View source for Kondo effect

{{short description|Physical phenomenon due to impurities}}
[[Image:Classickondo.png|thumb|320px|Kondo effect: How gold with a small amount of what were probably iron impurities behaves at low temperatures]]
[[File:Jun Kondo.jpg|200px|thumb|[[Jun Kondo]]]]
In [[physics]], the '''Kondo effect''' describes the scattering of [[conduction electrons]] in a metal due to [[Magnetic impurity|magnetic impurities]], resulting in a characteristic change i.e. a minimum in [[electrical resistivity]] with temperature.<ref>{{Cite journal |first=Alex C |last=Hewson |author2=Jun Kondo  |date=2009 |title=Kondo effect |journal=Scholarpedia |volume=4 |number=3 |page=7529 |doi=10.4249/scholarpedia.7529 |bibcode=2009SchpJ...4.7529H |author2-link=Jun Kondo |doi-access=free }}</ref>
The cause of the effect was first explained by [[Jun Kondo]], who applied third-order [[perturbation theory]] to the problem to account for scattering of [[Atomic orbital|s-orbital]] conduction electrons off d-orbital electrons localized at impurities ([[Kondo model]]). Kondo's calculation predicted that the scattering rate and the resulting part of the resistivity should increase logarithmically as the temperature approaches 0 K.<ref>{{cite journal|doi=10.1143/PTP.32.37|title=Resistance Minimum in Dilute Magnetic Alloys|date=1964|last1=Kondo|first1=Jun|journal=Progress of Theoretical Physics|volume=32|issue=1|pages=37–49|bibcode = 1964PThPh..32...37K |doi-access=free}}</ref> Experiments in the 1960s by [[Myriam Sarachik]] at [[Bell Laboratories]] provided the first data that confirmed the Kondo effect.<ref>{{Cite news|last=Chang|first=Kenneth|date=August 31, 2020|title=Myriam Sarachik Never Gave Up on Physics|language=en-US|work=The New York Times|url=https://www.nytimes.com/2020/08/31/science/myriam-sarachik-physics.html|access-date=October 13, 2021|issn=0362-4331|archive-date=August 31, 2020|archive-url=https://web.archive.org/web/20200831164630/https://www.nytimes.com/2020/08/31/science/myriam-sarachik-physics.html|url-status=live}}</ref> Extended to a lattice of ''magnetic impurities'', the Kondo effect likely explains the formation of [[heavy fermion|''heavy fermions'']] and [[Kondo insulator|''Kondo insulators'']] in intermetallic compounds, especially those involving rare earth elements such as [[cerium]], [[praseodymium]], and [[ytterbium]], and actinide elements such as [[uranium]]. The Kondo effect has also been observed in [[quantum dot]] systems.

==Theory==
The dependence of the resistivity <math>\rho</math> on temperature <math>T</math>, including the Kondo effect, is written as

:<math>\rho(T) = \rho_0 + aT^2 + c_m \ln\frac{\mu}{T} + bT^5,</math>

where <math>\rho_0</math> is the residual resistivity, the term <math>aT^2</math> shows the contribution from the [[Fermi liquid]] properties, and the term <math>bT^5</math> is from the lattice vibrations: <math>a</math>, <math>b</math>, <math>c_m</math> and <math>\mu</math> are constants independent of temperature. Jun Kondo derived the third term with logarithmic dependence on temperature and the experimentally observed concentration dependence.

==Background==
Kondo's solution was derived using [[perturbation theory]] resulting in a divergence as the temperature approaches 0 K, but later methods used non-perturbative techniques to refine his result. These improvements produced a finite resistivity but retained the feature of a resistance minimum at a non-zero temperature. One defines the '''Kondo temperature''' as the energy scale limiting the validity of the Kondo results. The [[Anderson impurity model]] and accompanying [[Kenneth G. Wilson|Wilsonian]] [[renormalization]] theory were an important contribution to understanding the underlying physics of the problem.<ref>{{cite journal|doi=10.1103/PhysRev.124.41|url=http://cos.cumt.edu.cn/jpkc/dxwl/zl/zl1/Physical%20Review%20Classics/condensed/061.pdf|title=Localized Magnetic States in Metals|date=1961|last1=Anderson|first1=P.|journal=Physical Review|volume=124|issue=1|pages=41–53|bibcode=1961PhRv..124...41A}}{{Dead link|date=May 2023 |bot=InternetArchiveBot |fix-attempted=yes }}</ref> Based on the [[Schrieffer–Wolff transformation]], it was shown that the Kondo model lies in the strong coupling regime of the Anderson impurity model. The Schrieffer–Wolff transformation<ref>{{cite journal|last=Schrieffer, J.R.|author2=Wolff, P.A. |title=Relation between the Anderson and Kondo Hamiltonians|journal=Physical Review|date=September 1966|volume=149|issue=2|pages=491–492|doi=10.1103/PhysRev.149.491|bibcode = 1966PhRv..149..491S |s2cid=55838235 }}</ref> projects out the high energy charge excitations in the Anderson impurity model, obtaining the Kondo model as an effective Hamiltonian.

