Crystal structure of LnFeAsOF, a 1111-type ferropnictide compound. Ln = lanthanide (La, Ce, Yb, Nd, Gd, Sm, etc.), Pn = pnictide (As, P, N, Bi, etc.)[1]
Iron-based superconductors (FeSC) are iron-containing chemical compounds whose superconducting properties were discovered in 2006.[2][3] The first of such superconducting compounds belong to the group of oxypnictides, which was known since 1995.[4] Until 2006, however, they were in the first stages of experimentation and implementation[5] and only the semiconductive properties of these compounds were known and patented.[6]Scientific American described subsequent research as follows:
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.[7]
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.[7]
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.[8] It has also been found that some iron chalcogens and crystallogens superconduct.[9][10]
Iron-based superconductors are classified according to their crystal structure and chemical formula into the following main families,
1111-type, with representative compounds LaFePO,[2] LaFeAsO,[3] SmFeAsO,[11] PrFeAsO,[12][13] and LaFeSiH.[14]
122-type such as BaFe2As2,[23] SrFe2As2[24] and CaFe2As2[25]
Superconductivity is obtained either in the parent phases of some of these systems (e.g. LaFePO,[2] LaFeSiH,[14] and LiFeAs[15][16][17]) or by means of doping or applied pressure.[8][26][27]
Undoped β-FeSe is the simplest iron-based superconductor but with distinct properties.[22] It has a critical temperature (Tc) of 8 K at normal pressure, and 36.7 K under high pressure[28] and by means of intercalation. The combination of both intercalation and higher pressure results in re-emerging superconductivity at Tc of up to 48 K (see,[22][29] 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.[41] Noteworthy is the synthesis of (Ca4Al2O6−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).[34] High-pressure technique also yields (Ca3Al2O5−y)(Fe2Pn2) (Pn = As and P), the first reported iron-based superconductors with the perovskite-based '32522' structure. The transition temperature (Tc) 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 Tc in iron-based superconductors.[33]
In 2009, it was shown that undoped iron pnictides had a magnetic quantum critical point deriving from competition between electronic localization and itinerancy.[42]
Phase diagram of the 122 family of ferro-pnictides complemented by the 122(Se) family as a generalized phase diagram for the iron based superconductors.[43]
Similarly to superconducting cuprates, the properties of iron based superconductors change dramatically with doping. Parent compounds of FeSC are usually metals (unlike the cuprates) but, similarly to cuprates, are ordered antiferromagnetically that often termed as a spin-density wave (SDW). Some parent compounds superconduct.[2][14][15][16][17] Otherwise, superconductivity emerges upon either hole or electron doping. In general, the phase diagram is similar to the cuprates.[43]
Simplified doping dependent phase diagrams of iron-based superconductors for both Ln-1111 and Ba-122 materials. The phases shown are the antiferromagnetic/spin density wave (AF/SDW) phase close to zero doping and the superconducting phase around optimal doping. The Ln-1111 phase diagrams for La[44] and Sm[45][46] were determined using muon spin spectroscopy, the phase diagram for Ce[47] was determined using neutron diffraction. The Ba-122 phase diagram is based on.[48]
Superconducting transition temperatures are listed in the tables (some at high pressure). BaFe1.8Co0.2As2 is predicted to have an upper critical field of 43 tesla from the measured coherence length of 2.8 nm.[36]
In 2011, Japanese scientists made a discovery which increased a metal compound's superconductivity by immersing iron-based compounds in hot alcoholic beverages such as red wine.[49][50] Earlier reports indicated that excess Fe is the cause of the bicollinear antiferromagnetic order and is not in favor of superconductivity. Further investigation revealed that weak acid has the ability to deintercalate the excess Fe from the interlayer sites. Therefore, weak acid annealing suppresses the antiferromagnetic correlation by deintercalating the excess Fe and, hence superconductivity is achieved.[51][52]
There is an empirical correlation of the transition temperature with electronic band structure: the Tc maximum is observed when some of the Fermi surface stays in proximity to Lifshitz topological transition.[43] Similar correlation has been later reported for high-Tc cuprates that indicates possible similarity of the superconductivity mechanisms in these two families of high temperature superconductors.[53]
Thin films
The critical temperature is increased further in thin-films of iron chalcogenides on suitable substrates. In 2015, a Tc of around 105–111 K was observed in thin films of iron selenide grown on strontium titanate.[54]
↑ 3.03.1Kamihara, Yoichi; Watanabe, Takumi; Hirano, Masahiro; Hosono, Hideo (2008). "Iron-Based Layered Superconductor La[O1−xFx]FeAs (x = 0.05–0.12) with Tc = 26 K". Journal of the American Chemical Society130 (11): 3296–3297. doi:10.1021/ja800073m. PMID18293989.
