Superfluidity

Called The superfluidity or superfluid, even superfluidity, superfluid or Hyperfluidität (English superfluidity ), is a macroscopic quantum effect and called in physics the state of a liquid in which it loses any internal friction. In addition superfluid substances have no entropy and an infinitely high thermal conductivity; It is therefore not possible to produce a temperature difference within a superfluid substance. The phenomenon of superfluidity was first described in 1938 by Pyotr Leonidovich Kapitsa, John F. Allen, and Don Misener. The branch of physics that deals with superfluidity, is the quantum hydrodynamics.

• 2.1 two-fluid model,
• 2.2 Quantum Mechanical Approach

Description

The phenomenon of superfluidity has so far been observed only in the helium isotopes 4He and 3He as well as in the lithium isotope 6Li. You go into the superfluid state when its temperature is the critical temperature of superfluidity, TSf, the so-called lambda point below. When 4He is TSf = 2.17 K. In the extremely rare isotope 3He TSf is 2.6 mK.

Superfluid 4 He is also referred to as helium -II, in contrast to normalfluidem ( liquid ) helium -I.

In the superfluid phase can be observed unusual phenomena:

• The liquid penetrates smoothly through the narrowest capillaries.
• Almost ideal thermal conductivity of the liquid by the effect of the second sound.
• Upon rotation of the container, the liquid does not rotate as a whole. For very slow rotation it just stops; with faster rotation is quantized mechanical vortices (similar to the magnetic flux vortices in superconductors or vertebrae in the bathtub ). These arrange themselves at a sufficiently high vortex density in a regular hexagonal lattice.
• The so-called fountain effect ( also fountains effect, Eng fountain effect. ): In a larger vessel with superfluid is partially immersed a small vessel, which has a base of capillaries and above a small opening. The little vessel has a small heater inside. If you turn this heater on, arises in the small vessel up to the opening, an overpressure, which can inject through this small hole liquid. This effect can be explained as follows: The heating converts helium II helium I to, because decreases for higher temperatures the ratio of helium II to He I. To compensate for the reducing the concentration of helium in the small vessel II, helium II from the large vessel flows smoothly. Conversely, however, can not flow back through the capillary helium I, since the friction will prevent this. Thus, due to excessive pressure.

Rollin film

The Rollin film is a about 100 atomic layers thick film of liquid around a body composed of the very low cohesive forces (attraction of liquid particles with each other ) in a superfluid and therefore compared to stronger adhesion forces ( attraction between the particles of the solid surface and the fluid particles ) results. It causes the Onnes effect.

Onnes effect

The Onnes effect, named after Heike Kamerlingh Onnes, describes the phenomenon of superfluid liquids, hinwegzufließen on higher ground obstacles. Here, the liquid moves slowly as a very thin film ( Rollin film) to vessel walls in the direction of higher temperatures high. This can be observed for example in superfluid helium. The effect is due to the fact that the internal friction (more precisely, their dynamic viscosity ) disappears in the superfluid, and the capillary forces on the vessel wall are greater than the gravitational forces and the adhesive resistance. Flow velocity of 0.2-0.4 m / s is typical. This property superfluid liquids can have a negative effect in the experiment, since even small leaks can cause the apparatus to significant losses of helium.

Explanations

The superfluidity can not yet fully explain theoretically. However, there are several approaches that describe the properties of the superfluid helium at least qualitatively.

Two - fluid model,

The two-fluid model (also two-fluid model) to explain the superfluidity is due to László Tisza and Lev Landau. Because in the temperature range of 1 K to lambda point indicates helium both superfluid and viscous properties, it is believed that make up the overall density of the fluid from a normal portion, which is progressively smaller with decreasing temperature, and a superfluid component. However, it is also possible excitations in superfluid share produce, which act as a viscosity of superfluid helium. If we take for example a floating body on superfluid helium, so this feels up to a certain speed limit ( the so-called Landau criterion) no friction. Above this speed, however, can be excited phonons rotons and at higher speeds, such as friction is applied to the body. When calculated in this case a limiting velocity of about 60 m / s In fact, it is in flow experiments found that the speed limit well below 1 cm / s, but with ions moving through superfluid helium, speeds of up to nearly 60 m / s. The cause is the excitation of quantized vortices in the superfluid, so-called vortices. This phenomenon is similar to the quantized excitation circuit currents in superconductors. The vortices are not to be confused with the roton here, since the latter represent a macroscopic excitation of the superfluid.

Quantum mechanical approach

Superfluid can be well understood in the model of Bose -Einstein condensation. Under this model, a macroscopic fraction of all bosons occupies the same quantum state. This allows all He particles condensed in this basic state, can be described by a single wave function. As well as laser light, and the quantum Hall effect, the superconducting phase may be understood as a macroscopic quantum state. The critical temperature for the phase transition to superfluid helium thereby obtained 3.1 K, which is qualitatively correct, but significantly higher than the measured 2.17 K. Furthermore, there is at T = 0 K, only 8 % of the atoms in the ground state not 100%, as the model of the Bose-Einstein theory predicts. Cause of these discrepancies is the atomic interaction of He atoms, which is set in the Bose -Einstein model to zero. In contrast, in the ( mentioned in the special article) Bose -Einstein condensation of rubidium and sodium gases in atom traps the interaction of the atoms involved is actually negligible.

So the model of the Bose -Einstein condensation is valid only qualitatively and quantitatively for the mentioned gases for helium liquids.

It should be noted that the Bose -Einstein condensation is not contrary to the two-fluid model. The proportion of particles which is condensed in basic mode, depends on the temperature. Below a critical temperature ( lambda point in 4He ), more and more particles occupy the ground state, the lower the temperature is. It can be condensed proportion than superfluid helium consider, on all remaining particles are normal liquid helium.

In contrast to the bosonic 4He atoms are in the atoms of the rare in nature occurring 3He are fermions. For these are not the Bose -Einstein statistics, but the Fermi -Dirac statistics is considered. Therefore, for the 3He atoms can not be applied the model of Bose -Einstein condensation. Nevertheless, we observe superfluid properties at 3He. However, this is not a contradiction, if one starts at the superfluidity of 3He not of isolated atoms, but by the coupling of two atoms, so that there is obtained analogously to the Cooper - pair formation in electron - bosonic superconductivity here 3He pairs with spin 1 ( one can understand that because of the weakness of this coupling, the transition temperature about one 1000th of the amounts of 4He ). Two 3He atoms can in this case take a slightly lower energy ( and therefore somewhat more probable ) state when their nuclear magnetic moments ( spins ) rectify (magnetic states) or oppositely directed ( nichtmagn. state).

Technical Applications

In physics and chemistry superfluid 4He is used in spectroscopy. The sample is washed in a liquid helium cryostat. By pumping the helium gas, the temperature is lowered below the lambda point and the helium is superfluid. The temperature depends on the pressure and can be adjusted in practice by various degrees of pumps from 1.1 to 2.1 K.

A far more elaborate technique is called superfluid helium droplet spectroscopy ( SHEDS ) and helium nano- droplet isolation ( Hendi ) Spectroscopy. Helium droplets used for this purpose are produced in an adiabatic expansion of helium in a vacuum apparatus, and have a temperature of only 370 mK. Molecules or clusters, which are dissolved in superfluid 4He, de facto free to rotate as if they were in the vacuum of space.

In the cooling system of the CERN LHC superfluid helium is used because of its relatively high thermal conductivity.