Superfluidity
When liquid helium-4 is cooled to a temperature close to absolute zero, it acquires an unusual set of properties known as superfluidity, and the material is said to be in a superfluid state. The superfluid flows without friction, and its viscosity is zero (or nearly zero).
Superfluidity was discovered by Pyotr Leonidovich Kapitsa, John F. Allen, and Don Misener in 1937. It is a major facet in the study of quantum hydrodynamics.
Background
Below its boiling point of 4.21 K and above a temperature of 2.1768 K (called the "lambda point" for helium), the helium-4 isotope exists in a normal, colorless liquid state, called helium I. Like other cryogenic liquids, helium I boils when heat is added to it.
When cooled below the lambda point, it enters a state called helium II, which is a superfluid. Helium II has several strange characteristics.
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.
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- For instance, it has high thermal conductivity (high ability to conduct heat) and cannot be boiled. When heated, it evaporates directly to form gas. When flowing through capillaries with a width as narrow as 10-7 to 10-8 meter, it has no measurable viscosity. However, when measurements were done between two moving discs, a viscosity comparable to that of gaseous helium was observed. [1]
Helium II also exhibits a "creeping" effect. When a surface extends past the level of helium II, the helium II moves along the surface, seemingly against the force of gravity. Helium II will escape from a vessel that is not sealed by creeping along the sides until it reaches a warmer region, where it evaporates. It moves in a film (called a "Rollin film") that is 30 nanometers thick, regardless of surface material. As a result of this creeping behavior and helium II's ability to leak rapidly through tiny openings, it is very difficult to confine liquid helium.
The thermal conductivity of helium II is greater than that of any other known substance, a million times that of helium I and several hundred times that of copper. This is because heat conduction occurs by an exceptional quantum-mechanical mechanism. When heat is introduced, it moves through helium II in the form of waves, at 20 meters per second at 1.8 K, in a phenomenon called second sound.[2]
The isotope helium-3 also has a superfluid phase, but only at much lower temperatures. As a result, less is known about such properties of helium-3.
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Superfluids, such as supercooled helium-4, exhibit many unusual properties.
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.
Theoretical explanation
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 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.
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 (compared to about 300,000,000 meters per second in a vacuum).
The Infrared Astronomical Satellite (IRAS), launched in January 1983 to gather infrared data, was cooled by 720 liters of superfluid helium, maintaining a temperature of 1.6K.
Recent discoveries
Recently, some physicists have 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 that 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 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 (about 1 percent) of the solid appears to become superfluid[3].
See also
- Absolute zero
- Helium
- Superdiamagnetism
- Bose-Einstein condensate
- Superconductivity
- Supersolid
Notes
ReferencesISBN links support NWE through referral fees
- Kleinert, Hagen. Gauge Fields in Condensed Matter, Vol. I, "SUPERFLOW AND VORTEX LINES," pp. 1–742, World Scientific (Singapore, 1989); Paperback ISBN 9971-5-0210-0. (Available online here.)
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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