Superfluid

Not Lambda

Physicists mentioned Ci above noted that in lower part of the temperature criticizes of 2,17 Kelvin, (either -270,98°C), which we call the point lambda (λ), the Hélium 4 underwent a Transition from phase. It passed from a liquid state to another to the appreciably different properties. Indeed, the experience, proven thereafter, showed that this new state of helium conducted the heat very well, which could be explained only by one low viscosity.

Experiments more specific to the Mécanique of the fluids showed then than the flow of this helium in a pipe was appreciably independent of the pressure applied to the walls of the pipe, and more independent of the section of the pipe in question!

This could be explained only by one complete absence of Viscosité, from where the name of superfluidity .

In short, a liquid is known as Superfluide if he does not oppose any resistance to the flow. Consequently, the solids which are driven inside do not undergo any Frottement viscous.

Property of superfluid

Other remarkable properties of superfluid are the existence of a thermal conductivity infinite and the presence of swirls having a Vorticity quantifée. From the theoretical point of view, one can describe the hydrodynamics of superfluid by model with two fluids, the normal fluid which has a not-null viscosity and the superfluid one of null viscosity. When the temperature decreases, the superfluid fraction increases and the normal fraction decreases. In lower part of the point λ, superfluid helium acquires the quality of Supraconducteur of heat, i.e. it does not support the least difference in temperature between two of its parts. Without what, helium is not exactly any more superfluid.

Model of the two fluids

The principle of this model says that superfluid helium is, actually, the interpenetration, the mixture, of two fluids to the very different properties. It would be composed of a normal fluid and a superfluid . The normal fluid is composed of atoms not having undergone condensation. They thus occupy, of the states different from the fundamental state and the atoms which compose it are thus localized. The superfluid one, it, is indentifié with the condensate. I.e. the whole of the atoms are compressed in the usual fundamental state. Contrary to the normal fluid, its particles are completely delocalized, from the point of view quantique.
The fact that helium is composed of these two elements, is of course only one model whose validity remains to be proven.

The other superfluid ones

In the Seventies, Douglas Osheroff, David Mr. Lee and Robert C. Richardson discovered a superfluid state for the rare isotope of helium, helium 3, at a temperature of 2mK approximately, quite lower than the temperature of superfluid transition measured in helium 4. For this discovery, the Nobel Prize of physics their was allotted in 1996.

The difference between the two Isotope S of helium is that the helium 4 atoms are Boson S, whereas the helium 3 atoms are Fermion S, with the result that them behavior at very low temperature follows radically different laws.

As suggested it Fritz London in the Forties, the formation of a superfluid state in helium 4 corresponds to a Condensation of Bump-Einstein of the helium atoms which are bosonic quantum particles. However, contrary to the case of ideal gas of Bump, in helium 4 the repulsion between the atoms is very strong, and even at very low temperature, only 10% of the atoms are in the condensate.

On the contrary, because of its character fermionic, helium 3 at very low temperature forms a Liquide of Fermi. It is the existence of a very weak attraction between the helium 3 atoms which produce a pairing of the atoms fermionic of helium 3 in lower part of the superfluid transition. A similar phenomenon led to the Supraconductivity in metals. In this last case, the particles forming a superfluid state are pairs of electrons (" pairs of Cooper"), and the absence of Viscosité results in an absence of electrical resistance. The theory of the superfluid phases of helium 3 is an extension of the Théorie BCS developed by Balian, Werthamer, Anderson, Brinkmann, Morel and Leggett.

References

Hydrodynamic with two fluids in superfluid:
Superfluidity:
Superfluidity: P.W. Anderson BASIC Concepts off Condensed Matter Physics (Addison-Wesley)
Superfluid: '' Diagrammes of Phase of helium 3 and helium 4 ''
'' Cours on line (in English) by NR. B. Kopnin on supraconductivity and superfluidity ''
Helium 3: '' Osheroff, Lee, and Richardson on the site of the foundation Nobel ''
Helium 3: '' Original Articles of Osheroff, Lee, Richardson ''

See too

Random links: