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| '''BCS theory''' is the first [[microscopic theory]] of [[superconductivity]] since its discovery in 1911. The theory describes superconductivity as a microscopic effect caused by a condensation of [[Cooper pair]]s into a [[boson]]-like state. The theory is also used in [[nuclear physics]] to describe the pairing interaction between [[nucleon]]s in an atomic [[Nucleus (atomic structure)|nucleus]]. It was proposed by [[John Bardeen]], [[Leon Neil Cooper]], and [[John Robert Schrieffer]] ("BCS") in 1957; they received the Nobel prize in physics for this theory in 1972.
| | 2006 will be the twenty fifth year of the 401k investment plan. Perhaps you have had more than one work in the last 25 years? If so, you then probably have several 401(k) strategy boating. <br><br>401(k) plans at the moment are over 25 years old. They looked a distinctive idea at first, but now just about every employer offers one. And Im sure I dont have to let you know that they"re a good way to save and earn money over the years. <br><br>The problem here is when you setup a 401k, you often diversify your program together with your boss. Demonstrably, you have to spend using the current options your company offers, which is good. Investing a little in the high risk, some in the risk, and some in the lower risk funds its typically the plan. You was a little more open on getting threat 20 years ago than you"re today. Perhaps now you"re a tad bit more conservative in your investment objectives. So you think you are diversified, right? <br><br>Not necessarily particularly if you"ve ten ideas with ten different employers. Remember when you set them up you tried to [http://Www.Sharkbayte.com/keyword/broaden broaden] every one. Well, twenty different strategies diversified exactly the same way means that your account is not really diversified at all. In case people desire to identify more on [http://www.xinxiseo.com/wells-fargo-401k-ideas-robbed-thousands-missing/ visit our site], there are many libraries you might think about pursuing. One employers average risk system may be another [http://Browse.deviantart.com/?q=employers+low employers low] risk plan. Your 401k 15 years ago where you committed to technology stocks was probably a top risk option. Now some of those hi-tech stocks would be the most conservative investments. <br><br>The only path to manage your numerous 401(k) ideas effortlessly is to combine them in to one program, under one investment portfolio and review it at the very least yearly. Among the advantages of strategies is they"re transferable. This unique [http://realsmartemail.com/all-you-need-to-learn-about-wholesale-sterling-silver-and-more/ advertisers] paper has a few salient aids for when to do it. The main thing is not actually to close a 401k and reinvest it, this can be a taxable event. So you can control your risk you can easily transfer your old 401k plans into a current or a new 401k. Get extra resources on this partner article directory - Click here: [http://www.imwwy.com/stylish-silver-earrings-for-anytime-and-constantly/ the infographic]. <br><br>That is one time when anything under one umbrella is the way to go..<br><br>If you loved this post and you would like to acquire far more info regarding [http://immenseintermis82.shutterfly.com group health insurance] kindly visit the web site. |
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| ==History==
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| Rapid progress in understanding superconductivity gained momentum in the mid-1950s. It began in the 1948 paper, "On the Problem of the Molecular Theory of Superconductivity"<ref>{{cite journal|last=London|first=F.|title=On the Problem of the Molecular Theory of Superconductivity|journal=Physical Review|date=September 1948|volume=74|issue=5|pages=562–573|doi=10.1103/PhysRev.74.562|accessdate=March 3, 2012|url=http://link.aps.org/doi/10.1103/PhysRev.74.562|bibcode = 1948PhRv...74..562L }}</ref> where [[Fritz London]] proposed that the [[Phenomenology (science)|phenomenological]] [[London equations]] may be consequences of the [[quantum coherence|coherence]] of a [[quantum state]]. In 1953, [[Brian Pippard]], motivated by penetration experiments, proposed that this would modify the London equations via a new scale parameter called the [[Superconducting coherence length|coherence length]]. John Bardeen then argued in the 1955 paper, "Theory of the Meissner Effect in Superconductors"<ref>{{cite journal|last=Bardeen|first=J.