Editing High-temperature superconductivity (section)

=== Iron-based ===
{{Main|Iron-based superconductor}}
[[File:Phase diagram of the 122 family of ferro-pnictides.png|thumb|right|upright=1.4|[[Phase diagram]] for high-temperature superconductors based on iron<ref name="Kordyuk2012"/>]]

Iron-based superconductors contain layers of [[iron]] and a [[pnictogen]]{{snd}}such as [[arsenic]] or [[phosphorus]]{{snd}}, a [[chalcogen]], or a [[crystallogen]]. This is currently the family with the second highest critical temperature, behind the cuprates. Interest in their superconducting properties began in 2006 with the discovery of superconductivity in LaFePO at {{cvt|4|K|C}}<ref name=Hoso2006>{{cite journal |last1=Kamihara |first1=Y.   |last2=Hiramatsu |first2=H.
 |last3=Hirano   |first3=M.   |last4=Kawamura  |first4=R.
 |last5=Yanagi   |first5=H.   |last6=Kamiya    |first6=T.
 |last7=Hosono   |first7=H. |year=2006
 |title=Iron-based layered superconductor: LaOFeP
 |journal=[[Journal of the American Chemical Society]]
 |volume=128  |issue=31  |pages=10012–10013
 |doi=10.1021/ja063355c
 |pmid=16881620
|bibcode=2006JAChS.12810012K }}
</ref> and gained much greater attention in 2008 after the analogous material LaFeAs(O,F)<ref name=Kami2008>
{{cite journal
 |last1=Kamihara |first1=Y.
 |last2=Watanabe |first2=T.
 |last3=Hirano   |first3=M.
 |last4=Hosono   |first4=H.
 |year=2008
 |title=Iron-Based Layered Superconductor La[O<sub>1−x</sub>F<sub>x</sub>]FeAs (x=0.05–0.12) with {{mvar|T}}<sub>c</sub>&nbsp;=&nbsp;26&nbsp;K
 |journal=[[Journal of the American Chemical Society]]
 |volume=130 |issue=11 |pages=3296–3297
 |doi=10.1021/ja800073m
 |pmid=18293989
}}
</ref> was found to superconduct at up to {{cvt|43|K|C}} under pressure.<ref name=Taka2008>
{{cite journal
 |last1=Takahashi |first1=H.    |last2=Igawa    |first2=K.
 |last3=Arii      |first3=K.    |last4=Kamihara |first4=Y.
 |last5=Hirano    |first5=M.    |last6=Hosono   |first6=H.
 |s2cid=498756
 |year=2008
 |title=Superconductivity at 43&nbsp;K in an iron-based layered compound LaO1-<sub>x</sub>F<sub>x</sub>FeAs
 |journal=[[Nature (journal)|Nature]]
 |volume=453  |issue=7193  |pages=376–378
 |doi=10.1038/nature06972  |pmid=18432191  |bibcode=2008Natur.453..376T
}}
</ref>
The highest critical temperatures in the iron-based superconductor family exist in thin films of FeSe,<ref name=Xue2012>
{{cite journal
 |last1=Wang  |first1=Qing-Yan    |last2=Li     |first2=Zhi 
 |last3=Zhang |first3=Wen-Hao     |last4=Zhang  |first4=Zuo-Cheng 
 |last5=Zhang |first5=Jin-Song    |last6=Li     |first6=Wei 
 |last7=Ding  |first7=Hao         |last8=Ou     |first8=Yun-Bo 
 |last9=Deng  |first9=Peng        |last10=Chang |first10=Kai 
 |last11=Wen  |first11=Jing       |last12=Song  |first12=Can-Li 
 |last13=He   |first13=Ke         |last14=Jia   |first14=Jin-Feng 
 |last15=Ji   |first15=Shuai-Hua  |last16=Wang  |first16=Ya-Yu
 |last17=Wang |first17=Li-Li      |last18=Chen  |first18=Xi 
 |last19=Ma   |first19=Xu-Cun     |last20=Xue   |first20=Qi-Kun
 |display-authors=6
 |year=2012
 |title=Interface-Induced high-temperature superconductivity in single unit-cell FeSe films on SrTiO<sub>3</sub> 
 |journal=Chin. Phys. Lett.
