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| In [[condensed matter physics]], '''Anderson localization''', also known as '''strong localization''', is the absence of diffusion of waves in a ''disordered'' medium. This phenomenon is named after the American physicist [[P. W. Anderson]], who was the first one to suggest the possibility of electron localization inside a semiconductor, provided that the degree of [[Randomness#In_the_physical_sciences|randomness]] of the [[impurities]] or [[crystallographic defect|defects]] is sufficiently large.<ref name=a58>{{ cite journal | last = Anderson | first = P. W. | authorlink = | coauthors = | year = 1958 | month = | title = Absence of Diffusion in Certain Random Lattices | journal = [[Physical Review|Phys. Rev.]] | volume = 109 | issue = 5| pages = 1492–1505 | doi = 10.1103/PhysRev.109.1492 | url = | accessdate = | quote = |bibcode = 1958PhRv..109.1492A }}</ref>
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| Anderson localization is a general wave phenomenon that applies to the transport of electromagnetic waves, acoustic waves, quantum waves, spin waves, etc. This phenomenon is to be distinguished from [[weak localization]], which is the precursor effect of Anderson localization (see below), and from [[Mott transition|Mott localization]], named after Sir [[Nevill Mott]], where the transition from metallic to insulating behaviour is ''not'' due to disorder, but to a strong mutual [[Coulomb repulsion]] of electrons.
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| ==Introduction==
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| In the original '''Anderson tight-binding model''', the evolution of the [[wave function]] ''ψ'' on the ''d''-dimensional lattice '''Z'''<sup>''d''</sup> is given by the [[Schrödinger equation]]
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| :<math> i \hbar \dot{\psi} = H \psi~, </math>
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| where the [[Hamiltonian (quantum mechanics)|Hamiltonian]] ''H'' is given by
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| :<math> (H \phi)(j) = E_j \phi(j) + \sum_{k \neq j} V(|k-j|) \phi(k)~, </math>
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| with ''E''<sub>''j''</sub> random and independent, and interaction ''V''(''r'') falling off as ''r''<sup>-2</sup> at infinity. For example, one may take ''E''<sub>''j''</sub> uniformly distributed in [−''W'', +''W''], and
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| :<math> V(|r|) = \begin{cases} 1, & |r| = 1 \\ 0, &\text{otherwise.} \end{cases} </math>
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| Starting with ''ψ''<sub>0</sub> localised at the origin, one is interested in how fast the probability distribution <math>|\psi|^2</math> diffuses. Anderson's analysis shows the following:
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| * if ''d'' is 1 or 2 and ''W'' is arbitrary, or if ''d'' ≥ 3 and ''W''/ħ is sufficiently large, then the probability distribution remains localized:
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| ::<math> \sum_{n \in \mathbb{Z}^d} |\psi(t,n)|^2 |n| \leq C </math>
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| :uniformly in ''t''. This phenomenon is called '''Anderson localization'''.
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| * if ''d'' ≥ 3 and ''W''/ħ is small,
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| :<math> \sum_{n \in \mathbb{Z}^d} |\psi(t,n)|^2 |n| \approx D \sqrt{t}~, </math>
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| :where ''D'' is the diffusion constant.
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| ==Analysis==
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| [[File:WF111-Anderson transition-multifractal.jpeg|thumbnail|Example of a multifractal electronic eigenstate at the Anderson localization transition in a system with 1367631 atoms.]]
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| The phenomenon of Anderson localization, particularly that of weak localization, finds its origin in the [[wave interference]] between multiple-scattering paths. In the strong scattering limit, the severe interferences can completely halt the waves inside the disordered medium.
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| For non-interacting electrons, a highly successful approach was put forward in 1979 by Abrahams ''et al.''<ref>{{cite journal|last1=Abrahams|first1=E.|last2=Anderson|first2=P.W.|last3=Licciardello|first3=D.C.|last4=Ramakrishnan|first4=T.V.|title=Scaling Theory of Localization: Absence of Quantum Diffusion in Two Dimensions|year=1979|journal=Phys. Rev. Lett.|volume=42|issue=10|pages=673–676|url=http://link.aps.org/doi/10.1103/PhysRevLett.42.673|doi=10.1103/PhysRevLett.42.673|bibcode = 1979PhRvL..42..673A }}</ref> This scaling hypothesis of localization suggests that a disorder-induced [[metal-insulator transition]] (MIT) exists for non-interacting electrons in three dimensions (3D) at zero magnetic field and in the absence of spin-orbit coupling. Much further work has subsequently supported these scaling arguments both analytically and numerically (Brandes ''et al.'', 2003; see Further Reading). In 1D and 2D, the same hypothesis shows that there are no extended states and thus no MIT. However, since 2 is the lower critical dimension of the localization problem, the 2D case is in a sense close to 3D: states are only marginally localized for weak disorder and a small magnetic field or [[spin-orbit coupling]] can lead to the existence of extended states and thus an MIT. Consequently, the localization lengths of a 2D system with potential-disorder can be quite large so that in numerical approaches one can always find a localization-delocalization transition when either decreasing system size for fixed disorder or increasing disorder for fixed system size.