|title=Theory of the Meissner Effect in Superconductors|journal=Physical Review|date=March 1955|volume=97|issue=6|pages=1724–1725|doi=10.1103/PhysRev.97.1724|accessdate=May 3, 2012|bibcode = 1955PhRv...97.1724B }}</ref> that such a modification naturally occurs in a theory with an energy gap. The key ingredient was Leon Neil Cooper's calculation of the bound states of electrons subject to an attractive force in his 1956 paper, "Bound Electron Pairs in a Degenerate Fermi Gas".<ref>{{cite journal|last=Cooper|first=Leon|title=Bound Electron Pairs in a Degenerate Fermi Gas|journal=Physical Review|date=November 1956|volume=104|issue=4|pages=1189–1190|doi=10.1103/PhysRev.104.1189|accessdate=May 3, 2012|issn=0031899X|bibcode = 1956PhRv..104.1189C }}</ref>
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| In 1957 Bardeen and Cooper assembled these ingredients and constructed such a theory, the BCS theory, with Robert Schrieffer. The theory was first published in April 1957 in the letter, "Microscopic theory of superconductivity".<ref>{{cite journal|last=Bardeen|first=J.|coauthors=Cooper, L. N., Schrieffer, J. R.|title=Microscopic Theory of Superconductivity|journal=Physical Review|date=April 1957|volume=106|issue=1|pages=162–164|doi=10.1103/PhysRev.106.162|url=http://prola.aps.org/pdf/PR/v106/i1/p162_1|accessdate=May 3, 2012|bibcode = 1957PhRv..106..162B }}</ref> The demonstration that the phase transition is second order, that it reproduces the [[Meissner effect]] and the calculations of specific heats and penetration depths appeared in the December 1957 article, "Theory of superconductivity".<ref name=BCS_theory>{{cite journal|last=Bardeen|first=J.|coauthors=Cooper, L. N.; Schrieffer, J. R.|title=Theory of Superconductivity|journal=Physical Review|date=December 1957|volume=108|issue=5|pages=1175–1204|doi=10.1103/PhysRev.108.1175|url=http://prola.aps.org/pdf/PR/v108/i5/p1175_1|accessdate=May 3, 2012|bibcode = 1957PhRv..108.1175B }}</ref> They received the [[Nobel Prize in Physics]] in 1972 for this theory. The 1950 [[Landau-Ginzburg theory]] of superconductivity is not cited in either of the BCS papers.
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| In 1986, [[high-temperature superconductivity]] was discovered (i.e. superconductivity at temperatures considerably above the previous limit of about 30 [[Kelvin|K]]; up to about 130 K). It is believed that BCS theory alone cannot explain this phenomenon and that other effects are at play.<ref>{{cite journal|last=Mann|first=A.|title=High Temperature Superconductivity at 25: Still In Suspense|journal=Nature|date=July 2011|volume=475|doi=10.1038/475280a|url=http://www.nature.com/news/2011/110720/full/475280a.html|accessdate=November 18, 2012|pmid=21776057|bibcode = 2011Natur.475..280M }}</ref> These effects are still not yet fully understood; it is possible that they even control superconductivity at low temperatures for some materials.
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| ==Overview==
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| At sufficiently low temperatures, electrons near the [[Fermi surface]] become unstable against the formation of Cooper pairs. Cooper showed such binding will occur in the presence of an attractive potential, no matter how weak. In conventional superconductors, an attraction is generally attributed to an electron-lattice interaction. The BCS theory, however, requires only that the potential be attractive, regardless of its origin. In the BCS framework, superconductivity is a macroscopic effect which results from the condensation of Cooper pairs. These have some bosonic properties, while bosons, at sufficiently low temperature, can form a large [[Bose-Einstein condensate]]. Superconductivity was simultaneously explained by [[Nikolay Bogoliubov]], by means of the so-called [[Bogoliubov transformation]]s.
