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| In [[physics]], '''Ginzburg–Landau theory''', named after [[Vitaly Lazarevich Ginzburg]] and [[Lev Landau]], is a mathematical theory used to describe [[superconductivity]]. In its initial form, it was postulated as a phenomenological model which could describe [[type-I superconductor]]s without examining their microscopic properties. Later, a version of Ginzburg–Landau theory was derived from the [[Bardeen-Cooper-Schrieffer]] microscopic theory by [[Lev Gor'kov]], thus showing that it also appears in some limit of microscopic theory and giving microscopic interpretation of all its parameters.
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| ==Introduction==
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| Based on [[Lev Davidovich Landau|Landau]]'s previously-established theory of second-order [[phase transition]]s, [[Vitaly Ginzburg|Ginzburg]] and [[Lev Landau|Landau]] argued that the [[Thermodynamic free energy|free energy]], ''F'', of a superconductor near the superconducting transition can be expressed in terms of a [[complex number|complex]] [[order parameter]] field, ''ψ'', which is nonzero below a phase transition into a superconducting state and is related to the density of the superconducting component, although no direct interpretation of this parameter was given in the original paper. Assuming smallness of ''|ψ|'' and smallness of its gradients, the free energy has the form of a field theory.
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| :<math> F = F_n + \alpha |\psi|^2 + \frac{\beta}{2} |\psi|^4 + \frac{1}{2m} \left| \left(-i\hbar\nabla - 2e\mathbf{A} \right) \psi \right|^2 + \frac{|\mathbf{B}|^2}{2\mu_0} </math>
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| where ''F<sub>n</sub>'' is the free energy in the normal phase, ''α'' and ''β'' in the initial argument were treated as phenomenological parameters, ''m'' is an effective mass, ''e'' is the charge of an electron, '''A''' is the [[magnetic potential#Magnetic vector potential|magnetic vector potential]], and <math>\mathbf{B}=\nabla \times \mathbf{A}</math> ('''B'''=curl('''A''')) is the magnetic field. By minimizing the free energy with respect to fluctuations in the order parameter and the vector potential, one arrives at the '''Ginzburg–Landau equations'''
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| :<math> \alpha \psi + \beta |\psi|^2 \psi + \frac{1}{2m} \left(-i\hbar\nabla - 2e\mathbf{A} \right)^2 \psi = 0 </math>
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| :<math> \mathbf{j} = \frac{2e}{m} \mathrm{Re} \left\{ \psi^* \left(-i\hbar\nabla - 2e \mathbf{A} \right) \psi \right\} </math>
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| where '''j''' denotes the dissipation-less electrical current density and ''Re'' the ''real part''. The first equation — which bears some similarities to the time-independent [[Schrödinger equation]], but is principally different due to a nonlinear term — determines the order parameter, ''ψ''. The second equation then provides the superconducting current.
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| ==Simple interpretation==
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| Consider a homogeneous superconductor where there is no superconducting current and the equation for ψ simplifies to:
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| :<math> \alpha \psi + \beta |\psi|^2 \psi = 0. \,</math>
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| This equation has a trivial solution: ψ = 0. This corresponds to the normal state of the superconductor, that is for temperatures, above the superconducting transition temperature, ''T>T''<sub>''c''</sub>.
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| Below the superconducting transition temperature, the above equation is expected to have a non-trivial solution (that is ψ ≠ 0). Under this assumption the equation above can be rearranged into:
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| :<math> |\psi|^2 = - \frac{\alpha} {\beta}.</math>
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| When the right hand side of this equation is positive, there is a nonzero solution for ψ (remember that the magnitude of a complex number can be positive or zero). This can be achieved by assuming the following temperature dependence of α: α(''T'') = α<sub>0</sub> (''T'' - ''T''<sub>''c''</sub>) with α<sub>0</sub> / β > 0: | |
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| *Above the superconducting transition temperature, ''T'' > ''T''<sub>''c''</sub>, the expression α(''T'') / β is positive and the right hand side of the equation above is negative. The magnitude of a complex number must be a non-negative number, so only ψ = 0 solves the Ginzburg–Landau equation.
