Editing High-temperature superconductivity (section)

=== 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>