Iron-based superconductor

File:LnFePnOFstructure.png (La, Ce, Yb, Nd, Gd, Sm, etc.), Pn = pnictide (As, P, N, Bi, etc.){{Cite journal | doi = 10.1088/1468-6996/16/3/033503| pmid = 27877784| title = Exploration of new superconductors and functional materials, and fabrication of superconducting tapes and wires of iron pnictides| journal = Science and Technology of Advanced Materials| volume = 16| issue = 3| page = 033503| year = 2015| last1 = Hosono | first1 = H. | last2 = Tanabe | first2 = K. | last3 = Takayama-Muromachi | first3 = E. | last4 = Kageyama | first4 = H. | last5 = Yamanaka | first5 = S. | last6 = Kumakura | first6 = H. | last7 = Nohara | first7 = M. | last8 = Hiramatsu | first8 = H. | last9 = Fujitsu | first9 = S. |pmc=5099821|bibcode = 2015STAdM..16c3503H |arxiv = 1505.02240}}]]

Iron-based superconductors (FeSC) are iron-containing chemical compounds whose superconducting properties were discovered in 2006.{{cite journal|title = Iron-Based Layered Superconductor: LaOFeP |author1=Kamihara, Yoichi |author2=Hiramatsu, Hidenori |author3=Hirano, Masahiro |author4=Kawamura, Ryuto |author5=Yanagi, Hiroshi |author6=Kamiya, Toshio |author7=Hosono, Hideo |journal = J. Am. Chem. Soc.|volume = 128|issue = 31|pages = 10012–10013|year = 2006|doi = 10.1021/ja063355c|pmid = 16881620}}{{cite journal|author1=Kamihara, Yoichi |author2=Watanabe, Takumi |author3=Hirano, Masahiro |author4=Hosono, Hideo |journal=Journal of the American Chemical Society|volume=130|pages=3296–3297|year=2008|doi=10.1021/ja800073m|pmid=18293989|title = Iron-Based Layered Superconductor La[O_1−xF_x]FeAs (x = 0.05–0.12) with T_c = 26 K|issue=11}} The first of such superconducting compounds belong to the group of oxypnictides, which was known since 1995.{{cite journal |author1=Zimmer, Barbara I. |author2=Jeitschko, Wolfgang |author3=Albering, Jörg H. |author4=Glaum, Robert |author5=Reehuis, Manfred |year=1995 |title=The rate earth transition metal phosphide oxides LnFePO, LnRuPO and LnCoPO with ZrCuSiAs type structure |journal=Journal of Alloys and Compounds |volume=229 |issue=2 |pages=238–242 |doi=10.1016/0925-8388(95)01672-4}} Until 2006, however, they were in the first stages of experimentation and implementation{{cite journal |author1=Ozawa, T C |author2=Kauzlarich, S M |year=2008 |title=Chemistry of layered d-metal pnictide oxides and their potential as candidates for new superconductors |journal=Sci. Technol. Adv. Mater. |volume=9 |issue=3 |page=033003 |arxiv=0808.1158 |bibcode=2008STAdM...9c3003O |doi=10.1088/1468-6996/9/3/033003 |pmc=5099654 |pmid=27877997}} {{Open access}} and only the semiconductive properties of these compounds were known and patented.Hosono, H. et al. (2006) [http://www.freepatentsonline.com/EP1868215.html Magnetic semiconductor material] European Patent Application EP1868215 Previously most high-temperature superconductors were cuprates containing copper - oxygen layers. Much of the interest in iron-based superconductors is precisely because of the differences from the cuprates, which may help lead to a theory of non-BCS-theory superconductivity.[http://www.sciam.com/article.cfm?id=iron-exposed-as-high-temp-superconductor "Iron Exposed as High-Temperature Superconductor"]. Scientific American. June 2008

Iron-based superconductors of the group of oxypnictides were initially called ferropnictides. The crystal structure of these compounds displays conducting layers of iron and a pnictogen (typically arsenic (As) and phosphorus (P)) separated by a charge-reservoir block. It has also been found that some iron chalcogens and crystallogens superconduct.{{cite journal|doi=10.1103/Physics.1.28|title=The iron age of superconductivity|year=2008|last1=Johannes|first1=Michelle|author1-link=Michelle D. Johannes|journal=Physics|volume=1|page=28|bibcode = 2008PhyOJ...1...28J |url=https://physics.aps.org/featured-article-pdf/10.1103/PhysRevB.78.134514|doi-access=free}}

{{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.