 |volume=29   |issue=3   |pages=037402
 |doi=10.1088/0256-307X/29/3/037402  |arxiv=1201.5694
 |bibcode=2012ChPhL..29c7402W   |s2cid=3858973 
}}
</ref><ref name=Liu2012>
{{cite journal
 |last1=Liu    |first1=Defa         |last2=Zhang |first2=Wenhao
 |last3=Mou    |first3=Daixiang     |last4=He    |first4=Junfeng
 |last5=Ou     |first5=Yun-Bo       |last6=Wang  |first6=Qing-Yan
 |last7=Li     |first7=Zhi          |last8=Wang  |first8=Lili
 |last9=Zhao   |first9=Lin          |last10=He   |first10=Shaolong
 |last11=Peng  |first11=Yingying    |last12=Liu  |first12=Xu
 |last13=Chen  |first13=Chaoyu      |last14=Yu   |first14=Li
 |last15=Liu   |first15=Guodong     |last16=Dong |first16=Xiaoli
 |last17=Zhang |first17=Jun         |last18=Chen |first18=Chuangtian
 |last19=Xu    |first19=Zuyan       |last20=Hu   |first20=Jiangping
 |last21=Chen  |first21=Xi          |last22=Ma   |first22=Xucun
 |last23=Xue   |first23=Qikun       |last24=Zhou |first24=X. J.
 |display-authors=6
 |year=2012
 |title=Electronic origin of high-temperature superconductivity in single-layer FeSe superconductor
 |journal=Nat. Commun.
 |volume=3  |issue=931  |pages=931
 |doi=10.1038/ncomms1946  |pmid=22760630 |arxiv=1202.5849
 |bibcode=2012NatCo...3..931L  |s2cid=36598762
}}
</ref><ref name=He2013>
{{cite journal
 |last1=He    |first1=Shaolong      |last2=He     |first2=Junfeng
 |last3=Zhang |first3=Wenhao        |last4=Zhao   |first4=Lin
 |last5=Liu   |first5=Defa          |last6=Liu    |first6=Xu
 |last7=Mou   |first7=Daixiang      |last8=Ou     |first8=Yun-Bo
 |last9=Wang  |first9=Qing-Yan      |last10=Li    |first10=Zhi
 |last11=Wang |first11=Lili         |last12=Peng  |first12=Yingying
 |last13=Liu  |first13=Yan          |last14=Chen  |first14=Chaoyu
 |last15=Yu   |first15=Li           |last16=Liu   |first16=Guodong
 |last17=Dong |first17=Xiaoli       |last18=Zhang |first18=Jun
 |last19=Chen |first19=Chuangtian   |last20=Xu    |first20=Zuyan
 |last21=Chen |first21=Xi           |last22=Ma    |first22=Xucun
 |last23=Xue  |first23=Qikun        |last24=Zhou  |first24=X. J.
 |display-authors=6
 |year=2013
 |title=Phase diagram and electronic indication of high-temperature superconductivity at 65&nbsp;K in single-layer FeSe films
 |journal=Nat. Mater.
 |volume=12 |issue=7 |pages=605–610
 |doi=10.1038/NMAT3648 |pmid=23708329
 |arxiv=1207.6823 |bibcode=2013NatMa..12..605H
|s2cid=119185689 }}
</ref> where a critical temperature in excess of {{cvt|100|K|C}} was reported in 2014.<ref name=Ge2014>
{{cite journal
 |year=2014
 |title=Superconductivity in single-layer films of FeSe with a transition temperature above 100&nbsp;K
 |journal=Nature Materials
 |volume=1406  |issue=3  |pages=285–9
 |arxiv=1406.3435  |bibcode=2015NatMa..14..285G
 |pmid=25419814 |doi=10.1038/nmat4153
 |last1=Ge   |first1=J. F.
 |last2=Liu  |first2=Z. L.
 |last3=Liu  |first3=C.
 |last4=Gao  |first4=C. L.
 |last5=Qian |first5=D.
 |last6=Xue  |first6=Q. K.
 |last7=Liu  |first7=Y.
 |last8=Jia  |first8=J. F.