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| *Below the superconducting transition temperature, ''T'' < ''T''<sub>''c''</sub>, the right hand side of the equation above is positive and there is a non-trivial solution for ψ. Furthermore
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| :<math> |\psi|^2 = - \frac{\alpha_{0} (T - T_{c})} {\beta},</math>
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| that is ψ approaches zero as ''T'' gets closer to ''T''<sub>''c''</sub> from below. Such a behaviour is typical for a second order phase transition. | |
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| In Ginzburg–Landau theory the electrons that contribute to superconductivity were proposed to form a [[superfluid]].<ref name=Ginzburg2004>{{cite journal |author=Ginzburg VL |title=On superconductivity and superfluidity (what I have and have not managed to do), as well as on the 'physical minimum' at the beginning of the 21 st century |journal=Chemphyschem. |volume=5 |issue=7 |month=July |pages=930–945 |year=2004 |pmid=15298379 |doi=10.1002/cphc.200400182 }}</ref> In this interpretation, |''ψ''|<sup>2</sup> indicates the fraction of electrons that has condensed into a superfluid.<ref name=Ginzburg2004/>
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| {{Gallery
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| |width=250
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| |height=200
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| |align=center
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| |File:Ginzburg Landau equation animation1.gif|Ginzburg Landau equation animation1
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| |File:Ginzburg Landau equation animation2.gif|Ginzburg Landau equation animation2
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| |File:Ginzburg Landau equation animation3.gif|Ginzburg Landau equation animation3
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| |File:Ginzburg Landau equation animation4.gif|Ginzburg Landau equation animation4
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| }}
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| ==Coherence length and penetration depth==
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| The Ginzburg–Landau equations predicted two new characteristic lengths in a superconductor which was termed '''[[Superconducting coherence length|coherence length]]''', ''ξ''. For ''T'' > ''T<sub>c</sub>'' (normal phase), it is given by
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| :<math> \xi = \sqrt{\frac{\hbar^2}{2 m |\alpha|}}. </math> | |
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| while for ''T'' < ''T<sub>c</sub>'' (superconducting phase), where it is more relevant, it is given by
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| :<math> \xi = \sqrt{\frac{\hbar^2}{4 m |\alpha|}}. </math>
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| It sets the exponential law according to which small perturbations of density of superconducting electrons
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| recover their equilibrium value ''ψ<sub>0</sub>''. Thus this theory proposed it characterized
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| all superconductors by two length scales. The second one is the '''penetration depth''', ''λ''.
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| It was previously introduced by the London brothers in their [[London theory]].
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| Expressed in terms of the parameters of Ginzburg-Landau model it is
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| :<math> \lambda = \sqrt{\frac{m}{4 \mu_0 e^2 \psi_0^2}}, </math>
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| where ''ψ<sub>0</sub>'' is the equilibrium value of the order parameter in the absence of an electromagnetic field. The penetration depth sets the exponential law according to which an external magnetic field decays inside the superconductor.
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| The ratio ''κ'' = ''λ/ξ'' is known as the '''Ginzburg–Landau parameter'''. It has been shown that [[Type I superconductor]]s are those with 0 < ''κ'' < 1/√2, and [[Type II superconductor]]s those with ''κ'' > 1/√2.
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| The exponential decay of the magnetic field is equivalent with the [[Higgs mechanism]] in high-energy physics. | |
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| ==Fluctuations in the Ginzburg-Landau model==
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| Taking into account fluctuations. For Type II superconductors, the [[phase transition]] from the normal state is of second order, as demonstrated by Dasgupta and Halperin. While for Type I superconductors it is of first order
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| as demonstrated by Halperin, Lubensky and Ma.
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| ==Classification of superconductors based on Ginzburg-Landau theory==
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| In the original paper Ginzburg and Landau observed the existence of two types of superconductors depending
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| on the energy of the interface between the normal and superconducting states.
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| The most important finding from Ginzburg–Landau theory was made by [[Alexei Alexeyevich Abrikosov|Alexei Abrikosov]] in 1957.
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| He used Ginzburg–Landau theory to explain experiments on superconducting alloys and thin films. He found that in a type-II superconductor in a high magnetic field – the field penetrates in the form of hexagonal lattice of quantized tubes of flux.
