Editing BCS theory (section)

==History==
Rapid progress in the understanding of superconductivity gained momentum in the mid-1950s. It began with 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|bibcode = 1948PhRv...74..562L }}</ref> where [[Fritz London]] proposed that the [[Phenomenology (particle physics)|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|bibcode = 1955PhRv...97.1724B }}<!--|access-date=May 3, 2012--></ref> that such a modification naturally occurs in a theory with an energy gap. The key ingredient was Leon 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|issn=0031-899X|bibcode = 1956PhRv..104.1189C |doi-access=free}}<!--|access-date=May 3, 2012--></ref>

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.|author2=Cooper, L. N.|author3=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|bibcode = 1957PhRv..106..162B |doi-access=free}}</ref> The demonstration that the phase transition is second order, that it reproduces the [[Meissner effect]] and the calculations of [[specific heat]]s and penetration depths appeared in the December 1957 article, "Theory of superconductivity".<ref name=BCS_theory>{{cite journal|last=Bardeen|first=J.|author2=Cooper, L. N. |author3=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|bibcode = 1957PhRv..108.1175B |doi-access=free}}</ref> They received the [[Nobel Prize in Physics]] in 1972 for this theory.

In 1986, [[high-temperature superconductivity]] was discovered in La-Ba-Cu-O, at temperatures up to 30&nbsp;K.<ref>{{cite journal|last=Bednorz|first=J. G.|author2=Müller, K. A.|s2cid=118314311|title=Possible high T<sub>c</sub> superconductivity in the Ba−La−Cu−O system|journal=Zeitschrift für Physik B: Condensed Matter|date=June 1986|volume=64|issue=2 |pages=189–193 |doi=10.1007/BF01303701|bibcode=1986ZPhyB..64..189B }}</ref> Following experiments determined more materials with transition temperatures up to about 130&nbsp;K, considerably above the previous limit of about 30&nbsp;[[Kelvin|K]]. It is experimentally very well known that the transition temperature strongly depends on pressure.  In general, it is believed that BCS theory alone cannot explain this phenomenon and that other effects are in 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|pmid=21776057|bibcode = 2011Natur.475..280M|issue=7356|pages=280–2|doi-access=}}</ref> These effects are still not yet fully understood; it is possible that they even control superconductivity at low temperatures for some materials.