Superfluid 27573 226167250 2008-07-17T03:20:53Z 65.35.23.30 /* Some Theory */ Added link to Landau [[Image:helium-II-creep.svg|thumb|right|200px|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.]] '''Superfluidity''' is a [[phase (matter)|phase of matter]] or description of [[heat capacity]] in which unusual effects are observed when [[liquids]], typically of [[helium-4]] or [[helium-3]], overcome [[friction]] by surface interaction when at a stage, known as the "[[lambda point]]" for helium-4, at which the liquid's [[viscosity]] becomes [[zero]]. Also known as a major facet in the study of [[quantum hydrodynamics]], it was discovered by [[Pyotr Kapitsa]], [[John F. Allen]], and [[Don Misener]] in [[1937]] and has been described through [[Phenomenology (science)|phenomenological]] and microscopic theories. In the 1950's Hall and Vinen performed experiments establishing the existence of quantized vortex lines. In the 1960's, Rayfield and Reif established the existence of quantized vortex rings. Packard has observed vortex rings directly, and Avenel and Varoquaux have studied the Josephson effect, in superfluid {{SimpleNuclide|Link|Helium|4}}. ==Some Theory== [[Lev Landau|L. D. Landau's]] phenomenological and semi-microscopic theory of superfluidity in {{SimpleNuclide|Helium|4}} earned him the Nobel Prize in Physics in 1964. Assuming that sound waves are the most important excitations in {{SimpleNuclide|Helium|4}} at low temperatures T, 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 T=0, and increases as T increases. 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 1940's, and Feynman, around 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. ==Background== Although the phenomenologies of the superfluid states of helium-4 and [[helium-3]] are very similar, the microscopic details of the transitions are very different. Helium-4 [[atoms]] are [[boson]]s, and their superfluidity can be understood in terms of the [[Bose statistics]] that they obey. Specifically, the superfluidity of helium-4 can be regarded as a consequence of [[Bose-Einstein condensate|Bose-Einstein condensation]] in an interacting system. On the other hand, helium-3 atoms are [[fermion]]s, and the superfluid transition in this system is described by a generalization of the [[BCS theory]] of superconductivity. In it, [[Cooper pair]]ing takes place between atoms rather than [[electrons]], and the attractive interaction between them is mediated by [[Spin (physics)|spin]] fluctuations rather than [[phonon]]s. See [[fermion condensate]]. A unified description of superconductivity and superfluidity is possible in terms of [[spontaneous symmetry breaking|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 gradient]] in a superfluid, much as it is impossible to set up a [[voltage]] difference in a [[superconductor]].) One of the most spectacular results of these properties is known as the [[thermomechanical]] or "fountain effect". If a [[capillary tube]] is placed into a bath of superfluid helium and then heated, even by shining a [[light]] on 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 [[Quantum vortex|quantized vortices]]. That is, when the container is rotated at [[speed]] below the first critical [[velocity]] (related to the [[quantum numbers]] for the [[chemical element|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. ==Applications== Recently in the field of [[chemistry]], superfluid helium-4 has been successfully used in [[spectroscopic]] techniques, as a [[quantum solvent]]. Referred to as Superfluid Helium Droplet Spectroscopy (SHeDS), it is of great interest in studies of [[gas]] molecules, as a single [[molecule]] solvated 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 B]] article). Recently, superfluids have been used to trap light and slow its [[speed of light|speed]]. In an experiment, performed by [[Lene Hau]], light was passed through 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.<ref>Lene Vestergaard Hau, S. E. Harris, Zachary Dutton, Cyrus H. Behroozi Light speed reduction to 17 metres per second in an ultracold atomic gas Nature 397, 594-598 (18 February 1999)</ref> This does not change the absolute value of [[speed of light|c]], nor is it completely new: any medium other than vacuum, such as water or glass, also slows down the propagation of light [[refractive index|in a certain fraction]]. 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 1.6K (-271.4 ­°C). ==Recent discoveries== [[Physicists]] have recently been able to create a Fermionic condensate from pairs of ultra-cold fermionic atoms. Under certain conditions, fermion pairs form [[diatomic molecule|diatomic molecules]] and undergo [[Bose–Einstein condensate|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, [[supersolid|super''solids'']] might have also 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 [http://www.phys.psu.edu/~chan/index_files/Page526.htm]. ==See also== * [[Superdiamagnetism]] * [[Bose-Einstein condensate]] * [[Superconductivity]] * [[Quantum vortex]] * [[Supersolid]] * [[superfluid film]] * [[Douglas D. Osheroff]] ==Notes== <references /> ==References== * 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]) ==External links== *[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. * http://www.aip.org/png/html/helium3.htm * http://www.aip.org/pt/vol-54/iss-2/p31.html * http://web.mit.edu/newsoffice/2005/matter.html * http://physicsweb.org/articles/world/11/6/3/1 {{Phase_of_matter}} <!-- Categories --> [[Category:Fundamental physics concepts]] [[Category:Condensed matter physics]] [[Category:Fluid dynamics]] [[Category:phases of matter]] [[Category:Amorphous liquids]] <!--Interwiki--> [[az:İfrat axıcılıq]] [[bs:Superfluid]] [[ca:Superfluid]] [[cs:Supratekutost]] [[de:Suprafluidität]] [[es:Superfluido]] [[et:Ülivoolavus]] [[fr:Superfluide]] [[ko:초유체]] [[id:Superfluida]] [[it:Superfluidità]] [[he:נוזל על]] [[nl:Superfluïditeit]] [[ja:超流動]] [[no:Superfluid]] [[pl:Nadciekłość]] [[pt:Superfluidez]] [[ru:Сверхтекучесть]] [[sk:Supratekutá látka]] [[fi:Supraneste]] [[sv:Suprafluiditet]] [[zh:超流体]]