↑Zimmer, Barbara I.; Jeitschko, Wolfgang; Albering, Jörg H.; Glaum, Robert; Reehuis, Manfred (1995). "The rate earth transition metal phosphide oxides LnFePO, LnRuPO and LnCoPO with ZrCuSiAs type structure". Journal of Alloys and Compounds229 (2): 238–242. doi:10.1016/0925-8388(95)01672-4.
↑ 12.012.1Ren, Z. A.; Yang, J.; Lu, W.; Yi, W.; Che, G. C.; Dong, X. L.; Sun, L. L.; Zhao, Z. X. (2008). "Superconductivity at 52 K in iron based F doped layered quaternary compound Pr[O1−xFx]FeAs". Materials Research Innovations12 (3): 105–106. doi:10.1179/143307508X333686. Bibcode: 2008MatRI..12..105R.
↑Ren, Zhi-An; Che, Guang-Can; Dong, Xiao-Li; Yang, Jie; Lu, Wei; Yi, Wei; Shen, Xiao-Li; Li, Zheng-Cai et al. (2008). "Superconductivity and phase diagram in iron-based arsenic-oxides ReFeAsO1−δ (Re = rare-earth metal) without fluorine doping". EPL83 (1). doi:10.1209/0295-5075/83/17002. Bibcode: 2008EL.....8317002R.
↑ 16.016.116.216.3Pitcher, Michael J.; Parker, Dinah R.; Adamson, Paul; Herkelrath, Sebastian J. C.; Boothroyd, Andrew T.; Ibberson, Richard M.; Brunelli, Michela; Clarke, Simon J. (2008). "Structure and superconductivity of LiFeAs". Chemical Communications (45): 5918–20. doi:10.1039/b813153h. PMID19030538.
↑ 18.018.1Chu, C.W.; Chen, F.; Gooch, M.; Guloy, A.M.; Lorenz, B.; Lv, B.; Sasmal, K.; Tang, Z.J. et al. (2009). "The synthesis and characterization of LiFeAs and NaFeAs". Physica C: Superconductivity469 (9–12): 326–331. doi:10.1016/j.physc.2009.03.016. Bibcode: 2009PhyC..469..326C.
↑ 19.019.1Parker, Dinah R.; Pitcher, Michael J.; Clarke, Simon J. (2008). "Structure and superconductivity of the layered iron arsenide NaFeAs". Chemical Communications2189 (16): 2189–91. doi:10.1039/B818911K. PMID19360189.
↑Zhang, S. J.; Wang, X. C.; Liu, Q. Q.; Lv, Y. X.; Yu, X. H.; Lin, Z. J.; Zhao, Y. S.; Wang, L. et al. (2009). "Superconductivity at 31 K in the "111"-type iron arsenide superconductor Na1−xFeAs induced by pressure". EPL88 (4). doi:10.1209/0295-5075/88/47008. Bibcode: 2009EL.....8847008Z.
↑Deng, Z.; Wang, X. C.; Liu, Q. Q.; Zhang, S. J.; Lv, Y. X.; Zhu, J. L.; Yu, R. C.; Jin, C. Q. (2009). "A new "111" type iron pnictide superconductor LiFeP". EPL87 (3). doi:10.1209/0295-5075/87/37004. Bibcode: 2009EL.....8737004D.
