Superfluidity

The superfluidity or superfluid also superfluidity , Super liquid or Hyperfluidität called ( English superfluidity ), is a macroscopic quantum effect and referred to in the physics the state of a liquid in which they every internal friction loses. In addition, superfluid materials have no entropy and almost ideal thermal conductivity ; it is therefore difficult to create a temperature difference within a superfluid substance. The phenomenon of superfluidity was first described in 1938 by Pyotr Leonidowitsch Kapiza , John F. Allen and Don Misener . Superfluids are examples of a quantum fluid .

description

The phenomenon of superfluidity has so far only been observed with the helium isotopes ⁴ He and ³ He and with the lithium isotope ⁶ Li. They go into the superfluid state when their temperature falls below the critical temperature of superfluidity, T _Sf , the so-called lambda point . With ⁴ He, T _Sf is 2.17  K , with the much rarer isotope ³ He at 0.0026 K.

Superfluid ⁴ He is also called helium-II, in contrast to normal fluid (liquid) helium-I.

In the superfluid phase one can observe unusual phenomena:

• The liquid penetrates smoothly through the narrowest capillaries .
• Almost ideal thermal conductivity of the liquid due to the effect of the second sound .
• When the container rotates, the liquid does not rotate as a whole. If the rotation is very slow, it simply stops; at faster rotation, quantized mechanical vortices form (similar to the magnetic flux vortices in the superconductor or vortices in the bath). If the vortex density is sufficiently high, these are arranged in a regular hexagonal grid.
• The so-called fountain effect (also known as fountain effect):

A small vessel with a bottom of capillaries and a small opening at the top is partially immersed in a larger vessel with superfluid. The small vessel has a small heater inside. If you switch on this heating, an overpressure is created in the small vessel at the top of the opening, which allows liquid to squirt through this small opening.

This effect can be explained as follows: The heater converts helium II into helium I, since the ratio of helium II to helium I is reduced for higher temperatures. To compensate for the decreasing concentration of helium II in the small vessel, helium II flows in from the large vessel without friction. Conversely, however, helium I cannot flow back through the capillaries, since the friction prevents this. This creates an overpressure.

Rollin movie

The Rollin film , named after a discovery by Bernard Vincent Rollin , physicist at the Clarendon Laboratory of the University of Oxford , and Franz Eugen Simon in 1937, is a liquid film about 100 atomic layers thick around a body that results from the very low cohesive forces (attraction liquid particles among each other) in a superfluid and the therefore stronger adhesion forces (attraction between the particles of the solid surface and the liquid particles). It creates the Onnes Effect .

Onnes effect

Helium II “crawls” up the wall of the inner vessel - after a certain time the liquid levels in the vessels would equalize. The Rollin film also covers the wall of the large container; if it were not closed, the liquid film would creep through every opening and the helium would gradually escape.

The Onnes Effect , named after Heike Kamerlingh Onnes , describes the phenomenon of superfluid liquids flowing over higher obstacles. The liquid moves as a very thin film ( rollin film ) slowly up the vessel walls towards higher temperatures. This can e.g. B. be observed in superfluid helium . The effect is due to the fact that the internal friction (more precisely: its dynamic viscosity ) disappears in the superfluid and the capillary forces on the vessel wall are stronger than the gravitational forces and the adhesive resistance. Flow rates of 20 to 40 cm / s are typical. This property of superfluid liquids can have a negative effect in experiments, as even minor leaks in the apparatus can lead to noticeable losses of helium.

Explanatory approaches

The superfluidity cannot yet be fully explained theoretically. However, there are various approaches that describe the properties of superfluid helium at least qualitatively.

Two fluid model

The two-fluid model (also two-fluid model ) for explaining superfluidity goes back to László Tisza and Lew Landau . Since helium exhibits both superfluid and viscous properties in the temperature range from 1 K to the lambda point , it is assumed that the total density of the liquid is made up of a normal portion, which becomes increasingly smaller as the temperature drops, and a superfluid portion. However, excitations in the superfluid component can also be generated, which act like a viscosity of superfluid helium. If you pull z. B. a floating body over superfluid helium, it feels no friction up to a certain limit speed (the so-called Landau criterion ). Above this speed, however, rotons and at even higher speeds phonons can be excited, which acts like friction on the body. Mathematically, this results in a limit speed of approx. 60 m / s. In fact, flow experiments show that the limit speed is well below 1 cm / s, but for ions moving through superfluid helium, speeds of just under 60 m / s are measured. The cause is the excitation of quantized eddies in the superfluid, so-called vortices . This phenomenon is comparable to the excitation of quantized circulating currents in superconductors . The vortices must not be confused with the rotons, since the former represent a macroscopic excitation of the superfluid.

Quantum mechanical approach

Superfluid can be well understood in the Bose-Einstein condensation model . According to this model, a macroscopic part of all bosons occupies the same quantum state . This means that all He particles that have condensed into this ground state can be described by a single wave function . Just like laser light and the quantum Hall effect , the superconducting phase can be understood as a macroscopic quantum state . The critical temperature for the phase transition to superfluid helium is 3.1 K, which is qualitatively correct, but is significantly higher than the measured 2.17 K. Furthermore, at 0 K only 8% of the atoms are in the ground state, not 100% as the Bose-Einstein theory model predicts. The cause of these discrepancies is the atomic interaction of the He atoms, which is set to zero in the Bose-Einstein model. In contrast, with the Bose-Einstein condensation of rubidium and sodium gases in atomic traps (mentioned in the special article), the interaction of the atoms involved is actually negligible.

