Editing High-temperature superconductivity (section)

== Properties ==
The "high-temperature" superconductor class has had many definitions.

The label high-{{mvar|T}}<sub>c</sub> should be reserved for materials with critical temperatures greater than the boiling point of [[liquid nitrogen]]. However, a number of materials{{snd}}including the original discovery and recently discovered pnictide superconductors{{snd}}have critical temperatures below {{cvt|77|K|C}} but nonetheless are commonly referred to in publications as high-{{mvar|T}}<sub>c</sub> class.<ref name=Norman2008>
{{cite journal
 |last=Norman  |first=Michael R.
 |year=2008
 |title=Trend: High-temperature superconductivity in the iron pnictides
 |journal=Physics
 |volume=1 |issue=21 |page=21
 |doi=10.1103/Physics.1.21 |doi-access=free |bibcode=2008PhyOJ...1...21N
}}
</ref><ref name=devereaux-stanford>
{{cite web
 |title=High-Temperature Superconductivity: The Cuprates
 |website=Devereaux group
 |publisher=Stanford University
 |url=http://www.stanford.edu/~tpd/research_hightc.html
 |access-date=March 30, 2012 |url-status=dead
 |archive-url=https://web.archive.org/web/20100615231514/http://www.stanford.edu/~tpd/research_hightc.html
 |archive-date=June 15, 2010
}}
</ref>

A substance with a critical temperature above the boiling point of liquid nitrogen, together with a high critical magnetic field and critical current density (above which superconductivity is destroyed), would greatly benefit technological applications. In magnet applications, the high critical magnetic field may prove more valuable than the high {{mvar|T}}<sub>c</sub> itself. Some cuprates have an upper critical field of about 100&nbsp;tesla. However, cuprate materials are brittle ceramics that are expensive to manufacture and not easily turned into wires or other useful shapes. Furthermore, high-temperature superconductors do not form large, continuous superconducting domains, rather clusters of microdomains within which superconductivity occurs. They are therefore unsuitable for applications requiring actual superconductive currents, such as magnets for [[Nuclear magnetic resonance|magnetic resonance]] spectrometers.<ref>
{{cite journal
 |first1=S.   |last1=Graser   |first2=P. J.|last2=Hirschfeld
 |first3=T.   |last3=Kopp     |first4=R.   |last4=Gutser
 |first5=B. M.|last5=Andersen |first6=J.   |last6=Mannhart
 |date=June 27, 2010
 |title=How grain boundaries limit supercurrents in high-temperature superconductors
 |journal=[[Nature Physics]]
 |volume=6  |issue=8  |pages=609–614
 |doi=10.1038/nphys1687  |arxiv=0912.4191  |bibcode=2010NatPh...6..609G
|s2cid=118624779 }}
</ref> For a solution to this (powders), see [[Superconducting wire|HTS wire]].

There has been considerable debate regarding high-temperature superconductivity coexisting with [[magnetism|magnetic ordering]] in YBCO,<ref name=Sann2004>
{{cite journal
 | last1=Sanna   | first1=S.
 | last2=Allodi  | first2=G.
 | last3=Concas  | first3=G.
 | last4=Hillier | first4=A.
 | last5=Renzi   | first5=R.
 | year=2004
 | title=Nanoscopic coexistence of magnetism and superconductivity in YBa<sub>2</sub>Cu<sub>3</sub>O<sub>6+x</sub> detected by muon spin rotation
 | journal=[[Physical Review Letters]]
 | volume=93  | issue=20  | page=207001
 | arxiv=cond-mat/0403608   | bibcode=2004PhRvL..93t7001S
 | doi=10.1103/PhysRevLett.93.207001   | pmid=15600957   | s2cid=34327069
}}
</ref> [[iron-based superconductor]]s, several ruthenocuprates and other exotic superconductors, and the search continues for other families of materials. HTS are [[Type-II superconductor]]s, which allow [[magnetic field]]s to penetrate their interior in [[quantum|quantized]] units of flux, meaning that much higher magnetic fields are required to suppress superconductivity. The layered structure also gives a directional dependence to the magnetic field response.

All known high-{{mvar|T}}<sub>c</sub> superconductors are Type-II superconductors. In contrast to [[Type-I superconductor]]s, which expel all magnetic fields due to the [[Meissner effect]], Type-II superconductors allow magnetic fields to penetrate their interior in quantized units of flux, creating "holes" or "tubes" of [[electrical conduction#Metals|normal metal]]lic regions in the superconducting bulk called [[Quantum vortex|vortices]]. Consequently, high-{{mvar|T}}<sub>c</sub> superconductors can sustain much higher magnetic fields.

=== Cuprates ===
{{Excerpt|Cuprate superconductor}}

=== 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.