↑Medvedev, S.; McQueen, T. M.; Troyan, I. A.; Palasyuk, T.; Eremets, M. I.; Cava, R. J.; Naghavi, S.; Casper, F. et al. (2009). "Electronic and Magnetic Phase Diagram of β-Fe1.01Se with superconductivity at 36.7 K under pressure". Nature Materials8 (8): 630–633. doi:10.1038/nmat2491. PMID19525948. Bibcode: 2009NatMa...8..630M.
↑Sun, Liling; Chen, Xiao-Jia; Guo, Jing; Gao, Peiwen; Huang, Qing-Zhen; Wang, Hangdong; Fang, Minghu; Chen, Xiaolong et al. (2012). "Re-emerging superconductivity at 48 kelvin in iron chalcogenides". Nature483 (7387): 67–69. doi:10.1038/nature10813. PMID22367543. Bibcode: 2012Natur.483...67S.
↑Prakash, J.; Singh, S. J.; Samal, S. L.; Patnaik, S.; Ganguli, A. K. (2008). "Potassium fluoride doped LaOFeAs multi-band superconductor: Evidence of extremely high upper critical field". EPL84 (5). doi:10.1209/0295-5075/84/57003. Bibcode: 2008EL.....8457003P.
↑Shirage, Parasharam M.; Miyazawa, Kiichi; Kito, Hijiri; Eisaki, Hiroshi; Iyo, Akira (2008). "Superconductivity at 43 K at ambient pressure in the iron-based layered compound La1−xYxFeAsOy". Physical Review B78 (17). doi:10.1103/PhysRevB.78.172503. Bibcode: 2008PhRvB..78q2503S.
↑Shirage, Parasharam M.; Miyazawa, Kiichi; Kihou, Kunihiro; Lee, Chul-Ho; Kito, Hijiri; Tokiwa, Kazuyasu; Tanaka, Yasumoto; Eisaki, Hiroshi et al. (2010). "Synthesis of ErFeAsO-based superconductors by the hydrogen doping method". EPL92 (5). doi:10.1209/0295-5075/92/57011. Bibcode: 2010EL.....9257011S.
↑ 33.033.1Shirage, Parasharam M.; Kihou, Kunihiro; Lee, Chul-Ho; Kito, Hijiri; Eisaki, Hiroshi; Iyo, Akira (2011). "Emergence of Superconductivity in "32522" Structure of (Ca3Al2O5−y)(Fe2Pn2) (Pn = As and P)". Journal of the American Chemical Society133 (25): 9630–3. doi:10.1021/ja110729m. PMID21627302.
↑ 34.034.1Shirage, Parasharam M.; Kihou, Kunihiro; Lee, Chul-Ho; Kito, Hijiri; Eisaki, Hiroshi; Iyo, Akira (2010). "Superconductivity at 28.3 and 17.1 K in (Ca4Al2O6−y)(Fe2Pn2) (Pn=As and P)". Applied Physics Letters97 (17): 172506. doi:10.1063/1.3508957. Bibcode: 2010ApPhL..97q2506S.
↑Yang, Jie; Li, Zheng-Cai; Lu, Wei; Yi, Wei; Shen, Xiao-Li; Ren, Zhi-An; Che, Guang-Can; Dong, Xiao-Li et al. (2008). "Superconductivity at 53.5 K in GdFeAsO1−δ". Superconductor Science and Technology21 (8). doi:10.1088/0953-2048/21/8/082001. Bibcode: 2008SuScT..21h2001Y.
↑ 36.036.1Yin, Yi; Zech, M.; Williams, T. L.; Wang, X. F.; Wu, G.; Chen, X. H.; Hoffman, J. E. (2009). "Scanning Tunneling Spectroscopy and Vortex Imaging in the Iron Pnictide Superconductor BaFe1.8Co0.2As2". Physical Review Letters102 (9). doi:10.1103/PhysRevLett.102.097002. PMID19392555. Bibcode: 2009PhRvL.102i7002Y.
