Superfluidity is a phase of matter or description of
heat capacityin which unusual effects are observed when liquids, typically of helium-4or helium-3, overcome frictionby surface interaction when at a stage, known as the " lambda point" for helium-4, at which the liquid's viscositybecomes zero. Also known as a major facet in the study of quantum hydrodynamics, it was discovered by Pyotr Kapitsa, John F. Allen, and Don Misenerin 1937and has been described through phenomenological and microscopic theories. In the 1950s Hall and Vinen performed experiments establishing the existence of quantized vortex lines. In the 1960s, Rayfield and Reif established the existence of quantized vortex rings. Packard has observed the intersection of vortex lines with the free surface of the fluid, and Avenel and Varoquaux have studied the Josephson effect in superfluid SimpleNuclide|Link|Helium|4.
L. D. Landau's phenomenological and semi-microscopic
theoryof superfluidity in SimpleNuclide|Helium|4 earned him the Nobel Prizein Physics in 1964. Assuming that sound waves are the most important excitations in SimpleNuclide|Helium|4 at low temperatures, he showed that SimpleNuclide|Helium|4 flowing past a wall would not spontaneously create excitations if the flow velocity was less than the sound velocity. In this model, the sound velocity is the "critical velocity" above which superfluidity is destroyed.
(SimpleNuclide|Helium|4 has a lower flow velocity than the sound velocity, but this model is useful to illustrate the concept.)
Landau also showed that the sound wave and other excitations could equilibrate with one another and flow separately from the rest of the SimpleNuclide|Helium|4 called the "condensate".
From the momentum and flow velocity of the excitations he could then define a "normal fluid" density, which is zero at zero temperature and increases with temperature. At the so-called Lambda temperature, where the normal fluid density equals the total density, the SimpleNuclide|Helium|4 is no longer superfluid.
To explain the early specific heat data on superfluid SimpleNuclide|Helium|4, Landau posited the existence of a type of excitation he called a "roton", but as better data became available he considered that the "roton" was the same as a high momentum version of sound.
Bijl in the 1940s, and
Feynmanaround 1955, developed microscopic theories for the roton, which was shortly observed with inelastic neutron experiments by Palevsky.
Landau thought that vorticity entered superfluid SimpleNuclide|Helium|4 by vortex sheets, but such sheets were shown to be unstable.
Onsager and, independently, Feynman showed that vorticity enters by quantized vortex lines. They also developed the idea of quantum vortex rings.
Although the phenomenologies of the superfluid states of helium-4 and
helium-3are very similar, the microscopic details of the transitions are very different. Helium-4 atomsare bosons, and their superfluidity can be understood in terms of the Bose statisticsthat 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 theoryof 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 gradientin a superfluid, much as it is impossible to set up a voltagedifference in a superconductor.) Application of heat to a spot in superfluid helium results in a wave of heat conduction at the relatively high velocity of 20 m/sec, called second sound.
One of the most spectacular results of these properties is known as the
thermomechanicalor "fountain effect". If a capillary tubeis placed into a bath of superfluid helium and then heated, even by shining a lighton 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
speedbelow the first critical velocity(related to the quantum numbersfor 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.
Recently in the field of
chemistry, superfluid helium-4 has been successfully used in spectroscopictechniques, as a quantum solvent. Referred to as Superfluid Helium Droplet Spectroscopy (SHeDS), it is of great interest in studies of gasmolecules, as a single moleculesolvated 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 Barticle).
Recently, superfluids have been used to trap light and slow its speed. In an experiment performed by
Lene Hau, light was passed through a Bose-Einstein condensed gas of sodium (analagous to a superfluid) and found to be slowed to 17 metres per second from its normal speed of 299,792,458 metres per second in vacuum. [Lene Vestergaard Hau, S. E. Harris, Zachary Dutton, Cyrus H. BehrooziLight speed reduction to 17 metres per second in an ultracold atomic gasNature 397, 594-598 (18 February 1999)] This does not change the absolute value of "c", nor is it completely new: any medium other than vacuum, such as water or glass, also slows down the propagation of light to "c"/"n" where "n" is the material's refractive index.
The Infrared Astronomical Satellite
IRAS, launched in January 1983to gather infrared datawas cooled by 720 litres of superfluid helium, maintaining a temperatureof 1.6 K (-271.4 °C)."'
Superfluid helium is used in the cooling system of
CERN's Large Hadron Collider( LHC) [2008 JINST 3 S08001] .
Physicistshave 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 pairswhich also exhibit superfluidity. This recent work with ultra-cold atomic gases has allowed scientiststo study the region in between these two extremes, known as the BEC-BCS crossover.
Additionally, super"solids" may also have been discovered in
2004by physicists at Penn State University. When helium-4 is cooled below about 200 mK under high pressures, a fraction(~1%) of the solidappears to become superfluid. [Moses Chan's Research Group. " [http://www.phys.psu.edu/~chan/index_files/Page526.htm Supersolid] ." "Penn State University," 2004.]
Douglas D. Osheroff
Timeline of low-temperature technology
* London, F. Superfluids (Wiley, New York, 1950).
* D.R. Tilley and J. Tilley, ``Superfluidity and Superconductivity," (IOP Publishing Ltd., Bristol, 1990).
Hagen Kleinert, "Gauge Fields in Condensed Matter", Vol. I, "SUPERFLOW AND VORTEX LINES", pp. 1–742, [http://www.worldscibooks.com/physics/0356.htm World Scientific (Singapore, 1989)] ; Paperback ISBN 9971-5-0210-0 (also available online [http://www.physik.fu-berlin.de/~kleinert/kleiner_reb1/contents1.html here] )
* [http://www.youtube.com/watch?v=2Z6UJbwxBZI&feature=related Video including superfluid helium's strange behavior]
* [http://ltl.tkk.fi/research/theory/helium.html Superfluid phases of helium]
* [http://www.lancs.ac.uk/depts/physics/research/condmatt/ult/index.html Lancaster University, Ultra Low Temperature Physics] - Superfluid helium-3 research group.
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