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| Most numerical approaches to the localization problem use the standard tight-binding Anderson [[Hamiltonian (quantum mechanics)|Hamiltonian]] with onsite-potential disorder. Characteristics of the electronic [[eigenstate]]s are then investigated by studies of participation numbers obtained by exact diagonalization, multifractal properties, level statistics and many others. Especially fruitful is the [[transfer-matrix method]] (TMM) which allows a direct computation of the localization lengths and further validates the scaling hypothesis by a numerical proof of the existence of a one-parameter scaling function. Direct numerical solution of Maxwell equations to demonstrate Anderson localization of light has been implemented (Conti and Fratalocchi, 2008). The phenomenon has also been observed in numerical simulation of the non-relativistic Schrödinger equation.
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| ==Experimental evidence==
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| Two reports of Anderson localization of light in 3D random media exist up to date (Wiersma ''et al.'', 1997 and Storzer ''et al.'', 2006; see Further Reading), even though absorption complicates interpretation of experimental results (Scheffold ''et al.'', 1999). Anderson localization can also be observed in a perturbed periodic potential where the transverse localization of light is caused by random fluctuations on a photonic lattice. Experimental realizations of transverse localization were reported for a 2D lattice (Schwartz ''et al.'', 2007) and a 1D lattice (Lahini ''et al.'', 2006). It has also been observed by localization of a [[Bose–Einstein condensate]] in a 1D disordered optical potential (Billy ''et al.'', 2008; Roati ''et al.'', 2008). Anderson localization of elastic waves in a 3D disordered medium has been reported (Hu ''et al.'', 2008). The observation of the MIT has been reported in a 3D model with atomic matter waves (Chabé ''et al.'', 2008). [[Random laser]]s can operate using this phenomenon.
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| ==Notes==
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| {{Reflist}}
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| ==Further reading==
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| *{{ Cite document
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| | last1=Brandes
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| | first1=T.
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| | last2=Kettemann
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| | first2=S.
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| | lastauthoramp=yes
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| | title= The Anderson Transition and its Ramifications --- Localisation, Quantum Interference, and Interactions
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| | publisher=Springer Verlag
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| | place=Berlin
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| | year=2003
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| | postscript=<!-- Bot inserted parameter. Either remove it; or change its value to "." for the cite to end in a ".", as necessary. -->{{inconsistent citations}}
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| }}
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| *{{ cite journal | last = Wiersma | first = Diederik S. | authorlink = | coauthors = ''et al.'' | year = 1997 | month = | title = Localization of light in a disordered medium | journal = [[Nature (journal)|Nature]] | volume = 390 | issue = 6661 | pages = 671–673 | doi = 10.1038/37757 | url = | accessdate = | quote = |bibcode = 1997Natur.390..671W }}
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| *{{ cite journal | last = Störzer | first = Martin | authorlink = | coauthors = ''et al.'' | year = 2006 | month = | title = Observation of the critical regime near Anderson localization of light | journal = [[Physical Review Letters|Phys. Rev. Lett.]] | volume = 96 | issue = 6| pages = 063904 | doi = 10.1103/PhysRevLett.96.063904 |pmid=16605998 | url = | accessdate = | quote = | bibcode=2006PhRvL..96f3904S|arxiv = cond-mat/0511284 }}
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| *{{ cite journal | last = Scheffold | first = Frank | authorlink = | coauthors = ''et al.'' | year = 1999 | month = | title = Localization or classical diffusion of light? | journal = Nature | volume = 398 | issue = 6724| pages = 206–207 | doi = 10.1038/18347 | url = | accessdate = | quote = |bibcode = 1999Natur.398..206S }}
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| *{{ cite journal | last = Schwartz| first = T. | authorlink = | coauthors = ''et al.'' | year = 2007 | month = | title = Transport and Anderson Localization in disordered two-dimensional Photonic Lattices | journal = Nature | volume = 446| issue = 7131| pages = 52–55 | doi = 10.1038/nature05623 | url = | accessdate = | quote = | pmid = 17330037 |bibcode = 2007Natur.446...52S }}