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|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

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{{blockquote|The crystalline material, known chemically as LaOFeAs, stacks iron and arsenic layers, where the electrons flow, between planes of lanthanum and oxygen. Replacing up to 11 percent of the oxygen with fluorine improved the compound – it became superconductive at 26 kelvin, the team reports in the March 19, 2008 Journal of the American Chemical Society. Subsequent research from other groups suggests that replacing the lanthanum in LaOFeAs with other rare earth elements such as cerium, samarium, neodymium and praseodymium leads to superconductors that work at 52 kelvin.}}

Iron-based superconductors are classified according to their crystal structure and chemical formula into the following main families,

• 1111-type, with representative compounds LaFePO, LaFeAsO, SmFeAsO, PrFeAsO,{{cite journal |author=Ren, Zhi-An |last2=Che |first2=Guang-Can |last3=Dong |first3=Xiao-Li |last4=Yang |first4=Jie |last5=Lu |first5=Wei |last6=Yi |first6=Wei |last7=Shen |first7=Xiao-Li |last8=Li |first8=Zheng-Cai |last9=Sun |first9=Li-Ling |last10=Zhou |first10=Fang |last11=Zhao |first11=Zhong-Xian |year=2008 |title=Superconductivity and phase diagram in iron-based arsenic-oxides ReFeAsO_1−δ (Re = rare-earth metal) without fluorine doping |journal=EPL |volume=83 |issue=1 |page=17002 |arxiv=0804.2582 |bibcode=2008EL.....8317002R |doi=10.1209/0295-5075/83/17002 |s2cid=96240327}} and LaFeSiH.
• 111-type such as LiFeAs, NaFeAs,{{cite journal |last1=Zhang |first1=S. J. |last2=Wang |first2=X. C. |last3=Liu |first3=Q. Q. |last4=Lv |first4=Y. X. |last5=Yu |first5=X. H. |last6=Lin |first6=Z. J. |last7=Zhao |first7=Y. S. |last8=Wang |first8=L. |last9=Ding |first9=Y. |last10=Mao |first10=H. K. |last11=Jin |first11=C. Q. |year=2009 |title=Superconductivity at 31 K in the "111"-type iron arsenide superconductor Na_1−xFeAs induced by pressure |journal=EPL |volume=88 |issue=4 |page=47008 |arxiv=0912.2025 |bibcode=2009EL.....8847008Z |doi=10.1209/0295-5075/88/47008 |s2cid=55588819}} and LiFeP.{{cite journal |last1=Deng |first1=Z. |last2=Wang |first2=X. C. |last3=Liu |first3=Q. Q. |last4=Zhang |first4=S. J. |last5=Lv |first5=Y. X. |last6=Zhu |first6=J. L. |last7=Yu |first7=R. C. |last8=Jin |first8=C. Q. |year=2009 |title=A new "111" type iron pnictide superconductor LiFeP |journal=EPL |volume=87 |issue=3 |page=37004 |arxiv=0908.4043 |bibcode=2009EL.....8737004D |doi=10.1209/0295-5075/87/37004 |s2cid=119227185}}
• 11-type FeSe
• 122-type such as BaFe₂As₂, SrFe₂As₂{{cite journal |last1=Sasmal |first1=K. |last2=Lv |first2=Bing |last3=Lorenz |first3=Bernd |last4=Guloy |first4=Arnold M. |last5=Chen |first5=Feng |last6=Xue |first6=Yu-Yi |last7=Chu |first7=Ching-Wu |year=2008 |title=Superconducting Fe-Based Compounds (A_1−xSr_x) Fe₂As₂ with A=K and Cs with Transition Temperatures up to 37 K |url=http://repository.ust.hk/ir/bitstream/1783.1-18311/1/PhysRevLett.101.107007.pdf |journal=Physical Review Letters |volume=101 |issue=10 |page=107007 |arxiv=0806.1301 |bibcode=2008PhRvL.101j7007S |doi=10.1103/physrevlett.101.107007 |pmid=18851250 |s2cid=2425512}} and CaFe₂As₂

Superconductivity is obtained either in the parent phases of some of these systems (e.g. LaFePO, LaFeSiH, and LiFeAs) or by means of doping or applied pressure.{{cite journal |author=Day, C. |year=2009 |title=Iron-based superconductors |journal=Physics Today |volume=62 |issue=8 |pages=36–40 |bibcode=2009PhT....62h..36D |doi=10.1063/1.3206093 |doi-access=free}}{{cite journal |author=Stewart, G. R. |year=2011 |title=Superconductivity in iron compounds |journal=Rev. Mod. Phys. |volume=83 |issue=4 |pages=1589–1652 |arxiv=1106.1618 |bibcode=2011RvMP...83.1589S |doi=10.1103/revmodphys.83.1589 |s2cid=119238477}}

