Meissner effect 1

Meissner effect

Diagram of the Meissner effect. Magnetic fieldlines, represented as arrows, are excluded from a

superconductor when it is below its criticaltemperature.

The Meissner effect is the expulsion of a magnetic field from asuperconductor during its transition to the superconducting state. TheGerman physicists Walther Meissner and Robert Ochsenfelddiscovered the phenomenon in 1933 by measuring the magnetic fielddistribution outside superconducting tin and lead samples.[1] Thesamples, in the presence of an applied magnetic field, were cooledbelow what is called their superconducting transition temperature.Below the transition temperature the samples cancelled nearly allmagnetic fields inside. They detected this effect only indirectly;because the magnetic flux is conserved by a superconductor, when theinterior field decreased the exterior field increased. The experimentdemonstrated for the first time that superconductors were more thanjust perfect conductors and provided a uniquely defining property ofthe superconducting state. Despite having been known for over 80years, no dynamical explanation of the Meissner effect exists withinthe conventional understanding of superconductivity.[2]

Explanation

A magnet levitating above a superconductorcooled by liquid nitrogen.

In a weak applied field, a superconductor "expels" nearly all magneticflux. It does this by setting up electric currents near its surface. Themagnetic field of these surface currents cancels the applied magneticfield within the bulk of the superconductor. As the field expulsion, orcancellation, does not change with time, the currents producing thiseffect (called persistent currents) do not decay with time. Therefore theconductivity can be thought of as infinite: a superconductor.Near the surface, within a distance called the London penetrationdepth, the magnetic field is not completely cancelled. Eachsuperconducting material has its own characteristic penetration depth.

Any perfect conductor will prevent any change to magnetic fluxpassing through its surface due to ordinary electromagnetic induction at zero resistance. The Meissner effect isdistinct from this: when an ordinary conductor is cooled so that it makes the transition to a superconducting state inthe presence of a constant applied magnetic field, the magnetic flux is expelled during the transition. This effectcannot be explained by infinite conductivity alone. Its explanation is more complex and was first given in theLondon equations by the brothers Fritz and Heinz London. It should thus be noted that the placement and subsequentlevitation of a magnet above an already superconducting material does not demonstrate the Meissner effect, while aninitially stationary magnet later being repelled by a superconductor as it is cooled through its critical temperaturedoes.

Meissner effect 2

Perfect diamagnetismSuperconductors in the Meissner state exhibit perfect diamagnetism, or superdiamagnetism, meaning that the totalmagnetic field is very close to zero deep inside them (many penetration depths from the surface). This means thattheir magnetic susceptibility, = −1. Diamagnetics are defined by the generation of a spontaneous magnetizationof a material which directly opposes the direction of an applied field. However, the fundamental origins ofdiamagnetism in superconductors and normal materials are very different. In normal materials diamagnetism arisesas a direct result of the orbital spin of electrons about the nuclei of an atom induced electromagnetically by theapplication of an applied field. In superconductors the illusion of perfect diamagnetism arises from persistentscreening currents which flow to oppose the applied field (the meissner effect); not solely the orbital spin.

ConsequencesThe discovery of the Meissner effect led to the phenomenological theory of superconductivity by Fritz and HeinzLondon in 1935. This theory explained resistanceless transport and the Meissner effect, and allowed the firsttheoretical predictions for superconductivity to be made. However, this theory only explained experimentalobservations—it did not allow the microscopic origins of the superconducting properties to be identified.Nevertheless, it became a requirement on all microscopic theories to be able to reproduce this effect. This was donesuccessfully by the BCS theory in 1957. However, both phenomenological Londons’ theory and microscopic BCSone describe the Meissner effect in its steady state only and cannot explain the transient stage when the supercurrentgrows from zero to its steady value. Indeed, under initial conditions of the Meissner effect, Lorentz force equals tozero, and there are no other electromotive forces in superconductor to accelerate the electrons. This fundamentalproblem of the conventional theory of the Meissner effect has been pointed out by J. E. Hirsch in The Lorentz forceand superconductivity.[3] He has also proposed the dynamical explanation of the Meissner effect in Spin Meissnereffect in superconductors and the origin of the Meissner effect.[4]

A tin cylinder—in a Dewar flask filled withliquid helium—has been placed between the

poles of an electromagnet. The magnetic fieldis about 8 milliteslas (80 G).

T=4.2 K, B=8 mT (80 G). Tin is in thenormally conducting state. The compass

needles indicate that magnetic fluxpermeates the cylinder.

The cylinder has beencooled from 4.2 K to1.6 K. The current inthe electromagnet has

been kept constant,but the tin becamesuperconducting at

about 3 K. Magneticflux has been expelledfrom the cylinder (the

Meissner effect).

Meissner effect 3

Paradigm for the Higgs mechanismThe Meissner effect of superconductivity serves as an important paradigm for the generation mechanism of a mass M(i.e. a reciprocal range, where h is Planck constant and c is speed of light) for a gauge field. Infact, this analogy is an abelian example for the Higgs mechanism, through which in high-energy physics the massesof the electroweak gauge particles, W± and Z are generated. The length is identical with "London's penetrationdepth" in the theory of superconductivity.

ObservationBefore the discovery of high-temperature superconductivity, observation of the Meissner effect was difficult,because the applied fields had to be relatively small (the measurements need to be made far from the phaseboundary). But with yttrium barium copper oxide, the effect can be demonstrated using liquid nitrogen. Permanentmagnets can be made to levitate.

References[1] Meissner, W.; R. Bochsenfeld (1912). "Ein neuer Effekt bei Eintritt der Supraleitfähigkeit" (http:/ / www. springerlink. com/ content/

l69w054091n24j14/ ?p=d517b9e40b344f9bb3fc19ee23a823b3& pi=4). Naturwissenschaften 21 (44): 787–788.Bibcode 1933NW.....21..787M. doi:10.1007/BF01504252. .