 |s2cid=119227626
 }}
</ref>

Since the original discoveries several families of iron-based superconductors have emerged:
* LnFeAs(O,F) or LnFeAsO<sub>1−x</sub> (Ln=lanthanide) with {{mvar|T}}<sub>c</sub> up to {{cvt|56|K|C}}, referred to as 1111 materials.<ref name=ren/> A [[fluoride]] variant of these materials was subsequently found with similar {{mvar|T}}<sub>c</sub> values.<ref name=Wu2008>
{{cite journal
 |last1=Wu    |first1=G.        |last2=Xie   |first2=Y. L.
 |last3=Chen  |first3=H.        |last4=Zhong |first4=M.
 |last5=Liu   |first5=R. H.     |last6=Shi   |first6=B. C.
 |last7=Li    |first7=Q. J.     |last8=Wang  |first8=X. F.
 |last9=Wu    |first9=T.        |last10=Yan  |first10=Y. J.
 |last11=Ying |first11=J. J.    |last12=Chen |first12=X. H.
 |display-authors=6
 |year=2009
 |title=Superconductivity at 56&nbsp;K in Samarium-doped SrFeAsF
 |journal=Journal of Physics: Condensed Matter
 |volume=21  |issue=3  |page=142203
 |arxiv=0811.0761  |doi=10.1088/0953-8984/21/14/142203
 |pmid=21825317  |bibcode=2009JPCM...21n2203W
 |s2cid=41728130
}}
</ref>
* (Ba,K)Fe<sub>2</sub>As<sub>2</sub> and related materials with pairs of iron-arsenide layers, referred to as 122 compounds. {{mvar|T}}<sub>c</sub> values range up to {{cvt|38|K|C}}.<ref name=Rotter2008>
{{cite journal
 |last1=Rotter   |first1=M.
 |last2=Tegel    |first2=M.
 |last3=Johrendt |first3=D.
 |year=2008
 |title=Superconductivity at 38&nbsp;K in the iron arsenide (Ba<sub>1−x</sub>K<sub>x</sub>)Fe<sub>2</sub>As<sub>2</sub>
 |journal=[[Physical Review Letters]]
 |volume=101   |issue=10   |page=107006
 |doi=10.1103/PhysRevLett.101.107006  |pmid=18851249  |arxiv=0805.4630
 |bibcode=2008PhRvL.101j7006R  |s2cid=25876149
}}
</ref><ref name=Sasmal2008>
{{cite journal
 |last1=Sasmal |first1=K.
 |last2=Lv     |first2=B.
 |last3=Lorenz |first3=B.
 |last4=Guloy  |first4=A. M.
 |last5=Chen   |first5=F.
 |last6=Xue    |first6=Y. Y.
 |last7=Chu    |first7=C. W.
 |year=2008
 |title=Superconducting Fe-based compounds (A<sub>1−x</sub>Sr<sub>x</sub>)Fe<sub>2</sub>As<sub>2</sub> with A=K and Cs with transition temperatures up to 37&nbsp;K
 |journal=[[Physical Review Letters]]
 |volume=101 |issue=10 |page=107007
 |doi=10.1103/PhysRevLett.101.107007  |pmid=18851250
 |bibcode=2008PhRvL.101j7007S  |arxiv=0806.1301
}}
</ref> These materials also superconduct when iron is replaced with [[cobalt]].
* LiFeAs and NaFeAs with {{mvar|T}}<sub>c</sub> up to around {{cvt|20|K|C}}. These materials superconduct close to stoichiometric composition and are referred to as 111 compounds.<ref name=Pitcher2008>
{{cite journal
 |last1=Pitcher    |first1=M. J.
 |last2=Parker     |first2=D. R.
 |last3=Adamson    |first3=P.
 |last4=Herkelrath |first4=S. J.
 |last5=Boothroyd  |first5=A. T.
 |last6=Ibberson   |first6=R. M.
 |last7=Brunelli   |first7=M.
 |last8=Clarke     |first8=S. J.
 |year=2008
 |title=Structure and superconductivity of LiFeAs
 |journal=[[Chemical Communications]]
 |volume=2008 |issue=45 |pages=5918–5920
 |doi=10.1039/b813153h |pmid=19030538 |arxiv=0807.2228 |s2cid=3258249
}}
</ref><ref name=Tapp2008>
{{cite journal
 |last1=Tapp |first1=Joshua H.
