View source for Physics:Superfluidity

{{Short description|Fluid which flows without losing kinetic energy}}
[[File:helium-II-creep.svg|thumb|right|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.]]
[[File:Liquid helium Rollin film.jpg|thumb|right|The liquid helium is in the superfluid phase. A thin invisible film creeps up the inside wall of the bowl and down on the outside. A drop forms. It will fall off into the liquid helium below. This will repeat until the cup is empty—provided the liquid remains superfluid.]]
{{Condensed matter physics}}

'''Superfluidity''' is the characteristic property of a [[Physics:Fluid|fluid]] with zero [[Physics:Viscosity|viscosity]] which therefore flows without any loss of [[Physics:Kinetic energy|kinetic energy]]. When stirred, a superfluid forms [[Vortex|vortices]] that continue to rotate indefinitely. Superfluidity occurs in two [[Physics:Isotope|isotope]]s of [[Chemistry:Helium|helium]] ([[Physics:Helium-3|helium-3]] and [[Physics:Helium-4|helium-4]]) when they are liquefied by cooling to cryogenic temperatures. It is also a property of various other exotic [[Physics:State of matter|states of matter]] theorized to exist in [[Physics:Astrophysics|astrophysics]], [[Physics:High-energy physics|high-energy physics]], and theories of [[Physics:Quantum gravity|quantum gravity]].<ref name=":0">{{Cite web|url=https://www.nobelprize.org/nobel_prizes/physics/laureates/1996/advanced.html|title=The Nobel Prize in Physics 1996 – Advanced Information|website=www.nobelprize.org|access-date=2017-02-10}}</ref> The theory of superfluidity was developed by Soviet theoretical physicists [[Biography:Lev Landau|Lev Landau]] and [[Biography:Isaak Khalatnikov|Isaak Khalatnikov]].

Superfluidity often co-occurs with [[Physics:Bose–Einstein condensate|Bose–Einstein condensation]], but neither phenomenon is directly related to the other; not all Bose–Einstein condensates can be regarded as superfluids, and not all superfluids are Bose–Einstein condensates.{{cn|date=May 2023}} Superfluids have some potential practical uses, such as dissolving substances in a [[Physics:Quantum solvent|quantum solvent]].

== Superfluidity of liquid helium ==
{{Main|Physics:Superfluid helium-4}}

Superfluidity was discovered in [[Physics:Helium-4|helium-4]] by [[Biography:Pyotr Kapitsa|Pyotr Kapitsa]]<ref>{{cite journal|last1=Kapitza|first1=P.|date=1938|title=Viscosity of Liquid Helium Below the λ-Point|journal=Nature|volume=141|issue=3558|page=74|bibcode=1938Natur.141...74K|doi=10.1038/141074a0|s2cid=3997900|doi-access=free}}</ref> and independently by [[Biography:John F. Allen (physicist)|John F. Allen]] and [[Physics:Don Misener|Don Misener]]<ref>{{cite journal|last1=Allen|first1=J. F.|last2=Misener|first2=A. D.|date=1938|title=Flow of Liquid Helium II|journal=Nature|volume=142|issue=3597|page=643|bibcode=1938Natur.142..643A|doi=10.1038/142643a0|s2cid=4135906}}</ref> in 1937. [[Biography:Heike Kamerlingh Onnes|Onnes]] possibly observed the superfluid phase transition on August 2 1911, the same day that he observed superconductivity in mercury.<ref>{{Cite journal |last1=van Delft |first1=Dirk |last2=Kes |first2=Peter |date=2010-09-01 |title=The discovery of superconductivity |url=https://pubs.aip.org/physicstoday/article/63/9/38/386608/The-discovery-of-superconductivityA-century-ago |journal=Physics Today |language=en |volume=63 |issue=9 |pages=38–43 |doi=10.1063/1.3490499 |bibcode=2010PhT....63i..38V |issn=0031-9228|doi-access=free }}</ref> It has since been described through phenomenology and microscopic theories. 