For He liquids, the Bose-Einstein condensation model only applies qualitatively, and also quantitatively for the gases mentioned.

It should be noted that the Bose-Einstein condensation does not contradict the two-fluid model. The proportion of particles that is condensed in the ground state depends on the temperature. The lower the temperature is below a critical temperature ( lambda point at ⁴ He), the more particles occupy the ground state. The condensed portion can be regarded as superfluid helium, the remaining particles are normal liquid helium.

In contrast to the bosonic ⁴ He - atoms are in the atoms of the rare naturally occurring ³ He to fermions . For these, the Bose-Einstein statistics do not apply , but the Fermi-Dirac statistics . The Bose-Einstein condensation model cannot therefore be used for the ³ He atoms. Nevertheless, even with ³ He, superfluid properties are observed . However, this is not a contradiction in terms if the superfluidity of ³ He is not based on isolated atoms, but on the coupling of two atoms, so that, analogous to Cooper pair formation in electron superconductivity, bosonic ³ He pairs with spin 1 are used here (one can understand that because of the weakness of this coupling, the transition temperature is about 1/1000 that of ⁴ He). Two ³ He atoms can assume a somewhat lower (and therefore somewhat more probable) state in terms of energy if their nuclear magnetic moments ( nuclear spins ) are aligned ( magnetic states) or opposite (non-magnetic state). The superfluid state in ³ He has a rich phase and excitation spectrum (see ³ He ). Superfluidity in ³ He was discovered in the 1970s by David Morris Lee , Douglas Dean Osheroff and Robert Coleman Richardson , who received the Nobel Prize for it, and the complex phase structure was investigated by Anthony J. Leggett (who also received the Nobel Prize for it).

In 1984 Juri Michailowitsch Bunkow , Igor A. Fomin and Wladimir Wladimirowitsch Dmitrijew discovered spin superfluidity in ³ He, which instead of mass flows as with normal superfluidity affected the magnetization and spin degrees of freedom .

Technical applications

In physics and chemistry , superfluid ⁴ He is used in spectroscopy . The sample is surrounded by liquid helium in a cryostat . By pumping out the gaseous helium, the temperature is lowered below the lambda point and the helium becomes superfluid. The temperature depends on the pressure and in practice can be set between 1.1 and 2.1 K by pumping at different levels.

A far more complex technique is called Superfluid Helium Droplet Spectroscopy (SHeDS) or Helium Nano Droplet Isolation (HeNDI) Spectroscopy . The helium droplets used for this are produced in an adiabatic expansion of helium in a vacuum apparatus and have a temperature of only 0.37 K. Molecules or clusters that are dissolved in superfluid ⁴ He can de facto rotate freely, as if they were in a space vacuum found.

In the cooling system of the LHC at CERN , superfluid helium is used due to its comparatively high thermal conductivity.

literature

• Antony M. Guénault: Basic superfluids. Taylor & Francis, London 2003, ISBN 0-7484-0891-6
• James F. Annett: Superconductivity, superfluids, and condensates. Oxford Univ. Press, Oxford 2005, ISBN 978-0-19-850756-7
• Christian Enss, Siegfried Hunklinger: Low-temperature physics. Springer, Berlin 2005, ISBN 978-3-540-23164-6
• Fritz London : Superfluids. Dover, New York, 1961
• KH Bennemann, et al .: Novel superfluids. 2nd volumes, Oxford Univ. Press, Oxford 2014, ISBN 978-0-19-872285-4 .
• Andreas Schmitt: Introduction to superfluidity. Springer, Cham 2015, ISBN 978-3-319-07946-2 .

Web links

Commons : Superfluidity  - collection of images, videos and audio files

Wiktionary: superfluidity  - explanations of meanings, word origins, synonyms, translations

• "Eddies clearly demonstrate the superfluidity of lithium" July 6, 2005 on Telepolis

Individual evidence

1. ^ Kapitza P: Viscosity of liquid helium below the λ-point . In: Nature . 141, 1938, p. 74. doi : 10.1038 / 141074a0 .
2. ^ Allen JF, Misener AD: Flow of liguid helium II . In: Nature . 141, 1938, p. 75. bibcode : 1938Natur.141 ... 75A .
3. ↑ Steven W. Van Sciver: Helium Cryogenics . Springer, New York 2012. ISBN 978-1-4419-9979-5 . There the chapter “Helium as a Quantum Fluid”, pp. 163–226, here p. 182.
4. ^ DR Allum, PVE McClintock: The Breakdown of Superfluidity in Liquid ⁴ He: An Experimental Test of Landau's Theory . In: Philosophical Transactions of the Royal Society of London. Series A, Mathematical and Physical Sciences . 284, 1977, pp. 179-224. doi : 10.1098 / rsta.1977.0008 . , accessed August 10, 2012
5. ↑ LHC: the guide , CERN brochure