidity of helium-4 can be regarded as a consequence of
Bose-Einstein condensation in an interacting system. On the other hand, helium-3 atoms are
fermions, and the superfluid transition in this system is described by a generalization of the
BCS theory of superconductivity. In it,
Cooper pairing takes place between atoms rather than
electrons, and the attractive interaction between them is mediated by
spin fluctuations rather than
phonons. See
fermion condensate. A unified description of superconductivity and superfluidity is possible in terms of
gauge symmetry breaking.
Superfluids, such as supercooled helium-4, exhibit many unusual properties. A superfluid acts as if it were a mixture of a normal component, with all the properties associated with normal fluid, and a superfluid component. The superfluid component has zero
viscosity, zero
entropy, and infinite
thermal conductivity. (It is thus impossible to set up a
temperature gradient in a superfluid, much as it is impossible to set up a
voltage difference in a superconductor.) One of the most spectacular results of these properties is known as the
thermomechanical or "fountain effect". If a
capillary tube is placed into a bath of superfluid helium and then heated, even by shining a
light on it, the superfluid helium will flow up through the tube and out the top as a result of the
Clausius-Clapeyron relation. A second unusual effect is that superfluid helium can form a layer, a single atom thick, up the sides of any container in which it is placed.
A more fundamental property than the disappearance of viscosity becomes visible if superfluid is placed in a rotating container. Instead of rotating uniformly with the container, the rotating state consists of
quantized vortices. That is, when the container is rotated at
speed below the first critical
velocity (related to the
quantum numbers for the
element in question) the liquid remains perfectly stationary. Once the first critical velocity is reached, the superfluid will very quickly begin spinning at the critical speed. The speed is quantized - i.e. it can only spin at certain speeds.
Applications
Recently in the field of
chemistry, superfluid helium-4 has been successfully used in
spectroscopic techniques, as a
quantum solvent. Referred to as Superfluid Helium Droplet Spectroscopy (SHeDS), it is of great interest in studies of
gas molecules, as a single
molecule solvated in a superfluid medium allows a molecule to have effective rotational freedom - allowing it to behave exactly as it would in the "gas" phase.
Superfluids are also used in high-precision devices, such as
gyroscopes, which allow the measurement of some theoretically predicted gravitational effects (for an example see the
Gravity Probe B article).
Recently, superfluids have been used to trap and slow the
speed of light. In an experiment, performed by
Lene Hau, light was passed through a superfluid and found to be slowed to 17 meters per second
[1] (normally 299,792,458 meters per second).
The
Infrared Astronomical Satellite (
IRAS), launched in January
1983 to gather infrared
data was cooled by 720 litres of superfluid helium, maintaining a
temperature of 1.6K.
Recent discoveries
Physicists have recently been able to create a Fermionic condensate from pairs of ultra-cold fermionic atoms. Under certain conditions, fermion pairs form
diatomic molecules and undergo
Bose–Einstein condensation. At the other limit, the fermions (most notably superconducting electrons) form
Cooper pairs which also exhibit superfluidity. This recent work with ultra-cold atomic gases has allowed
scientists to study the region in between these two extremes, known as the
BEC-BCS crossover.
Additionally,
super''solids'' might have also been discovered, in
2004, by physicists at
Penn State University. When helium-4 is cooled, below about 200 mK under high pressures, a
fraction (~1%) of the
solid appears to become superfluid
[1].
Books
★
Hagen Kleinert, ''Gauge Fields in Condensed Matter'', Vol. I, "SUPERFLOW AND VORTEX LINES", pp. 1–742,
World Scientific (Singapore, 1989); Paperback ISBN 9971-5-0210-0 (also available online
here)
See also
★
Superdiamagnetism
★
Bose-Einstein condensate
★
Superconductivity
★
Quantum vortex
★
Supersolid
★
superfluid film
★
Douglas D. Osheroff
External links
★
Superfluid phases of helium
★
Lancaster University, Ultra Low Temperature Physics - Superfluid helium-3 research group.
★ http://www.aip.org/png/html/helium3.htm
★ http://www.aip.org/pt/vol-54/iss-2/p31.html
★ http://web.mit.edu/newsoffice/2005/matter.html
★ http://physicsweb.org/articles/world/11/6/3/1
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