Editing Unconventional superconductor (section)

== History ==

The superconducting properties of  CeCu<sub>2</sub>Si<sub>2</sub>, a type of
[[heavy fermion material]], were reported in 1979 by [[Frank Steglich]].<ref>{{cite journal |doi=10.1103/PhysRevLett.43.1892|title=Superconductivity in the Presence of Strong Pauli Paramagnetism: CeCu2Si2 |year=1979 |last1=Steglich |first1=F. |first2=J. |journal=Physical Review Letters |volume=43 |pages=1892–1896 |last2=Aarts |last3=Bredl |first3=C.D. |last4=Lieke |first4=W. |last5=Meschede |first5=D. |last6=Franz |first6=W. |last7=Schäfer |first7=H. |bibcode=1979PhRvL..43.1892S |issue=25 |hdl=1887/81461 |s2cid=123497750 |hdl-access=free }}</ref> For a long time it was believed that CeCu<sub>2</sub>Si<sub>2</sub> was a singlet d-wave superconductor, but since the mid-2010s, this notion has been strongly contested.<ref>{{Cite journal |last1=Kittaka|first1=Shunichiro|last2=Aoki|first2=Yuya|last3=Shimura|first3=Yasuyuki|last4=Sakakibara|first4=Toshiro|last5=Seiro|first5=Silvia|last6=Geibel|first6=Christoph|last7=Steglich|first7=Frank|last8=Ikeda|first8=Hiroaki|last9=Machida|first9=Kazushige|date=2014-02-12|title=Multiband Superconductivity with Unexpected Deficiency of Nodal Quasiparticles in CeCu<sub>2</sub>Si<sub>2</sub>|journal=Physical Review Letters|volume=112|issue=6|pages=067002|doi=10.1103/PhysRevLett.112.067002|bibcode=2014PhRvL.112f7002K|pmid=24580704|arxiv=1307.3499|s2cid=13367098 }}</ref> In  the early eighties, many more unconventional, [[heavy fermion]] [[Superconductivity|superconductors]] were discovered, including UBe<sub>13</sub>,<ref>{{cite journal |doi=10.1103/PhysRevLett.50.1595|title=UBe_{13}: An Unconventional Actinide Superconductor|year=1983|last1=Ott|first1=H. R.|last2=Rudigier|first2=H.|last3=Fisk|first3=Z.|last4=Smith|first4=J.|journal=Physical Review Letters|volume=50|issue=20|pages=1595–1598|bibcode=1983PhRvL..50.1595O|url=https://escholarship.org/uc/item/18w6w653 }}</ref> [[UPt3|UPt<sub>3</sub>]]<ref>{{cite journal|doi=10.1103/PhysRevLett.52.679|title=Possibility of Coexistence of Bulk Superconductivity and Spin Fluctuations in UPt<sub>3</sub>|year=1984|last1=Stewart|first1=G. R.|first2=Z.|first3=J. O.|first4=J. L.|journal=Physical Review Letters|volume=52|pages=679–682|last2=Fisk|last3=Willis|last4=Smith|bibcode=1984PhRvL..52..679S|issue=8|s2cid=73591098 |url=http://www.escholarship.org/uc/item/6px8s7q3}}</ref> and URu<sub>2</sub>Si<sub>2</sub>.<ref>{{cite journal|doi=10.1103/PhysRevLett.55.2727|pmid=10032222|title=Superconducting and Magnetic Transitions in the Heavy-Fermion System URu_{2}Si_{2}|year=1985|last1=Palstra|first1=T. T. M.|last2=Menovsky|first2=A. A.|last3=Berg|first3=J. van den|last4=Dirkmaat|first4=A. J.|last5=Kes|first5=P. H.|last6=Nieuwenhuys|first6=G. J.|last7=Mydosh|first7=J. A.|journal=Physical Review Letters|volume=55|issue=24|pages=2727–2730|bibcode=1985PhRvL..55.2727P|url=https://www.rug.nl/research/portal/en/publications/superconducting-and-magnetic-transitions-in-the-heavyfermion-system-uru2si2(ade9b7b6-f4eb-4973-bfbe-7837626183c0).html}}</ref> In each of these materials, the anisotropic nature of the pairing was implicated by the power-law dependence of the [[nuclear magnetic resonance]] (NMR) relaxation rate and specific heat capacity on temperature. The presence of nodes in the superconducting gap of UPt<sub>3</sub> was confirmed in 1986 from the polarization dependence of the ultrasound attenuation.<ref>{{cite journal|doi=10.1103/PhysRevLett.56.1078|title=Anisotropy of Transverse Sound in the Heavy-Fermion Superconductor UPt<sub>3</sub>
|year=1986|first4=D.|last1=Shivaram|last4=Hinks|first1=B. S.|first2=Y. H.|first3=T.F.|journal=Physical Review Letters|volume=56|pages=1078–1081|last2=Jeong|last3=Rosenbaum|pmid=10032562|bibcode=1986PhRvL..56.1078S|issue=10|url=https://authors.library.caltech.edu/47038/1/PhysRevLett.56.1078.pdf}}</ref>