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| In many superconductors, the attractive interaction between electrons (necessary for pairing) is brought about indirectly by the interaction between the electrons and the vibrating crystal lattice (the [[phonon]]s). Roughly speaking the picture is the following:
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| :An electron moving through a conductor will attract nearby positive charges in the lattice. This deformation of the lattice causes another electron, with opposite spin, to move into the region of higher positive charge density. The two electrons then become correlated. Because there are a lot of such electron pairs in a superconductor, these pairs overlap very strongly and form a highly collective condensate. In this "condensed" state, the breaking of one pair will change the energy of the entire condensate - not just a single electron, or a single pair. Thus, the energy required to break any single pair is related to the energy required to break ''all'' of the pairs (or more than just two electrons). Because the pairing increases this energy barrier, kicks from oscillating atoms in the conductor (which are small at sufficiently low temperatures) are not enough to affect the condensate as a whole, or any individual "member pair" within the condensate. Thus the electrons stay paired together and resist all kicks, and the electron flow as a whole (the current through the superconductor) will not experience resistance. Thus, the collective behavior of the condensate is a crucial ingredient necessary for superconductivity.
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| ===More details===
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| BCS theory starts from the assumption that there is some attraction between electrons, which can overcome the [[Coulomb repulsion]]. In most materials (in low temperature superconductors), this attraction is brought about indirectly by the coupling of electrons to the [[crystal lattice]] (as explained above). However, the results of BCS theory do ''not'' depend on the origin of the attractive interaction. For instance, Cooper pairs have been observed in [[Ultracold atom|ultracold gases]] of [[Fermion]]s where a homogeneous [[magnetic field]] has been tuned to their [[Feshbach resonance]]. The original results of BCS (discussed below) described an [[Atomic orbital|s-wave]] superconducting state, which is the rule among low-temperature superconductors but is not realized in many unconventional superconductors such as the [[Atomic orbital|d-wave]] high-temperature superconductors.
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| Extensions of BCS theory exist to describe these other cases, although they are insufficient to completely describe the observed features of high-temperature superconductivity.
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| BCS is able to give an approximation for the quantum-mechanical many-body state of the
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| system of (attractively interacting) electrons inside the metal. This state is
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| now known as the BCS state. In the normal state of a metal, electrons move independently, whereas in the BCS state, they are bound into Cooper pairs by the attractive interaction.
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| The BCS formalism is based on the reduced potential for the electrons attraction.
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| Within this potential, a variational [[ansatz]] for the wave function is proposed. This ansatz
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| was later shown to be exact in the dense limit of pairs. Note that the continuous crossover between the dilute and dense regimes of attracting pairs of fermions is still an open problem, which now attracts a lot of attention within the field of ultracold gases.
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| === Underlying evidence ===
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| The hyperphysics website pages at [[Georgia State University]] summarize some key background to BCS theory as follows:<ref>http://hyperphysics.phy-astr.gsu.edu/hbase/solids/bcs.html</ref>
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| :* '''Evidence of a [[band gap]] at the Fermi level''' (described as "a key piece in the puzzle") - the existence of a critical temperature and critical magnetic field implied a band gap, and suggested a [[phase transition]], but single [[electron]]s are forbidden from condensing to the same energy level by the [[Pauli exclusion principle]]. The site comments that "a drastic change in conductivity demanded a drastic change in electron behavior". Conceivably, pairs of electrons might perhaps act like [[boson]]s instead, which are bound by [[Bose-Einstein statistics|different condensate rules]] and do not have the same limitation.
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| :* '''[[Isotope effect]] on the critical temperature, suggesting lattice interactions'''.
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| :* '''An [[exponential]] rise in [[heat capacity]] near the critical temperature for some superconductors''' - An exponential increase in heat capacity near the critical temperature also suggests an energy bandgap for the superconducting material. As superconducting [[vanadium]] is warmed toward its critical temperature, its heat capacity increases massively in a very few degrees; this suggests an energy gap being bridged by thermal energy.
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| :* '''The lessening of the measured energy gap towards the critical temperature''' - this suggests a type of situation where some kind of [[binding energy]] exists but it is gradually weakened as the critical temperature is approached. A binding energy suggests two or more particles or other entities that are bound together in the superconducting state. This helped to support the idea of bound particles - specifically electron pairs - and together with the above helped to paint a general picture of paired electrons and their lattice interactions.
