Superfluid

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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 characterised by the complete absence of viscosity. Thus superfluids, placed in a closed loop, can flow endlessly without friction. Superfluidity was discovered by Pyotr Leonidovich Kapitsa, John F. Allen, and Don Misener in 1937. The study of superfluidity is called quantum hydrodynamics. Phenomenological theory of superfluidity in helium-4 was created by Lev Landau, whereas Nikolay Bogoliubov first suggested simple microscopical theory.

[edit] 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 in a bath of superfluid helium, and the tube is heated (even by shining a light on it), the superfluid helium will flow up through the tube and out the top (this is 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 it is placed in.

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 instantaneously starts spinning at the critical speed. The speed is quantized - i.e. it can only spin at certain speeds.

[edit] 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 example see Gravity Probe B article.

[edit] 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, supersolids may 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].

[edit] 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)

[edit] See also

• Superdiamagnetism
• Bose-Einstein condensate
• Superconductivity
• Quantum vortex
• Supersolid
• superfluid film

[edit] 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