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| *{{ cite journal | last = Lahini| first = Y. | authorlink = | coauthors = ''et al.'' | year = 2006 | month = | title = Direct Observation of Anderson Localized Modes and the Effect of Nonlinearity | journal = Photonic Metamaterials: From Random to Periodic (META), Grand Bahama Island, The Bahamas, June 5, 2006, Postdeadline Papers | volume = | issue = | pages = | doi = | url = http://www.opticsinfobase.org/abstract.cfm?URI=META-2006-ThC4 | accessdate = | quote = | pmid = | bibcode=}}
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| *{{ cite journal | last = Billy | first = Juliette | authorlink = | coauthors = ''et al.'' | year = 2008 | month = | title = Direct observation of Anderson localization of matter waves in a controlled disorder | journal = Nature | volume = 453 | issue = 7197 | pages = 891–894 | doi = 10.1038/nature07000 | url = | accessdate = | quote = | pmid = 18548065 |bibcode = 2008Natur.453..891B |arxiv = 0804.1621 }}
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| *{{ cite journal | last = Roati | first = Giacomo | authorlink = | coauthors = ''et al.'' | year = 2008 | month = | title = Anderson localization of a non-interacting Bose-Einstein condensate | journal = Nature | volume = 453 | issue = 7197 | pages = 895–898 | doi = 10.1038/nature07071 | url = | accessdate = | quote = | pmid = 18548066 |bibcode = 2008Natur.453..895R |arxiv = 0804.2609 }}
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| *{{ cite journal | last = Ludlam | first = J. J. | authorlink = | coauthors = ''et al.'' | year = 2005 | month = | title = Universal features of localized eigenstates in disordered systems | journal = Journal of Physics: Condensed Matter | volume = 17 | issue = 30| pages = L321–L327 | doi = 10.1088/0953-8984/17/30/L01 | url = | accessdate = | quote = |bibcode = 2005JPCM...17L.321L }}
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| *{{ cite journal | last = Conti | first = C | authorlink = | coauthors = A. Fratalocchi | year = 2008 | month = | title = Dynamic light diffusion, three-dimensional Anderson localization and lasing in inverted opals | journal = [[Nature Physics]] | volume = 4 | issue = 10| pages = 794–798 | doi = 10.1038/nphys1035 | url = | accessdate = | quote = |bibcode = 2008NatPh...4..794C |arxiv = 0802.3775 }}
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| *{{ cite journal | last = Hu | first = Hefei | authorlink = | coauthors = ''et al.'' | year = 2008 | month = | title = Localization of ultrasound in a three-dimensional elastic network | journal = Nature Physics | volume = 4| issue = 12| pages = 945| doi = 10.1038/nphys1101 | url = | accessdate = | quote = |bibcode = 2008NatPh...4..945H |arxiv = 0805.1502 }}
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| *{{ cite journal | last = Chabé| first = J. | authorlink = | coauthors = ''et al.'' | year = 2008 | month = | title = Experimental Observation of the Anderson Metal-Insulator Transition with Atomic Matter Waves | journal = Phys. Rev. Lett. | volume = 101| issue = 25| pages = 255702| doi = 10.1103/PhysRevLett.101.255702 | url = | accessdate = | quote = | pmid = 19113725 | bibcode=2008PhRvL.101y5702C|arxiv = 0709.4320 }}
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| ==External links==
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| *[http://ptonline.aip.org/journals/doc/PHTOAD-ft/vol_62/iss_8/24_1.shtml Fifty years of Anderson localization] ''Physics Today'', August 2009.
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| *[http://www2.warwick.ac.uk/fac/sci/csc/images/wf111.jpg Example of an electronic eigenstate at the MIT in a system with 1367631 atoms] Each cube indicates by its size the probability to find the electron at the given position. The color scale denotes the position of the cubes along the axis into the plane
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| *[http://www2.warwick.ac.uk/fac/sci/physics/research/theory/research/disqs/media Videos of multifractal electronic eigenstates at the MIT]
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| *[http://lpmmc.grenoble.cnrs.fr/spip.php?article408 Anderson localization of elastic waves]
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| *[http://www.opfocus.org/index.php?topic=story&v=1&s=1 Popular scientific article on the first experimental observation of Anderson localization in matter waves]
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| [[Category:Mesoscopic physics]]
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| [[Category:Condensed matter physics]]
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Tennis Coach Archie from Bois-des-Filion, has interests for instance motorbikes, new launch property singapore and video games. Has recently finished a journey to Inner City and Harbour.
Check out my homepage; Www.irkstroika.ru