↑Satoru Matsuishi; Yasunori Inoue; Takatoshi Nomura; Hiroshi Yanagi; Masahiro Hirano; Hideo Hosono (2008). "Superconductivity Induced by Co-Doping in Quaternary Fluoroarsenide CaFeAsF". J. Am. Chem. Soc.130 (44): 14428–14429. doi:10.1021/ja806357j. PMID18842039.
↑Mizuguchi, Yoshikazu; Tomioka, Fumiaki; Tsuda, Shunsuke; Yamaguchi, Takahide; Takano, Yoshihiko (2008). "Superconductivity at 27 K in tetragonal FeSe under high pressure". Appl. Phys. Lett.93 (15): 152505. doi:10.1063/1.3000616. Bibcode: 2008ApPhL..93o2505M.
↑Yates, K A; Usman, I T M; Morrison, K; Moore, J D; Gilbertson, A M; Caplin, A D; Cohen, L F; Ogino, H et al. (2010). "Evidence for nodal superconductivity in Sr2ScFePO3". Superconductor Science and Technology23 (2). doi:10.1088/0953-2048/23/2/022001. Bibcode: 2010SuScT..23b2001Y.
↑Luetkens, H; Klauss, H. H.; Kraken, M; Litterst, F. J.; Dellmann, T; Klingeler, R; Hess, C; Khasanov, R et al. (2009). "Electronic phase diagram of the LaO1−xFxFeAs superconductor". Nature Materials8 (4): 305–9. doi:10.1038/nmat2397. PMID19234445. Bibcode: 2009NatMa...8..305L.
↑Drew, A. J.; Niedermayer, Ch; Baker, P. J.; Pratt, F. L.; Blundell, S. J.; Lancaster, T; Liu, R. H.; Wu, G et al. (2009). "Coexistence of static magnetism and superconductivity in SmFeAsO1−xFx as revealed by muon spin rotation". Nature Materials8 (4): 310–314. doi:10.1038/nmat2396. PMID19234446. Bibcode: 2009NatMa...8..310D.
↑Sanna, S.; De Renzi, R.; Lamura, G.; Ferdeghini, C.; Palenzona, A.; Putti, M.; Tropeano, M.; Shiroka, T. (2009). "Competition between magnetism and superconductivity at the phase boundary of doped SmFeAsO pnictides". Physical Review B80 (5). doi:10.1103/PhysRevB.80.052503. Bibcode: 2009PhRvB..80e2503S.
↑Zhao, J; Huang, Q; de la Cruz, C; Li, S; Lynn, J. W.; Chen, Y; Green, M. A.; Chen, G. F. et al. (2008). "Structural and magnetic phase diagram of CeFeAsO1−xFx and its relation to high-temperature superconductivity". Nature Materials7 (12): 953–959. doi:10.1038/nmat2315. PMID18953342. Bibcode: 2008NatMa...7..953Z.
↑Chu, Jiun-Haw; Analytis, James; Kucharczyk, Chris; Fisher, Ian (2009). "Determination of the phase diagram of the electron doped superconductor Ba(Fe1−xCox)2As2". Physical Review B79 (1). doi:10.1103/PhysRevB.79.014506. Bibcode: 2009PhRvB..79a4506C.
↑Deguchi, K; Sato, D; Sugimoto, M; Hara, H; Kawasaki, Y; Demura, S; Watanabe, T; Denholme, S J et al. (2012). "Clarification as to why alcoholic beverages have the ability to induce superconductivity in Fe1+dTe1−xSx". Superconductor Science and Technology25 (8). doi:10.1088/0953-2048/25/8/084025. Bibcode: 2012SuScT..25h4025D.
↑A. A. Kordyuk (2018). "Electronic band structure of optimal superconductors: from cuprates to ferropnictides and back again (Review Article)". Low Temp. Phys.44 (6): 477–486. doi:10.1063/1.5037550. Bibcode: 2018LTP....44..477K.