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| ==Successes of the BCS theory==
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| BCS derived several important theoretical predictions that are independent of the details of the interaction, since the quantitative predictions mentioned below hold for any sufficiently weak attraction between the electrons and this last condition is fulfilled for many low temperature superconductors - the so-called weak-coupling case. These have been confirmed in numerous experiments:
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| * The electrons are bound into Cooper pairs, and these pairs are correlated due to the [[Pauli exclusion principle]] for the electrons, from which they are constructed. Therefore, in order to break a pair, one has to change energies of all other pairs. This means there is an energy gap for single-particle excitation, unlike in the normal metal (where the state of an electron can be changed by adding an arbitrarily small amount of energy). This energy gap is highest at low temperatures but vanishes at the transition temperature when superconductivity ceases to exist. The BCS theory gives an expression that shows how the gap grows with the strength of the attractive interaction and the (normal phase) single particle [[density of states]] at the [[Fermi level]]. Furthermore, it describes how the density of states is changed on entering the superconducting state, where there are no electronic states any more at the Fermi level. The energy gap is most directly observed in tunneling experiments<ref name="Giaever">Ivar Giaever - Nobel Lecture. Nobelprize.org. Retrieved 16 Dec 2010. http://nobelprize.org/nobel_prizes/physics/laureates/1973/giaever-lecture.html</ref> and in reflection of microwaves from superconductors.
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| * BCS theory predicts the dependence of the value of the energy gap E at temperature T on the critical temperature T<sub>c</sub>. The ratio between the value of the energy gap at zero temperature and the value of the superconducting transition temperature (expressed in energy units) takes the universal value of 3.5, independent of material. Near the critical temperature the relation asymptotes to
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| ::<math>E=3.52k_BT_c\sqrt{1-(T/T_c)}</math>
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| :which is of the form suggested the previous year by [[M. J. Buckingham]] in [http://prola.aps.org/abstract/PR/v101/i4/p1431_1 Very High Frequency Absorption in Superconductors] based on the fact that the superconducting phase transition is second order, that the superconducting phase has a mass gap and on Blevins, Gordy and Fairbank's experimental results the previous year on the absorption of millimeter waves by superconducting [[tin]].
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| * Due to the energy gap, the specific heat of the superconductor is suppressed strongly ([[exponential decay|exponentially]]) at low temperatures, there being no thermal excitations left. However, before reaching the transition temperature, the specific heat of the superconductor becomes even higher than that of the normal conductor (measured immediately above the transition) and the ratio of these two values is found to be universally given by 2.5.
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| * BCS theory correctly predicts the Meissner effect, i.e. the expulsion of a magnetic field from the superconductor and the variation of the penetration depth (the extent of the screening currents flowing below the metal's surface) with temperature. This had been demonstrated experimentally by [[Walther Meissner]] and [[Robert Ochsenfeld]] in their 1933 article [http://adsabs.harvard.edu/abs/1933NW.....21..787M Ein neuer Effekt bei Eintritt der Supraleitfähigkeit].
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| * It also describes the variation of the [[upper critical field|critical magnetic field]] (above which the superconductor can no longer expel the field but becomes normal conducting) with temperature. BCS theory relates the value of the critical field at zero temperature to the value of the transition temperature and the density of states at the Fermi level.
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| * In its simplest form, BCS gives the superconducting transition temperature ''T''<sub>c</sub>in terms of the electron-phonon coupling potential ''V'' and the [[Debye frequency|Debye]] cutoff energy ''E''<sub>D</sub>:<ref name=BCS_theory/>
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| ::<math>k_B\,T_c = 1.14E_D\,{e^{-1/N(0)\,V}},</math>
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| :where ''N''(0) is the electronic density of states at the Fermi level. For more details, see [[Cooper pairs]].
