Superfluid

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.

Superfluiditatea este o fază a materiei în care anumite fluide suprarăcite, în special heliu 4 și heliu 3, manifestă un comportament straniu, ca și când forțele de atracție și frecare interne nu ar exista, fenomen accentuat până la un punct, cunoscut ca "punctul lambda", pentru heliu 4, la care viscozitatea lichidului devine zero.

Această proprietate reprezintă un interes major în domeniul hidrodinamicii cuantice, a fost descoperită de Piotr Kapița, John F. Allen și Don Misener în 1937 și a fost descrisă prin intermediul fenomenologiei și a teorilor macroscopice. În anii 1950, Hall și Vinen au întreprins experimente pentru a demonstra existența liniilor discrete de vorticitate. În anii 1960, Rayfield și Reif au demonstrat existența inelelor discrete de vorticitate. Packard a observat intersecția liniilor de vorticitate cu suprafața liberă a fluidului, iar Avenel și Varoquaux au studiat efectul Josephson la superfluide Format:SimpleNuclide.

Câteva explicări teoretice

L. D. Landau's phenomenological and semi-microscopic theory of superfluidity in Format:SimpleNuclide earned him the Nobel Prize in Physics in 1964. Assuming that sound waves are the most important excitations in Format:SimpleNuclide at low temperatures, he showed that Format:SimpleNuclide 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.

Aplicații

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 1.6 K (-271.4 ­°C).

Superfluid helium is used in the cooling system of CERN's Large Hadron Collider (LHC).

Descoperiri recente

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 also have 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.

Vezi și

• Superdiamagnetism
• Condensatul Bose-Einstein
• Superconductibilitate
• Vortex cuantic
• Supersolid
• Peliculă superfluidă
• Douglas D. Osheroff
• Timeline of low-temperature technology
• Second sound

Referințe

• 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, World Scientific (Singapore, 1989); Paperback ISBN 9971-5-0210-0 (also available online here)

Legături externe

• Video including superfluid helium's strange behavior
• Superfluid phases of helium
• Lancaster University, Ultra Low Temperature Physics - Superfluid helium-3 research group.
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