[[Image:kscheme.jpg|thumb|320px|Schematic of the weakly coupled high temperature situation in which the magnetic moments of conduction electrons in the metal host pass by the impurity magnetic moment at speeds of v<sub>F</sub>, the Fermi velocity, experiencing only a mild antiferromagnetic correlation in the vicinity of the impurity. In contrast, as the temperature tends to zero the impurity magnetic moment and one conduction electron moment bind very strongly to form an overall non-magnetic state.]]

The Kondo effect can be considered as an example of [[asymptotic freedom]], i.e. a situation where the coupling becomes non-perturbatively strong at low temperatures and low energies. In the Kondo problem, the coupling refers to the interaction between the localized magnetic impurities and the itinerant electrons.

== Examples ==
Extended to a lattice of magnetic ions, the Kondo effect likely explains the formation of [[heavy fermion]]s and [[Kondo insulator]]s in intermetallic compounds, especially those involving rare earth elements such as [[cerium]], [[praseodymium]], and [[ytterbium]], and actinide elements such as [[uranium]]. In [[heavy fermion]] materials, the non-perturbative growth of the interaction leads to quasi-electrons with masses up to thousands of times the free electron mass, i.e., the electrons are dramatically slowed by the interactions. In a number of instances they are [[superconductors]]. It is believed that a manifestation of the Kondo effect is necessary for understanding the unusual metallic delta-phase of [[plutonium]].{{Citation needed|date=April 2020}}

The Kondo effect has been observed in [[quantum dot]] systems.<ref>{{cite journal|doi=10.1126/science.281.5376.540|pmid=9677192|first=Sara M.|last=Cronenwett|date=1998 |title=A Tunable Kondo Effect in Quantum Dots|journal=Science|volume=281|number=5376|pages=540–544|arxiv = cond-mat/9804211 |bibcode = 1998Sci...281..540C |s2cid=5139144}}</ref><ref>{{Cite journal|url=http://kouwenhovenlab.tudelft.nl/wp-content/uploads/2011/10/97-revival-of-the-kondo.pdf|title=Revival of the Kondo}}</ref> In such systems, a quantum dot with at least one unpaired electron behaves as a magnetic impurity, and when the dot is coupled to a metallic conduction band, the conduction electrons can scatter off the dot. This is completely analogous to the more traditional case of a magnetic impurity in a metal.

Band-structure hybridization and flat band topology in Kondo insulators have been imaged in [[angle-resolved photoemission spectroscopy]] experiments.<ref>{{Cite journal|last1=Neupane|first1=Madhab|last2=Alidoust|first2=Nasser|last3=Belopolski|first3=Ilya|last4=Bian|first4=Guang|last5=Xu|first5=Su-Yang|last6=Kim|first6=Dae-Jeong|last7=Shibayev|first7=Pavel P.|last8=Sanchez|first8=Daniel S.|last9=Zheng|first9=Hao|last10=Chang|first10=Tay-Rong|last11=Jeng|first11=Horng-Tay|display-authors=etal|date=2015-09-18|title=Fermi surface topology and hot spot distribution in the Kondo lattice system CeB<sub>6</sub>|journal=Physical Review B|volume=92|issue=10|pages=104420|doi=10.1103/PhysRevB.92.104420|arxiv=1411.0302|bibcode=2015PhRvB..92j4420N|doi-access=free}}</ref><ref>{{Cite journal|last1=Neupane|first1=M.|last2=Alidoust|first2=N.|last3=Xu|first3=S.-Y.|last4=Kondo|first4=T.|last5=Ishida|first5=Y.|last6=Kim|first6=D. J.|last7=Liu|first7=Chang|last8=Belopolski|first8=I.|last9=Jo|first9=Y. J.|last10=Chang|first10=T.-R.|last11=Jeng|first11=H.-T.|date=2013|title=Surface electronic structure of the topological Kondo-insulator candidate correlated electron system SmB<sub>6</sub>|journal=Nature Communications|language=en|volume=4|issue=1|page=2991|doi=10.1038/ncomms3991|pmid=24346502|arxiv=1312.1979|bibcode=2013NatCo...4.2991N|issn=2041-1723|doi-access=free}}</ref><ref>{{Citation|last1=Hasan|first1=M. Zahid|title=Topological Insulators, Topological Dirac semimetals, Topological Crystalline Insulators, and Topological Kondo Insulators|date=2015|url=https://onlinelibrary.wiley.com/doi/abs/10.1002/9783527681594.ch4|work=Topological Insulators|pages=55–100|publisher=John Wiley & Sons, Ltd.|language=en|doi=10.1002/9783527681594.ch4|isbn=978-3-527-68159-4|access-date=2020-04-26|last2=Xu|first2=Su-Yang|last3=Neupane|first3=Madhab}}</ref>