Undoped β-FeSe is the simplest iron-based superconductor but with distinct properties.{{Cite journal|author1=Yu. V. Pustovit |author2=A. A. Kordyuk |year=2016 |title=Metamorphoses of electronic structure of FeSe-based superconductors (Review article) |journal=Low Temp. Phys. |volume=42 |issue=11 |arxiv=1608.07751|bibcode=2016LTP....42..995P |doi=10.1063/1.4969896 |pages=995–1007 |s2cid=119184569 }} It has a critical temperature (T_c) of 8 K at normal pressure, and 36.7 K under high pressure{{cite journal|doi=10.1038/nmat2491|pmid=19525948|title=Electronic and Magnetic Phase Diagram of β-Fe_1.01Se with superconductivity at 36.7 K under pressure|year=2009|last1=Medvedev|first1=S.|last2=McQueen|first2=T. M.|last3=Troyan|first3=I. A.|last4=Palasyuk|first4=T.|last5=Eremets|author5-link=Mikhail Eremets|first5=M. I.|last6=Cava|first6=R. J.|last7=Naghavi|first7=S.|last8=Casper|first8=F.|last9=Ksenofontov|first9=V.|last10=Wortmann|first10=G.|last11=Felser|first11=C.|journal=Nature Materials|volume=8|issue=8|pages=630–633|arxiv = 0903.2143 |bibcode = 2009NatMa...8..630M |s2cid=117714394 }} and by means of intercalation. The combination of both intercalation and higher pressure results in re-emerging superconductivity at T_c of up to 48 K (see,{{Cite journal|author1=Sun, Liling |author2=Chen, Xiao-Jia |author3= Guo, Jing |author4= Gao, Peiwen |author5= Huang, Qing-Zhen |author6= Wang, Hangdong |author7= Fang, Minghu |author8= Chen, Xiaolong |author9= Chen, Genfu |author10= Wu, Qi |author11= Zhang, Chao |author12= Gu, Dachun |author13= Dong, Xiaoli |author14= Wang, Lin |author15= Yang, Ke |author16= Li, Aiguo |author17= Dai, Xi |author18= Mao, Ho-kwang |author19=Zhao, Zhongxian |year=2012 |title= Re-emerging superconductivity at 48 kelvin in iron chalcogenides |journal= Nature |volume= 483 | number = 7387|pages = 67–69|doi= 10.1038/nature10813 |pmid=22367543 |arxiv= 1110.2600 |bibcode=2012Natur.483...67S }} and references therein).

Compared with other families, the synthesis of the 122 compounds is relatively easy which facilitates the investigation of these systems.

Compounds such as Sr₂ScFePO₃ discovered in 2009 are referred to as the '42622' family, as FePSr₂ScO₃.{{cite journal | doi=10.1088/0953-2048/23/2/022001 | title=Evidence for nodal superconductivity in Sr₂ScFePO₃ | year=2010 | last1=Yates | first1=K A | last2=Usman | first2=I T M | last3=Morrison | first3=K | last4=Moore | first4=J D | last5=Gilbertson | first5=A M | last6=Caplin | first6=A D | last7=Cohen | first7=L F | last8=Ogino | first8=H | last9=Shimoyama | first9=J | journal=Superconductor Science and Technology | volume=23 | issue=2 | page=022001 | arxiv = 0908.2902 | bibcode = 2010SuScT..23b2001Y| s2cid=119248392 }} Noteworthy is the synthesis of (Ca₄Al₂O_6−y)(Fe₂Pn₂) (or Al-42622(Pn); Pn = As and P) using high-pressure synthesis technique. Al-42622(Pn) exhibit superconductivity for both Pn = As and P with the transition temperatures of 28.3 K and 17.1 K, respectively. The a-lattice parameters of Al-42622(Pn) (a = 3.713 Å and 3.692 Å for Pn = As and P, respectively) are smallest among the iron-pnictide superconductors. Correspondingly, Al-42622(As) has the smallest As–Fe–As bond angle (102.1°) and the largest As distance from the Fe planes (1.5 Å). High-pressure technique also yields (Ca₃Al₂O_5−y)(Fe₂Pn₂) (Pn = As and P), the first reported iron-based superconductors with the perovskite-based '32522' structure. The transition temperature (T_c) is 30.2 K for Pn = As and 16.6 K for Pn = P. The emergence of superconductivity is ascribed to the small tetragonal a-axis lattice constant of these materials. From these results, an empirical relationship was established between the a-axis lattice constant and T_c in iron-based superconductors.

In 2009, it was shown that undoped iron pnictides had a magnetic quantum critical point deriving from competition between electronic localization and itinerancy.{{Cite journal|last1=Dai|first1=Jianhui|last2=Si|first2=Qimiao|last3=Zhu|first3=Jian-Xin|last4=Abrahams|first4=Elihu|date=2009-03-17|title=Iron pnictides as a new setting for quantum criticality|journal=Proceedings of the National Academy of Sciences|language=en|volume=106|issue=11|pages=4118–4121|doi=10.1073/pnas.0900886106|issn=0027-8424|pmc=2657431|pmid=19273850|arxiv=0808.0305|bibcode=2009PNAS..106.4118D|doi-access=free}}

File:Phase diagram of the 122 family of ferro-pnictides.png