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| ==Landau–Ginzburg theories in string theory==
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| In [[particle physics]], any [[quantum field theory]] with a unique classical [[vacuum state]] and a [[potential energy]] with a [[degenerate critical point]] is called a Landau–Ginzburg theory. The generalization to N=(2,2) [[supersymmetry|supersymmetric theories]] in 2 spacetime dimensions was proposed by [[Cumrun Vafa]] and [[Nicholas Warner (physicist)|Nicholas Warner]] in the November 1988 article [http://www.slac.stanford.edu/spires/find/hep/www?j=PHLTA,B218,51 Catastrophes and the Classification of Conformal Theories], in this generalization one imposes that the [[superpotential]] possess a degenerate critical point. The same month, together with [[Brian Greene]] they argued that these theories are related by a [[renormalization group flow]] to [[sigma model]]s on [[Calabi–Yau manifold]]s in the paper [http://www.slac.stanford.edu/spires/find/hep/www?j=NUPHA,B324,371 Calabi–Yau Manifolds and Renormalization Group Flows]. In his 1993 paper [http://www.slac.stanford.edu/spires/find/hep/www?eprint=hep-th/9301042 Phases of N=2 theories in two-dimensions], [[Edward Witten]] argued that Landau–Ginzburg theories and sigma models on Calabi–Yau manifolds are different phases of the same theory. These models were later used to describe the low energy dynamics of 4 dimensional gauge theories with monopoles as well as brane constructions.{{harvtxt|Gaiotto|Gukov|Seiberg|2013}}
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| ==See also==
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| * [[Gross–Pitaevskii equation]]
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| * [[Landau theory]]
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| * [[Reaction–diffusion]] systems
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| ==References==
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| {{reflist}}
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| === Papers ===
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| * V.L. Ginzburg and L.D. Landau, ''Zh. Eksp. Teor. Fiz.'' '''20''', 1064 (1950). English translation in: L. D. Landau, Collected papers (Oxford: Pergamon Press, 1965) p. 546
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| * A.A. Abrikosov, ''Zh. Eksp. Teor. Fiz.'' '''32''', 1442 (1957) (English translation: ''Sov. Phys. JETP'' '''5''' 1174 (1957)].) Abrikosov's original paper on vortex structure of [[Type-II superconductor]]s derived as a solution of G–L equations for κ > 1/√2
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| * L.P. Gor'kov, ''Sov. Phys. JETP'' '''36''', 1364 (1959)
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| * A.A. Abrikosov's 2003 Nobel lecture: [http://nobelprize.org/nobel_prizes/physics/laureates/2003/abrikosov-lecture.pdf pdf file] or [http://nobelprize.org/nobel_prizes/physics/laureates/2003/abrikosov-lecture.html video]
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| * V.L. Ginzburg's 2003 Nobel Lecture: [http://nobelprize.org/nobel_prizes/physics/laureates/2003/ginzburg-lecture.pdf pdf file] or [http://nobelprize.org/nobel_prizes/physics/laureates/2003/ginzburg-lecture.html video]
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| *{{Citation | last1=Gaiotto | first1=David | last2=Gukov | first2=Sergei | last3=Seiberg | first3=Nathan | author1-link=David Gaiotto | author2-link=Sergei Gukov | author3-link=Nathan Seiberg | title=Surface Defects and Resolvents | url=http://arxiv.org/pdf/1307.2578v1.pdf | year=2013 | journal=[[Journal of High Energy Physics]] }}
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| ===Books===
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| * D. Saint-James, G. Sarma and E. J. Thomas, ''Type II Superconductivity'' Pergamon (Oxford 1969)
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| * M. Tinkham, ''Introduction to Superconductivity'', McGraw–Hill (New York 1996)
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| * [[Pierre-Gilles de Gennes|de Gennes, P.G.]], ''Superconductivity of Metals and Alloys'', Perseus Books, 2nd Revised Edition (1995), ISBN 0-201-40842-2 (this book is heavily based on G–L theory)
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| * [[Hagen Kleinert]], ''Gauge Fields in Condensed Matter'', Vol. I [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]'')
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| {{DEFAULTSORT:Ginzburg-Landau theory}}
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| [[Category:Superconductivity]]
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