 |last2=Tang |first2=Zhongjia
 |last3=Lv |first3=Bing
 |last4=Sasmal |first4=Kalyan
 |last5=Lorenz |first5=Bernd
 |last6=Chu |first6=Paul C.W.
 |last7=Guloy |first7=Arnold M.
 |year=2008
 |title=LiFeAs: An intrinsic FeAs-based superconductor with {{mvar|T}}<sub>c</sub>=18&nbsp;K
 |journal=[[Physical Review B]]
 |volume=78  |issue=6  |page=060505
 |doi=10.1103/PhysRevB.78.060505 |bibcode=2008PhRvB..78f0505T |arxiv=0807.2274
 |s2cid = 118379012
}}
</ref><ref name=Parker2008>
{{cite journal
 |last1=Parker    |first1=D. R.
 |last2=Pitcher   |first2=M. J.
 |last3=Baker     |first3=P. J.
 |last4=Franke    |first4=I.
 |last5=Lancaster |first5=T.
 |last6=Blundell  |first6=S. J.
 |last7=Clarke    |first7=S. J.
 |year=2009
 |title=Structure, antiferromagnetism and superconductivity of the layered iron arsenide NaFeAs
 |journal=[[Chemical Communications]]
 |volume=2009  |issue=16  |pages=2189–2191
 |doi=10.1039/b818911k |pmid=19360189 |arxiv=0810.3214 |s2cid=45189652
}}
</ref>
* FeSe with small off-[[stoichiometry]] or [[tellurium]] doping.<ref name=Hsu2008>
{{cite journal
 |last1=Hsu   |first1=F. C.    |last2=Luo   |first2=J. Y.
 |last3=Yeh   |first3=K. W.    |last4=Chen  |first4=T. K.
 |last5=Huang |first5=T. W.    |last6=Wu    |first6=P. M.
 |last7=Lee   |first7=Y. C.    |last8=Huang |first8=Y. L.
 |last9=Chu   |first9=Y. Y.    |last10=Yan  |first10=D. C.
 |last11=Wu   |first11=M. K.   |display-authors=6
 |year=2008
 |title=Superconductivity in the PbO-type structure α-FeSe
 |journal=[[Proceedings of the National Academy of Sciences of the United States of America]]
 |volume=105  |issue=38  |pages=14262–14264
 |doi=10.1073/pnas.0807325105  |pmid=18776050  |pmc=2531064
 |bibcode=2008PNAS..10514262H
|doi-access=free }}
</ref>
* LaFeSiH with {{mvar|T}}<sub>c</sub> around {{cvt|11|K|C}} in its stoichiometric composition.<ref name=Bernardini2018>
{{cite journal
 |last1=Bernardini   |first1=F.
 |last2=Garbarino    |first2=G.
 |last3=Sulpice      |first3=A.
 |last4=Núñnez-Regueiro   |first4=M.
 |last5=Gaudin       |first5=E.
 |last6=Chevalier    |first6=B.
 |last7=Measson    |first7=M.-A.
 |last8=Cano       |first8=A.
 |last9=Tencé    |first9=S.
|display-authors=1
 |year=2018
 |title=Iron-based superconductivity extended to the novel silicide LaFeSiH
 |journal=[[Physical Review B]]
 |volume=97   |issue=10   |page=100504
 |doi=10.1103/PhysRevB.97.100504
 |arxiv=1701.05010
|bibcode=2018PhRvB..97j0504B
 }}
</ref> This superconducting crystallogenide has oxide and fluoride variants LaFeSiO<sub>x</sub> and LaFeSiF<sub>x</sub>.<ref name=Hansen2022>
{{cite journal
 |last1=Hansen   |first1=M. F.
 |last2=x   |first2=x
|display-authors=1
 |year=2022
 |title=Superconductivity in the crystallogenide LaFeSiO1−δ with squeezed FeSi layers
 |journal=[[npj Quantum Materials]]
 |volume=7  |page=86
 |doi=10.1038/s41535-022-00493-z
 |arxiv=2206.11690
}}
</ref><ref name=Vaney2022>
{{cite journal
 |last1=Vaney   |first1=J. B.