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| * The BCS theory reproduces the '''isotope effect''', which is the experimental observation that for a given superconducting material, the critical temperature is inversely proportional to the mass of the [[isotope]] used in the material. The isotope effect was reported by two groups on the 24th of March 1950, who discovered it independently working with different [[mercury (element)|mercury]] isotopes, although a few days before publication they learned of each other's results at the ONR conference in [[Atlanta, Georgia]]. The two groups are [[Emanuel Maxwell]], who published his results in [http://prola.aps.org/abstract/PR/v78/i4/p477_1 Isotope Effect in the Superconductivity of Mercury] and C. A. Reynolds, B. Serin, W. H. Wright, and L. B. Nesbitt who published their results 10 pages later in [http://prola.aps.org/abstract/PR/v78/i4/p487_1 Superconductivity of Isotopes of Mercury]. The choice of isotope ordinarily has little effect on the electrical properties of a material, but does affect the frequency of lattice vibrations. This effect suggests that superconductivity is related to vibrations of the lattice. This is incorporated into BCS theory, where lattice vibrations yield the binding energy of electrons in a Cooper pair.
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| ==See also==
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| * [[Superconductivity]]
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| * [[Magnesium diboride]], considered a BCS superconductor
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| * [[Quasiparticle]]
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| ==References==
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| {{reflist|colwidth=30em}}
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| The BCS Papers:
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| * L. N. Cooper, "Bound Electron Pairs in a Degenerate Fermi Gas", [http://prola.aps.org/abstract/PR/v104/i4/p1189_1 ''Phys. Rev'' '''104''', 1189 - 1190 (1956)].
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| * J. Bardeen, L. N. Cooper, and J. R. Schrieffer, "Microscopic Theory of Superconductivity", [http://prola.aps.org/abstract/PR/v106/i1/p162_1 ''Phys. Rev.'' '''106''', 162 - 164 (1957)].
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| * J. Bardeen, L. N. Cooper, and J. R. Schrieffer, "Theory of Superconductivity", [http://link.aps.org/abstract/PR/v108/p1175 ''Phys. Rev.'' '''108''', 1175 (1957)].
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| ==Further reading==
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| * John Robert Schrieffer, ''Theory of Superconductivity'', (1964), ISBN 0-7382-0120-0
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| * [[Michael Tinkham]], ''Introduction to Superconductivity'', ISBN 0-486-43503-2
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| * [[Pierre-Gilles de Gennes]], ''Superconductivity of Metals and Alloys'', ISBN 0-7382-0101-4.
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| * {{Cite book |author=[[Leon Cooper|Cooper, Leon N]] ; Feldman, Dmitri (Eds.) |title=[[BCS: 50 Years (book)]] |publisher=[[World Scientific]] |year=2010 |isbn=978-981-4304-64-1 }}
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| ==External links==
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| * ScienceDaily: [http://www.sciencedaily.com/releases/2006/08/060817101658.htm Physicist Discovers Exotic Superconductivity] ([[University of Arizona]]) August 17, 2006
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| * [http://hyperphysics.phy-astr.gsu.edu/hbase/solids/bcs.html Hyperphysics page on BCS ]
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| * [http://ffden-2.phys.uaf.edu/212_fall2003.web.dir/T.J_Barry/bcstheory.html BCS History ]
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| * [http://www.aip.org/history/mod/superconductivity/03.html Dance Analogy] of BCS theory as explained by Bob Schrieffer (audio recording)
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| {{four-fermion interactions}}
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| [[Category:Superconductivity]]
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2006 will be the twenty fifth year of the 401k investment plan. Perhaps you have had more than one work in the last 25 years? If so, you then probably have several 401(k) strategy boating.
401(k) plans at the moment are over 25 years old. They looked a distinctive idea at first, but now just about every employer offers one. And Im sure I dont have to let you know that they"re a good way to save and earn money over the years.
The problem here is when you setup a 401k, you often diversify your program together with your boss. Demonstrably, you have to spend using the current options your company offers, which is good. Investing a little in the high risk, some in the risk, and some in the lower risk funds its typically the plan. You was a little more open on getting threat 20 years ago than you"re today. Perhaps now you"re a tad bit more conservative in your investment objectives. So you think you are diversified, right?
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