=== Magnesium diboride ===
[[Magnesium diboride]] is occasionally referred to as a high-temperature superconductor<ref name=preuss>{{cite web|last=Preuss|first=Paul|title=A Most Unusual Superconductor and How It Works|url=http://www.lbl.gov/Science-Articles/Archive/MSD-superconductor-Cohen-Louie.html|publisher=Berkeley Lab|access-date=March 12, 2012|url-status=live|archive-url=https://web.archive.org/web/20120703001012/http://www.lbl.gov/Science-Articles/Archive/MSD-superconductor-Cohen-Louie.html|archive-date=July 3, 2012 }}</ref> because its {{mvar|T}}<sub>c</sub> value of {{cvt|39|K|C}} is above that historically expected for [[BCS theory|BCS]] superconductors. However, it is more generally regarded as the highest {{mvar|T}}<sub>c</sub> conventional superconductor, the increased {{mvar|T}}<sub>c</sub> resulting from two separate bands being present at the [[Fermi level]].

=== Carbon-based ===
In 1991 Hebard et al. discovered [[Fullerides|Fulleride]] superconductors,<ref name=Heba1991>
{{cite journal
 |last1=Hebard |first1=A. F.
 |last2=Rosseinsky |first2=M. J.
 |last3=Haddon |first3=R. C.
 |last4=Murphy |first4=D. W.
 |last5=Glarum |first5=S. H.
 |last6=Palstra |first6=T. T. M.
 |last7=Ramirez |first7=A. P.
 |last8=Kortan |first8=A. R.
 |year=1991
 |title=Superconductivity at 18&nbsp;K in potassium-doped C<sub>60</sub>
 |journal=[[Nature (journal)|Nature]]
 |volume=350  |issue=6319  |pages=600–601
 |doi=10.1038/350600a0 |bibcode=1991Natur.350..600H  |s2cid=4350005
 |url=https://pure.rug.nl/ws/files/14557724/1991NatureHebard.pdf
|hdl=11370/3709b8a7-6fc1-4b32-8842-ce9b5355b5e4|hdl-access=free}}
</ref> where alkali-metal atoms are intercalated into C<sub>60</sub> molecules.

In 2008 Ganin et al. demonstrated superconductivity at temperatures of up to {{cvt|38|K|C}} for Cs<sub>3</sub>C<sub>60</sub>.<ref name=ganin08>
{{cite journal
 |last1=Ganin |first1=A. Y.
 |last2=Takabayashi |first2=Y.
 |last3=Khimyak |first3=Y. Z.
 |last4=Margadonna |first4=S.
 |last5=Tamai |first5=A.
 |last6=Rosseinsky |first6=M. J.
 |last7=Prassides |first7=K.
 |year=2008
 |title=Bulk superconductivity at 38&nbsp;K in a molecular system
 |journal=[[Nature Materials]]
 |volume=7 |issue=5 |pages=367–71
 |doi=10.1038/nmat2179 |pmid=18425134 |bibcode=2008NatMa...7..367G
}}
</ref>

P-doped [[Graphane]] was proposed in 2010 to be capable of sustaining high-temperature superconductivity.<ref name=savini10>{{Cite journal |last1=Savini |first1=G. |last2=Ferrari |first2=A. C. |last3=Giustino |first3=F. |year=2010 |title=First-principles prediction of doped graphane as a high-temperature electron-phonon superconductor |journal=[[Physical Review Letters]] |volume=105 |issue=3 |pages=037002 |arxiv=1002.0653 |bibcode=2010PhRvL.105c7002S |doi=10.1103/PhysRevLett.105.037002 |pmid=20867792|s2cid=118466816 }}</ref>

On 31st of December 2023 "Global Room-Temperature Superconductivity in Graphite" was published in the journal "Advanced Quantum Technologies" claiming to demonstrate superconductivity at room temperature and ambient pressure in [[Highly oriented pyrolytic graphite]] with dense arrays of nearly parallel line defects.<ref>{{cite journal | url=https://onlinelibrary.wiley.com/doi/10.1002/qute.202300230?ref=upstract.com | doi=10.1002/qute.202300230 | title=Global Room-Temperature Superconductivity in Graphite | date=2024 | last1=Kopelevich | first1=Yakov | last2=Torres | first2=José | last3=Da Silva | first3=Robson | last4=Oliveira | first4=Felipe | last5=Diamantini | first5=Maria Cristina | last6=Trugenberger | first6=Carlo | last7=Vinokur | first7=Valerii | journal=Advanced Quantum Technologies | volume=7 | issue=2 | arxiv=2208.00854 }}</ref>