 |last2=x   |first2=x
|display-authors=1
 |year=2022
 |title=Topotactic fluorination of intermetallics as an efficient route towards quantum materials
 |journal=[[Nature Communications]]
 |volume=13  
 |page=1462
 |doi=10.1038/s41467-022-29043-8
|pmid=35304455
 |pmc=8933527
 |bibcode=2022NatCo..13.1462V
 }}
</ref> 

Most undoped iron-based superconductors show a tetragonal-orthorhombic structural [[phase transition]] followed at lower temperature by magnetic ordering, similar to the cuprate superconductors.<ref name=Zhao2008>
{{cite journal
 |last1=Zhao       |first1=J.     |last2=Huang |first2=Q.
 |last3=de la Cruz |first3=C.     |last4=Li    |first4=S.
 |last5=Lynn       |first5=J. W.  |last6=Chen  |first6=Y.
 |last7=Green      |first7=M. A.  |last8=Chen  |first8=G. F.
 |last9=Li         |first9=G.     |last10=Li   |first10=Z.
 |last11=Luo       |first11=J. L. |last12=Wang |first12=N. L.
 |last13=Dai       |first13=P.    |display-authors=6
 |year=2008
 |title=Structural and magnetic phase diagram of CeFeAsO<sub>1−x</sub>F<sub>x</sub> and its relation to high-temperature superconductivity
 |journal=[[Nature Materials]]
 |volume=7  |issue=12  |pages=953–959
 |doi=10.1038/nmat2315 |pmid=18953342 |bibcode=2008NatMa...7..953Z
 |arxiv=0806.2528 |s2cid=25937023
}}
</ref> However, they are poor metals rather than Mott insulators and have five [[electronic band structure|band]]s at the Fermi surface rather than one.<ref name=Kordyuk2012>
{{Cite journal
 |last1      = Kordyuk |first1 = A. A.
 |year       = 2012
 |title      = Iron-based superconductors: Magnetism, superconductivity, and electronic structure (Review Article)
 |journal    = Low Temp. Phys.
 |volume     = 38   |issue = 9   |pages = 888–899
 |arxiv      = 1209.0140   |bibcode = 2012LTP....38..888K
 |doi        = 10.1063/1.4752092   |s2cid = 117139280
 |url        = http://www.imp.kiev.ua/~kord/papers/box/2012_LTP_Kordyuk.pdf
 |url-status = live
 |archive-url = https://web.archive.org/web/20150511190854/http://www.imp.kiev.ua/~kord/papers/box/2012_LTP_Kordyuk.pdf
 |archive-date = May 11, 2015  |df = dmy-all
}}
</ref> 
The phase diagram emerging as the iron-arsenide layers are doped is remarkably similar, with the superconducting phase close to or overlapping the magnetic phase. Strong evidence that the {{mvar|T}}<sub>c</sub> value varies with the As–Fe–As bond angles has already emerged and shows that the optimal {{mvar|T}}<sub>c</sub> value is obtained with undistorted FeAs<sub>4</sub> tetrahedra.<ref name=Lee2008>
{{cite journal
 |last1=Lee |first1=Chul-Ho
 |last2=Iyo |first2=Akira
 |last3=Eisaki |first3=Hiroshi
 |last4=Kito |first4=Hijiri
 |last5=Teresa Fernandez-Diaz |first5=Maria
 |last6=Ito |first6=Toshimitsu
 |last7=Kihou |first7=Kunihiro
 |last8=Matsuhata |first8=Hirofumi
 |last9=Braden |first9=Markus
 |last10=Yamada |first10=Kazuyoshi
 |display-authors=6
 |year=2008
 |title=Effect of structural parameters on superconductivity in fluorine-free LnFeAsO<sub>1−y</sub> (Ln=La, Nd)
 |journal=[[Journal of the Physical Society of Japan]]
 |volume=77  |issue=8  |page=083704
 |doi=10.1143/JPSJ.77.083704  |bibcode=2008JPSJ...77h3704L  |arxiv=0806.3821
 |s2cid = 119112251
}}
</ref> The symmetry of the pairing wavefunction is still widely debated, but an extended ''s''-wave scenario is currently favoured.