The first unconventional triplet superconductor, organic material (TMTSF)<sub>2</sub>PF<sub>6</sub>, was discovered by [[Denis Jerome]], [[Klaus Bechgaard]] and coworkers in 1980 (TMTSF = Tetramethyltetraselenafulvalenium, see [[Fulvalene]]).<ref>{{cite journal |doi=10.1051/jphyslet:0198000410409500 |title=Superconductivity in a synthetic organic conductor (TMTSF)2PF 6 |year=1980 |last1=Jérome |first1=D. |first2=A. |first3=M. |first4=K. |journal=Journal de Physique Lettres |volume=41 |pages=95 |last2=Mazaud |last3=Ribault |last4=Bechgaard |issue=4 |url=https://hal.archives-ouvertes.fr/jpa-00231730/file/ajp-jphyslet_1980_41_4_95_0.pdf }}</ref> Experimental works by [[Paul Chaikin]]'s and Michael Naughton's groups as well as theoretical analysis of their data by [[Andrei Lebed]] have firmly confirmed unconventional nature of superconducting pairing in (TMTSF)<sub>2</sub>X (X=PF<sub>6</sub>, ClO<sub>4</sub>, etc.) organic materials.<ref>{{cite journal |doi=10.1103/PhysRevLett.46.852|title=Zero-Pressure Organic Superconductor: Di-(Tetramethyltetraselenafulvalenium)-Perchlorate [(TMTSF)2ClO4]|year=1981|last1=Bechgaard|first1=Klaus|first2=Claus S.|journal=Physical Review Letters|volume=46|pages=852|last2=Carneiro|last3=Olsen|first3=Malte|last4=Rasmussen|first4=Finn|last5=Jacobsen|first5=Claus|bibcode=1981PhRvL..46..852B|issue=13|url=http://orbit.dtu.dk/files/4927636/Bech.pdf}}</ref>

High-temperature singlet d-wave superconductivity was discovered by [[J.G. Bednorz]] and [[Karl Alexander Müller|K.A. Müller]] in 1986, who also discovered that the [[lanthanum]]-based [[cuprate superconductor|cuprate]] [[perovskite]] material LaBaCuO<sub>4</sub>  develops superconductivity  at a critical temperature (''T''<sub>c</sub>) of approximately 35&nbsp;[[kelvin|K]] (-238 degrees [[Celsius]]). This was well above the highest critical temperature known at the time (''T''<sub>c</sub> = 23&nbsp;K), and thus the new family of materials was called [[High-temperature superconductivity|high-temperature superconductors]]. Bednorz and Müller received the [[Nobel Prize in Physics]] for this discovery in 1987. Since then, many other [[High-temperature superconductivity|high-temperature superconductors]] have been synthesized.

LSCO (La<sub>2−''x''</sub>Sr<sub>''x''</sub>CuO<sub>4</sub>) was discovered the same year (1986). Soon after, in January 1987, [[yttrium barium copper oxide]] (YBCO) was discovered to have a ''T''<sub>c</sub> of 90&nbsp;K, the first material to achieve superconductivity above the boiling point of [[liquid nitrogen]] (77&nbsp;K).<ref>{{cite journal |title=Superconductivity at 93 K in a new mixed-phase Yb-Ba-Cu-O compound system at ambient pressure |author=K. M. Wu |journal=Phys. Rev. Lett. |volume=58 |issue=9 |year=1987 |pages=908–910 |doi=10.1103/PhysRevLett.58.908|pmid=10035069 |bibcode=1987PhRvL..58..908W |display-authors=etal |doi-access=free }}</ref> This was highly significant from the point of view of the [[technological applications of superconductivity]] because liquid nitrogen is far less expensive than [[liquid helium]], which is required to cool [[conventional superconductor]]s down to their critical temperature. In 1988 [[bismuth strontium calcium copper oxide]] (BSCCO) with ''T''<sub>c</sub> up to 107&nbsp;K,<ref>{{cite journal|title = A New High-''T''<sub>c</sub> Oxide Superconductor without a Rare Earth Element|author1=H. Maeda |author2=Y. Tanaka |author3=M. Fukutumi |author4=T. Asano  |name-list-style=amp |journal=Jpn. J. Appl. Phys. |volume=27 |issue=2 |pages=L209–L210 |year=1988 |doi=10.1143/JJAP.27.L209 |bibcode=1988JaJAP..27L.209M |doi-access=free }}</ref> and [[thallium barium calcium copper oxide]] (TBCCO) (T=thallium) with ''T''<sub>c</sub> of 125&nbsp;K were discovered. The current record critical temperature is about ''T''<sub>c</sub>&nbsp;= 133&nbsp;K (−140&nbsp;°C) at standard pressure, and somewhat higher critical temperatures can be achieved at high pressure. Nevertheless, at present it is considered unlikely that cuprate perovskite materials will achieve room-temperature superconductivity.