In liquid helium-4, the superfluidity occurs at far higher temperatures than it does in [[Physics:Helium-3|helium-3]]. Each atom of helium-4 is a [[Physics:Boson|boson]] particle, by virtue of its integer spin. A helium-3 atom is a [[Physics:Fermion|fermion]] particle; it can form bosons only by pairing with another particle like itself at much lower temperatures. The discovery of superfluidity in helium-3 was the basis for the award of the 1996 Nobel Prize in Physics.<ref name=":0" /> This process is similar to the [[Physics:Cooper pair|electron pair]]ing in [[Physics:Superconductivity|superconductivity]].

== Ultracold atomic gases ==
Superfluidity in an ultracold [[Physics:Fermi gas|fermionic gas]] was experimentally proven by [[Biography:Wolfgang Ketterle|Wolfgang Ketterle]] and his team who observed [[Physics:Quantum vortex|quantum vortices]] in [[Physics:Lithium-6|lithium-6]] at a temperature of 50 nK at [[Organization:Massachusetts Institute of Technology|MIT]] in April 2005.<ref>{{cite web| website=mit.edu |url = http://web.mit.edu/newsoffice/2005/matter.html| title = MIT physicists create new form of matter| date=22 June 2005 | access-date = November 22, 2010}}</ref><ref>{{Cite journal | last1 = Grimm | first1 = R. | title = Low-temperature physics: A quantum revolution | doi = 10.1038/4351035a | journal = Nature | volume = 435 | issue = 7045 | pages = 1035–1036 | year = 2005 | pmid =  15973388|bibcode = 2005Natur.435.1035G | s2cid = 7262637 | doi-access = free }}</ref>  Such vortices had previously been observed in an ultracold bosonic gas using [[Physics:Rubidium-87|rubidium-87]] in 2000,<ref>{{Cite journal | last1 = Madison | first1 = K. | last2 = Chevy | first2 = F. | last3 = Wohlleben | first3 = W. | last4 = Dalibard | first4 = J. | title = Vortex Formation in a Stirred Bose–Einstein Condensate | doi = 10.1103/PhysRevLett.84.806 | journal = Physical Review Letters | volume = 84 | issue = 5 | pages = 806–809 | year = 2000 | pmid =  11017378|arxiv = cond-mat/9912015 |bibcode = 2000PhRvL..84..806M | s2cid = 9128694 }}</ref> and more recently in [[Two-dimensional gas|two-dimensional gas]]es.<ref>{{Cite journal | last1 = Burnett | first1 = K. | title = Atomic physics: Cold gases venture into Flatland | doi = 10.1038/nphys704 | journal = Nature Physics | volume = 3 | issue = 9 | pages = 589 | year = 2007 |bibcode = 2007NatPh...3..589B | doi-access = free }}</ref> As early as 1999, [[Biography:Lene Hau|Lene Hau]] created such a condensate using [[Chemistry:Sodium|sodium]] atoms<ref>{{Cite journal | last1 = Hau | first1 = L. V. | last2 = Harris | first2 = S. E. | last3 = Dutton | first3 = Z. | last4 = Behroozi | first4 = C. H. | journal = Nature | volume = 397 | issue = 6720 | pages = 594–598| doi = 10.1038/17561 | year = 1999 |title=Light speed reduction to 17 metres per second in an ultracold atomic gas|bibcode = 1999Natur.397..594V | s2cid = 4423307 | url = http://nrs.harvard.edu/urn-3:HUL.InstRepos:3636967 }}</ref> for the purpose of slowing light, and later stopping it completely.<ref>{{cite web|url=http://www.physicscentral.com/explore/people/hau.cfm |title=Lene Hau |publisher= | website= Physicscentral.com |access-date=2013-02-10}}</ref>  Her team subsequently used this system of compressed light<ref>{{Cite journal| url= https://groups.seas.harvard.edu/haulab/publications/pdf/HauScientificAmerican2003.pdf|title=Frozen Light|journal=Scientific American|pages=44–51|date=2003|first= Lene Vestergaard | last= Hau}}</ref> to generate the superfluid analogue of shock waves and tornadoes:<ref>{{cite web |url= http://www.siam.org/meetings/nw06/hau.php | title= Shocking Bose–Einstein Condensates with Slow Light | last=Hau| first= Lene |website= SIAM.org |publisher= Society for Industrial and Applied Mathematics | date= September 9–12, 2006}}</ref>