=== Nickelates ===
In 1999, Anisimov et al. conjectured superconductivity in nickelates, proposing nickel oxides as direct analogs to the cuprate superconductors.<ref>
{{cite journal
 |last1=Anisimov  |first1=V. I.
 |last2=Bukhvalov |first2=D.
 |last3=Rice      |first3=T. M.
 |date=15 March 1999
 |title=Electronic structure of possible nickelate analogs to the cuprates
 |journal=Physical Review B
 |volume=59  |issue=12  |pages=7901–7906
 |bibcode=1999PhRvB..59.7901A  |doi=10.1103/PhysRevB.59.7901
}}
</ref> Superconductivity in an infinite-layer nickelate, Nd<sub>0.8</sub>Sr<sub>0.2</sub>NiO<sub>2</sub>, was reported at the end of 2019 with a superconducting transition temperature between {{cvt|9|and|15|K|C}}.<ref>
{{cite journal
 |last1=Li   |first1=D.
 |last2=Lee  |first2=K.
 |last3=Wang |first3=B. Y.
 |display-authors=etal
 |year=2019
 |title=Superconductivity in an infinite-layer nickelate
 |url= https://www.nature.com/articles/s41586-019-1496-5
 |journal= Nature
 |volume= 572 |issue= 7771
 |pages= 624–627
 |doi=  10.1038/s41586-019-1496-5
 |pmid= 31462797
 |bibcode= 2019Natur.572..624L
 |osti=1562463
 |s2cid= 201656573
}}
</ref><ref>
{{cite journal
 |last1=Botana     |first1=A. S.
 |last2=Bernardini |first2=F.
 |last3=Cano       |first3=A.
 |year=2021
 |title=Nickelate superconductors: an ongoing dialog between theory and experiments
 |journal=  Journal of Experimental and Theoretical Physics|volume= 132 |issue=4
 |pages= 618–627 |doi= 10.1134/S1063776121040026
 |arxiv=2012.02764
 |bibcode=2021JETP..132..618B
 |s2cid=255191342
}}
</ref> This superconducting phase is observed in oxygen-reduced thin films created by the pulsed laser deposition of Nd<sub>0.8</sub>Sr<sub>0.2</sub>NiO<sub>3</sub> onto SrTiO<sub>3</sub> substrates that is then reduced to Nd<sub>0.8</sub>Sr<sub>0.2</sub>NiO<sub>2</sub> via annealing the thin films at {{convert|260|-|280|C|K|order=flip}} in the presence of CaH<sub>2</sub>.<ref>
{{cite journal
 |last1= Wu        |first1= Xianxin     |last2= Di Sante |first2= Domenico
 |last3= Schwemmer |first3= Tilman      |last4= Hanke    |first4= Werner
 |last5= Hwang     |first5= Harold Y.   |last6= Raghu    |first6= Srinivas
 |last7= Thomale   |first7= Ronny
 |date= 24 February 2020
 |title=Robust d<sub>x<sup>2</sup>-y<sup>2</sup></sub>-wave superconductivity of infinite-layer nickelates
 |journal= Physical Review B
 |volume= 101 |issue= 6 |page= 060504
 |doi=10.1103/PhysRevB.101.060504 |arxiv= 1909.03015
 |bibcode= 2020PhRvB.101f0504W |s2cid= 202537199
 |url= https://journals.aps.org/prb/abstract/10.1103/PhysRevB.101.060504
}}
</ref> The superconducting phase is only observed in the oxygen reduced film and is not seen in oxygen reduced bulk material of the same stoichiometry, suggesting that the strain induced by the oxygen reduction of the Nd<sub>0.8</sub>Sr<sub>0.2</sub>NiO<sub>2</sub> thin film changes the phase space to allow for superconductivity.<ref>
{{cite journal
 |last1= Li |first1= Q.
 |last2= He |first2= C.
 |display-authors=etal
 |year=2020
 |title= Absence of supercondutivity in bulk Nd<sub>1−x</sub>Sr<sub>x</sub>NiO<sub>2</sub>
 |journal= Communications Materials
 |volume= 1 |issue= 1
 |page= 16
 |doi= 10.1038/s43246-020-0018-1 |arxiv= 1911.02420
 |bibcode= 2020CoMat...1...16L
 |doi-access= free  |s2cid= 208006588
}}
</ref>
Of important is further to extract access hydrogen from the reduction with CaH<sub>2</sub>, otherwise [[Topotactic transition|topotactic]] hydrogen may prevent superconductivity.
<ref>
{{cite journal
 |last1= Si |first1= L.
 |last2= Xiao |first2= W.
 |last3= Kaufmann |first3= J.
 |last4= Tomczak |first4= J. M.
 |last5= Lu |first5= X.
 |last6= Zhong |first6= Z.
 |last7= Held |first7= K.
 |display-authors=etal
 |year=2020
 |title= Topotactic Hydrogen in Nickelate Superconductors and Akin Infinite-Layer Oxides ABO<sub>2</sub>
 |url= https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.124.166402
 |journal= Physical Review Letters
 |volume= 124 |issue= 1
 |page= 166402 
 |doi= 10.1103/PhysRevLett.124.166402
 |pmid= 32383925
 |arxiv= 1911.06917  
 |bibcode= 2020PhRvL.124p6402S
 |s2cid= 208139397
 }}
</ref>