[2] . http:/ / iopscience. iop. org/ 0295-5075/ 81/ 6/ 67003/ fulltext/ .[3] J. E. Hirsch (2003). "The Lorentz force and superconductivity". Phys. Lett. A 315 (6): 474. arXiv:cond-mat/0305542.

Bibcode 2003PhLA..315..474H. doi:10.1016/S0375-9601(03)01107-1.[4] J. E. Hirsch (2008). "Spin Meissner effect in superconductors and the origin of the Meissner effect". Europhys. Lett. 81 (6): 67003.

Bibcode 2008EL.....8167003H. doi:10.1209/0295-5075/81/67003.

• Michael Tinkham (2004). Introduction to Superconductivity. Dover Books on Physics (2nd ed.).ISBN 9780486435039.. A good technical reference.

• Fritz Wolfgang London (1950). "Macroscopic Theory of Superconductivity". Superfluids. Structure of matterseries. 1. OCLC 257588418.. Revised 2nd edition, Dover (1960) ISBN 9780486600444. By the man whoexplained the Meissner effect. pp. 34–37 gives a technical discussion of the Meissner effect for a superconductingsphere.

• Wayne M. Saslow (2002). Electricity, Magnetism, and Light. Academic. ISBN 9780126194555.OCLC 51032778.. pp. 486–489 gives a simple mathematical discussion of the surface currents responsible for theMeissner effect, in the case of a long magnet levitated above a superconducting plane.

External links• Maglev Trains (http:/ / www. magnet. fsu. edu/ education/ tutorials/ slideshows/ maglev/ index. html) Audio

slideshow from the National High Magnetic Field Laboratory discusses magnetic levitation, the Meissner Effect,magnetic flux trapping and superconductivity.

• Meissner Effect (Science from scratch) (http:/ / www. youtube. com/ watch?v=44mVZdnR6Yc) Short video fromImperial College London about the Meissner effect and levitating trains of the future.

• Introduction to superconductivity (http:/ / www. alfredleitner. com/ superconductors. html) Video about Type 1Superconductors: R=0/Transition temperatures/B is a state variable/Meissner effect/Energy gap (Giaever)/BCSmodel.

• Meissner Effect (Hyperphysics) (http:/ / hyperphysics. phy-astr. gsu. edu/ hbase/ solids/ meis. html)• Possible root of the Meissner effect (http:/ / arxiv. org/ abs/ 1006. 2347)

Article Sources and Contributors 4

Article Sources and ContributorsMeissner effect  Source: http://en.wikipedia.org/w/index.php?oldid=477204688  Contributors: Agnistus, Aleitner, Alureiter, AnonNemoVoid, BailesB, BananaFiend, Bokbindaren, BryanDerksen, CapitalR, Choihei, ChristophDemmer, Circuitboardsushi, Coemgenus, Cyfal, DaGizza, Daggerstab, David R. Ingham, David.Monniaux, Diamonddavej, Discospinster, Dodecagon,Dsmurat, Ebear422, Eleider, Enno, EoGuy, Erasmusbee, Ewulp, Eynar, FlorianMarquardt, Friendofthefoot, Geelen, Gene Nygaard, Harp, Headbomb, HereToHelp, Iceman9985, Igodard,Jacquesbezuidenhout, Jaguar2k, Jernejl, Johnman239, Johnvonahlen, Jorgecarleitao, Jqt, Julesd, K Eliza Coyne, Kent Witham, Kthemank, Larryisgood, LeadSongDog, Louise Rill, Mayur, Mea,Meisam.fa, Michael Hardy, Milez, Mr Evil, Mxn, Nethershaw, Netscott, Nibios, Ninly, Nobody267, Ojigiri, Ost316, Oxymoron83, Perspicacite, Petiatil, Pgm944, Pieter Kuiper, Quarky2001,Qwerty Binary, Qwerty937, R0uge, RJFJR, RekishiEJ, Robma, RockMagnetist, Rorro, Saperaud, Sardanaphalus, Shantavira, Siddharth srinivasan, Sim man, Spinningspark, Sunilvarma.d,Suspekt, Svdmolen, Syndicate, Tetracube, The Thing That Should Not Be, Tim Starling, Tobias Bergemann, User071, Vanderdecken, Verdatum, Versus22, Vijayaraj83, Vze2wgsm1, WLU,WMSwiki, Waleswatcher, Why Not A Duck, Wikiborg, William Avery, Wricardoh, Xenocide321, YBCO, Zetawoof, Zomno, 137 anonymous edits

Image Sources, Licenses and ContributorsImage:EfektMeisnera.svg  Source: http://en.wikipedia.org/w/index.php?title=File:EfektMeisnera.svg  License: Public Domain  Contributors: Piotr Jaworski, PioM EN DE PL; POLAND/PoznańImage:Meissner effect p1390048.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Meissner_effect_p1390048.jpg  License: Creative Commons Attribution-ShareAlike 3.0 Unported Contributors: Mai-Linh DoanImage:Tin_4.2K_Electromagnet.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Tin_4.2K_Electromagnet.jpg  License: Public Domain  Contributors: [I, Alfred Leitner took thisphotograph as apart of my movie on Type I Superconductors]Image:Tin_80gauss_4.2K.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Tin_80gauss_4.2K.jpg  License: Public Domain  Contributors: I, AlfredLeitner, took this photograph aspart of my movie about Type I SuperconductivityImage:Tin_80gauss_1.6K.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Tin_80gauss_1.6K.jpg  License: Public Domain  Contributors: I, AlfredLeitner, took this photograph aspart of my movie about Type I Superconductivity

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