On the other hand, other unconventional superconductors have been discovered. These include some that do not superconduct at high temperatures, such as [[strontium ruthenate|strontium ruthenate Sr<sub>2</sub>RuO<sub>4</sub>]], but that, like high-temperature superconductors, are unconventional in other ways. (For example, the origin of the attractive force leading to the formation of [[Cooper pair]]s may be different from the one postulated in [[BCS theory]].) In addition to this, superconductors that have unusually high values of ''T''<sub>c</sub> but that are not cuprate perovskites have been discovered. Some of them may be extreme examples of [[conventional superconductor]]s (this is suspected of [[magnesium diboride]], MgB<sub>2</sub>, with ''T''<sub>c</sub> = 39&nbsp;K). Others could display more unconventional features.

In 2008 a new class that does not include copper (layered [[oxypnictide]] superconductors), for example LaOFeAs, was discovered.<ref name = "Superconductivity">{{cite journal |title = Superconductivity at 43K in an iron-based layered compound LaO<sub>1−''x''</sub>F<sub>''x''</sub>FeAs | author1 = Hiroki Takahashi | author2 = Kazumi Igawa | author3 = Kazunobu Arii | author4 = Yoichi Kamihara | author5 = Masahiro Hirano|author6= Hideo Hosono | journal = Nature|volume = 453|pages = 376–378|year = 2008|doi = 10.1038/nature06972|pmid = 18432191|issue = 7193|bibcode=2008Natur.453..376T|s2cid = 498756}}</ref><ref>{{cite web |url=http://www.sciam.com/article.cfm?id=iron-exposed-as-high-temp-superconductor |title=A New Iron Age: New class of superconductor may help pin down mysterious physics |first=Charles Q. |last=Choi |date=June 1, 2008 |work=Scientific American |access-date=2009-10-29}}</ref><ref>{{Cite web |url=https://www.sciencedaily.com/releases/2008/05/080528140242.htm |title=New High-Temperature Superconductors Are Iron-based With Unusual Magnetic Properties |website=ScienceDaily |date=June 1, 2008 |author=National Institute of Standards and Technology}}</ref> An oxypnictide of [[samarium]] seemed to have a ''T''<sub>c</sub> of about 43&nbsp;K, which was higher than predicted by BCS theory.<ref>{{cite journal | last1=Chen | first1=X. H. | last2=Wu | first2=T. | last3=Wu | first3=G. | last4=Liu | first4=R. H. | last5=Chen | first5=H. | last6=Fang | first6=D. F. | title=Superconductivity at 43 K in SmFeAsO<sub>1−x</sub>F<sub>X</sub> | journal=Nature | year=2008 | volume=453 | issue=7196 | pages=761–762 | doi=10.1038/nature07045 | pmid=18500328 | arxiv=0803.3603 | bibcode=2008Natur.453..761C | s2cid=205213713 }}</ref> Tests at up to 45&nbsp;[[tesla (unit)|T]]<ref>[http://www.planetanalog.com/news/showArticle.jhtml?articleID=208401297 High-temp superconductors pave way for 'supermagnets']{{Dead link|date=July 2018 |bot=InternetArchiveBot |fix-attempted=no }}</ref><ref>{{cite journal |doi=10.1038/nature07058|title=Very High Field Two-band Superconductivity in LaFeAsO0.89F0.11 at very high magnetic fields|year=2008|last1=Hunte|first1=F.|first2=J.|first3=A.|first4=D. C.|first5=R.|first6=A. S.|first7=M. A.|first8=B. C.|first9=D. K.|last10=Mandrus|first10=D.|journal=Nature|volume=453|pages=903–5|pmid=18509332|last2=Jaroszynski|last3=Gurevich|last4=Larbalestier|last5=Jin|last6=Sefat|last7=McGuire|last8=Sales|last9=Christen|issue=7197|bibcode = 2008Natur.453..903H |display-authors=8|arxiv=0804.0485|s2cid=115211939 }}</ref> suggested the upper critical field of LaFeAsO<sub>0.89</sub>F<sub>0.11</sub> to be around 64&nbsp;T. Some other [[iron-based superconductor]]s do not contain oxygen.