In 2012, Beri and Cooper proposed a topological Kondo effect could be found with [[Majorana fermion]]s,<ref>{{Cite journal|last1=Béri|first1=B.|last2=Cooper|first2=N. R.|year=2012|title=Topological Kondo Effect with Majorana Fermions|journal=Physical Review Letters|volume=109|issue=15|pages=156803|doi=10.1103/PhysRevLett.109.156803|pmid=23102351|arxiv=1206.2224|bibcode=2012PhRvL.109o6803B|s2cid=45712589}}</ref> while it has been shown that [[Quantum simulator|quantum simulations]] with [[ultracold atom]]s may also demonstrate the effect.<ref>{{Cite journal|last1=Buccheri|first1=F.|last2=Bruce|first2=G. D.|last3=Trombettoni|first3=A.|last4=Cassettari|first4=D.|last5=Babujian|first5=H.|last6=Korepin|first6=V. E.|last7=Sodano|first7=P.|date=2016-01-01|title=Holographic optical traps for atom-based topological Kondo devices|journal=New Journal of Physics|language=en|volume=18|issue=7|pages=075012|doi=10.1088/1367-2630/18/7/075012|issn=1367-2630|arxiv=1511.06574|bibcode=2016NJPh...18g5012B|s2cid=118610269}}</ref>

In 2017, teams from the Vienna University of Technology and Rice University conducted experiments into the development of new materials made from the metals cerium, bismuth and palladium in specific combinations and theoretical work experimenting with models of such structures, respectively. The results of the experiments were published in December 2017<ref>{{Cite journal|last1=Dzsaber|first1=S.|last2=Prochaska|first2=L.|last3=Sidorenko|first3=A.|last4=Eguchi|first4=G.|last5=Svagera|first5=R.|last6=Waas|first6=M.|last7=Prokofiev|first7=A.|last8=Si|first8=Q.|last9=Paschen|first9=S.|date=2017-06-16|title=Kondo Insulator to Semimetal Transformation Tuned by Spin-Orbit Coupling|journal=Physical Review Letters|language=en|volume=118|issue=24|pages=246601|doi=10.1103/PhysRevLett.118.246601|pmid=28665644|arxiv=1612.03972|bibcode=2017PhRvL.118x6601D|issn=0031-9007|doi-access=free}}</ref> and, together with the theoretical work,<ref>{{Cite journal|last1=Lai|first1=H.H.|last2=Grefe|first2=S.E.|last3=Paschen|first3=S.|last4=Si|first4=Q.|year=2012|title=Weyl–Kondo semimetal in heavy-fermion systems|journal=Proceedings of the National Academy of Sciences of the United States of America |volume=115|issue=1|pages=93–97|doi=10.1073/pnas.1715851115|pmid=29255021|pmc=5776817|arxiv=1206.2224|bibcode=2018PNAS..115...93L|doi-access=free}}</ref> lead to the discovery of a new state,<ref>Gabbatiss, J. (2017) [https://www.independent.co.uk/news/science/quantum-material-physics-weyl-kondo-semimetal-vienna-university-technology-a8121141.html "Scientists discover entirely new material that cannot be explained by classical physics"], ''The Independent''</ref> a correlation-driven [[Weyl semimetal]]. The team dubbed this new [[quantum material]] Weyl-Kondo [[semimetal]].

==References==
{{Reflist}}

==External links==
*[http://journals.jps.jp/toc/jpsj/2005/74/1#Special Kondo Effect - 40 Years after the Discovery] - special issue of the Journal of the Physical Society of Japan
*''The Kondo Problem to Heavy Fermions'' - Monograph on the Kondo effect by A.C. Hewson ({{ISBN|0-521-59947-4}})
*''Exotic Kondo Effects in Metals'' - Monograph on newer versions of the Kondo effect in non-magnetic contexts especially ({{ISBN|0-7484-0889-4}})
*[http://www.nature.com/nature/journal/v410/n6830/abs/410793a0.html ''Correlated electrons in δ-plutonium within a dynamical mean-field picture''], Nature '''410''', 793 (2001). Nature article exploring the links of the Kondo effect and plutonium

{{Condensed matter physics topics}}
{{Quantum field theories}}

[[Category:Electrical resistance and conductance]]
[[Category:Correlated electrons]]
[[Category:Electric and magnetic fields in matter]]
[[Category:Physical phenomena]]