{{quote | These dramatic excitations result in the formation of [[Physics:Soliton|soliton]]s that in turn decay into [[Physics:Quantum vortex|quantized vortices]]—created far out of equilibrium, in pairs of opposite circulation—revealing directly the process of superfluid breakdown in Bose–Einstein condensates. With a double light-roadblock setup, we can generate controlled collisions between shock waves resulting in completely unexpected, nonlinear excitations. We have observed hybrid structures consisting of vortex rings embedded in dark solitonic shells. The vortex rings act as 'phantom propellers' leading to very rich excitation dynamics. | Lene Hau | SIAM Conference on Nonlinear Waves and Coherent Structures}}

== Superfluids in astrophysics ==
The idea that superfluidity exists inside [[Astronomy:Neutron star|neutron star]]s was first proposed by [[Biography:Arkady Migdal|Arkady Migdal]].<ref>{{cite journal|author=A. B. Migdal|journal= Nucl. Phys. |volume=13|pages= 655–674 |date=1959|doi=10.1016/0029-5582(59)90264-0|title=Superfluidity and the moments of inertia of nuclei|issue=5|bibcode = 1959NucPh..13..655M }}</ref><ref>{{cite journal|author=A. B. Migdal|journal= Soviet Phys. JETP |volume=10|page= 176 |date=1960|language=en|url=http://www.jetp.ac.ru/cgi-bin/e/index/e/10/1/p176?a=list|title=Superfluidity and the Moments of Inertia of Nuclei|issue= 5 |doi= 10.1016/0029-5582(59)90264-0 |bibcode= 1959NucPh..13..655M }}</ref> By analogy with electrons inside [[Physics:Superconductor|superconductor]]s forming [[Physics:Cooper pair|Cooper pair]]s because of electron-lattice interaction, it is expected that [[Physics:Nucleon|nucleon]]s in a neutron star at sufficiently high density and low temperature can also form Cooper pairs because of the long-range attractive nuclear force and lead to superfluidity and superconductivity.<ref>{{cite book|author=U. Lombardo|author2=H.-J. Schulze|name-list-style=amp|series=Lecture Notes in Physics|title=Physics of Neutron Star Interiors|chapter=Superfluidity in Neutron Star Matter|volume=578|date=2001|pages=30–53|arxiv=astro-ph/0012209|doi=10.1007/3-540-44578-1_2|isbn=978-3-540-42340-9|s2cid=586149}}</ref>

== In high-energy physics and quantum gravity ==
{{Main|Physics:Superfluid vacuum theory}}

[[Physics:Superfluid vacuum theory|Superfluid vacuum theory]] (SVT) is an approach in [[Physics:Theoretical physics|theoretical physics]] and [[Physics:Quantum mechanics|quantum mechanics]] where the physical [[Physics:Vacuum|vacuum]] is viewed as superfluid.

The ultimate goal of the approach is to develop scientific models that unify quantum mechanics (describing three of the four known fundamental interactions) with [[Physics:Gravity|gravity]]. This makes SVT a candidate for the theory of [[Physics:Quantum gravity|quantum gravity]] and an extension of the [[Physics:Standard Model|Standard Model]].

It is hoped that development of such theory would unify into a single consistent model of all fundamental interactions,
and to describe all known interactions and elementary particles as different manifestations of the same entity, superfluid vacuum.