{{As of|2009}}, the highest-temperature superconductor (at ambient pressure) is mercury barium calcium copper oxide (HgBa<sub>2</sub>Ca<sub>2</sub>Cu<sub>3</sub>O<sub>''x''</sub>), at 138&nbsp;K and is held by a cuprate-perovskite material,<ref>{{cite journal |author1 = P. Dai|author2 = B. C. Chakoumakos|author3 = G. F. Sun|author4 = K. W. Wong |author5 = Y. Xin |author6 = D. F. Lu|title = Synthesis and neutron powder diffraction study of the superconductor HgBa<sub>2</sub>Ca<sub>2</sub>Cu<sub>3</sub>O<sub>8+δ</sub> by Tl substitution|journal = Physica C|volume = 243|year = 1995|pages = 201–206|doi = 10.1016/0921-4534(94)02461-8|issue = 3–4|bibcode=1995PhyC..243..201D }}</ref> possibly 164 K under high pressure.<ref>{{cite journal |author1=L. Gao |author2=Y. Y. Xue |author3=F. Chen |author4=Q. Xiong |author5=R. L. Meng |author6=D. Ramirez |author7=C. W. Chu |author8=J. H. Eggert |author9=H. K. Mao  |name-list-style=amp |title = Superconductivity up to 164 K in HgBa<sub>2</sub>Ca<sub>m-1</sub>Cu<sub>m</sub>O<sub>2m+2+δ</sub> (m=1, 2, and 3) under quasihydrostatic pressures|journal = Phys. Rev. B|volume = 50|year = 1994|pages = 4260–4263|doi = 10.1103/PhysRevB.50.4260|issue = 6|pmid=9976724 |bibcode=1994PhRvB..50.4260G }}</ref>

Other unconventional superconductors not based on cuprate structure have too been found.<ref name = "Superconductivity" /> Some have unusually high values of the [[critical temperature]], ''T''<sub>c</sub>, and hence they are sometimes also called high-temperature superconductors.

=== Graphene ===

In 2017, [[Scanning tunneling microscope|scanning tunneling microscopy]] and spectroscopy experiments on [[graphene]] proximitized to the electron-doped (non-chiral) ''d''-wave superconductor Pr<sub>2−''x''</sub>Ce<sub>''x''</sub>CuO<sub>4</sub> (PCCO) revealed evidence for an unconventional superconducting density of states induced in graphene.<ref>{{Cite journal |last1=Di Bernardo|first1=A.|last2=Millo|first2=O.|last3=Barbone|first3=M.|last4=Alpern|first4=H.|last5=Kalcheim|first5=Y.|last6=Sassi|first6=U.|last7=Ott|first7=A. K.|last8=Fazio|first8=D. De|last9=Yoon|first9=D.|date=2017-01-19|title=p-wave triggered superconductivity in single-layer graphene on an electron-doped oxide superconductor|journal=Nature Communications|language=en|volume=8|doi=10.1038/ncomms14024|issn=2041-1723|page=14024|arxiv=1702.01572|bibcode=2017NatCo...814024D|pmid=28102222|pmc=5253682 }}</ref> Publications in March 2018 provided evidence for unconventional [[Bilayer graphene#Superconductivity in twisted bilayer graphene|superconducting properties of a graphene bilayer]] where [[Twistronics|one layer was offset]] by a "magic angle" of 1.1° relative to the other.<ref>{{Cite journal |last=Gibney|first=Elizabeth|author-link=Elizabeth Gibney|date=5 March 2018|title=Surprise graphene discovery could unlock secrets of superconductivity|department=News|journal=Nature|volume=555|issue=7695|pages=151&ndash;2|quote=Physicists now report that arranging two layers of atom-thick graphene so that the pattern of their carbon atoms is offset by an angle of 1.1º makes the material a superconductor.|bibcode=2018Natur.555..151G|doi=10.1038/d41586-018-02773-w|pmid=29517044|doi-access=free }}</ref>