On the macro-scale a larger similar phenomenon has been suggested as happening in the murmurations of [[Biology:Starling|starling]]s. The rapidity of change in flight patterns mimics the phase change leading to superfluidity in some liquid states.<ref>{{cite journal |title=Information transfer and behavioural inertia in starling flocks |journal=Nature Physics |volume=10 |issue=9 |pages=615–698 |year=2014 |last1=Attanasi |first1=A. |last2=Cavagna |first2=A. |last3=Del Castello |first3=L. |last4=Giardina |first4=I. |last5=Grigera |first5=T. S. |last6=Jelić |first6=A. |last7=Melillo |first7=S. |last8=Parisi |first8=L. |last9=Pohl |first9=O. |last10=Shen |first10=E. |last11=Viale |first11=M. |doi=10.1038/nphys3035 |bibcode=2014NatPh..10..691A |pmid=25264452 |pmc=4173114 |arxiv=1303.7097}}</ref>

Light behaves like a superfluid in various applications such as Poisson's Spot. As the liquid helium shown above, light will travel along the surface of an obstacle before continuing along its trajectory. Since light is not affected by local gravity its "level" becomes its own trajectory and velocity. Another example is how a beam of light travels through the hole of an aperture and along its backside before diffraction.

== See also ==
* [[Physics:Boojum (superfluidity)|Boojum (superfluidity)]]
* [[Physics:Condensed matter physics|Condensed matter physics]]
* [[Physics:Macroscopic quantum phenomena|Macroscopic quantum phenomena]]
* [[Physics:Quantum hydrodynamics|Quantum hydrodynamics]]
* [[Physics:Slow light|Slow light]]
* [[Physics:Superconductivity|Superconductivity]]
* [[Physics:Supersolid|Supersolid]]

== References ==
{{reflist}}

== Further reading ==
* {{Cite book|first=Isaac M.|last=Khalatnikov|title=An introduction to the theory of superfluidity|publisher=CRC Press |date=2018 |isbn=978-0-42-997144-0 |url=https://books.google.com/books?id=QXFQDwAAQBAJ}}
* {{Cite book|first=James F.|last=Annett |date=2005 |title=Superconductivity, superfluids, and condensates |publisher=Oxford Univ. Press |location=Oxford |isbn=978-0-19-850756-7 |url=https://books.google.com/books?id=WZcXmBrZIc8C}}
* {{Cite book|first=Tony|last=Guénault|title=Basic superfluids|publisher=Taylor & Francis|location=London |date=2003|isbn=0-7484-0891-6 |url=https://books.google.com/books?id=zTRAicK9G64C}}
* Svistunov, B. V., Babaev E. S., Prokof'ev N. V. [https://www.crcpress.com/Superfluid-States-of-Matter/Svistunov-Babaev-Prokofev/p/book/9781439802755 ''Superfluid States of Matter'']
* {{Cite book|first=Grigory E.|last=Volovik|title=The Universe in a helium droplet |series=Int. Ser. Monogr. Phys. |volume=117 |date=2003 |pages=1–507 |isbn=978-0-19-850782-6 |url=https://books.google.com/books?id=0pEUDAAAQBAJ}}

== External links ==
* Video: [http://www.alfredleitner.com/p/liquid-helium.html Demonstration of superfluid helium] (Alfred Leitner, 1963, 38 min.)
* [https://physicsworld.com/a/superfluidity-seen-in-a-2d-fermi-gas/ Superfluidity seen in a 2d fermi gas] recent 2021 observation relevant for [[Physics:Cuprate superconductor|Cuprate superconductors]]

{{States of matter}}
{{emerging technologies|topics=yes|robotics=yes|manufacture=yes|materials=yes}}


[[Category:Superfluidity]]

[[Category:Physical phenomena]]
[[Category:Fluid dynamics]]
[[Category:Liquid helium]]
[[Category:Phases of matter]]

{{Sourceattribution|Superfluidity}}