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[[File:AEFe2Pn2 structure.png|thumb|Crystal structure of 122-type AEFe2Pn2 superconductors, AE = alkaline earth metal (Ca, Se, etc.), Pn = pnictide (As, P, etc.)<ref name=rev1>{{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| article-number = 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}}</ref>]]
The '''122 iron arsenide''' unconventional superconductors are part of a new class of [[Physics:Iron-based superconductor|iron-based superconductor]]s. They form in the tetragonal I4/mmm, ThCr2Si2 type, crystal structure. The shorthand name "122" comes from their [[Chemistry:Stoichiometry|stoichiometry]]; the 122s have the [[Chemistry:Chemical formula|chemical formula]] AEFe2Pn2, where AE stands for [[Chemistry:Alkaline earth metal|alkaline earth metal]] (Ca, Ba Sr or Eu) and Pn is pnictide (As, P, etc.).<ref name=rev1 /><ref name=Krey>{{Cite journal | doi = 10.1103/PhysRevB.78.184517|arxiv = 0807.3032 |bibcode = 2008PhRvB..78r4517K| title =Pressure-induced volume-collapsed tetragonal phase of CaFe2As2 as seen via neutron scattering| journal = Physical Review B| volume = 78| issue = 18|article-number = 184517 | year = 2008| last1 = Kreyssig | first1 = A.| last2 = Green | first2 = M. A.| last3 = Lee | first3 = Y.| last4 = Samolyuk | first4 = G. D.| last5 = Zajdel | first5 = P.| last6 = Lynn | first6 = J. W.| last7 = Bud'ko | first7 = S. L.| last8 = Torikachvili | first8 = M. S.| last9 = Ni | first9 = N.| last10 = Nandi | first10 = S.| last11 = Leão | first11 = J. B.| last12 = Poulton | first12 = S. J.| last13 = Argyriou | first13 = D. N.| last14 = Harmon | first14 = B. N.| last15 = McQueeney | first15 = R. J.| last16 = Canfield | first16 = P. C.| last17 = Goldman | first17 = A. I.|s2cid = 118703521 }}</ref><ref name=Tegel>{{Cite journal | doi = 10.1088/0953-8984/20/45/452201| title = Structural and magnetic phase transitions in the ternary iron arsenides SrFe2As2 and EuFe2As2| journal = Journal of Physics: Condensed Matter| volume = 20| issue = 45| article-number = 452201| year = 2008| last1 = Tegel | first1 = M. | last2 = Rotter | first2 = M. | last3 = Weiß | first3 = V. | last4 = Schappacher | first4 = F. M. | last5 = Pöttgen | first5 = R. | last6 = Johrendt | first6 = D. | bibcode = 2008JPCM...20S2201T|arxiv = 0806.4782 | s2cid = 15023921}}</ref> These materials become superconducting under pressure and also upon doping.<ref>{{Cite journal | doi = 10.1143/APEX.1.081702| title = Superconductivity at 26 K in (Ca1−xNax)Fe2As2| journal = Applied Physics Express| volume = 1| article-number = 081702| year = 2008| last1 = Shirage | first1 = P. M. | last2 = Miyazawa | first2 = K. | last3 = Kito | first3 = H. | last4 = Eisaki | first4 = H. | last5 = Iyo | first5 = A. | issue = 8| bibcode = 2008APExp...1h1702M| s2cid = 94498268}}</ref><ref name=Park>{{Cite journal | doi = 10.1088/0953-8984/20/32/322204| title = Pressure-induced superconductivity in CaFe2As2| journal = Journal of Physics: Condensed Matter| volume = 20| issue = 32| article-number = 322204| year = 2008| last1 = Park | first1 = T. | last2 = Park | first2 = E. | last3 = Lee | first3 = H. | last4 = Klimczuk | first4 = T. | last5 = Bauer | first5 = E. D. | last6 = Ronning | first6 = F. | last7 = Thompson | first7 = J. D. | bibcode = 2008JPCM...20F2204P| arxiv = 0807.0800 | s2cid = 94568851}}</ref><ref name=Alireza2008>{{Cite journal | doi = 10.1088/0953-8984/21/1/012208|arxiv = 0807.1896 |bibcode = 2009JPCM...21a2208A | title = Superconductivity up to 29 K in SrFe2As2 and BaFe2As2 at high pressures| journal = Journal of Physics: Condensed Matter| volume = 21| article-number = 012208| year = 2009| last1 = Alireza | first1 = P. L. | last2 = Ko | first2 = Y. T. C. | last3 = Gillett | first3 = J. | last4 = Petrone | first4 = C. M. | last5 = Cole | first5 = J. M. | last6 = Lonzarich | first6 = G. G. | last7 = Sebastian | first7 = S. E. |issue = 1 |pmid = 21817209 |s2cid = 206027136 }}</ref><ref name=Anupam2009>{{Cite journal | doi = 10.1088/0953-8984/21/26/265701|arxiv = 0812.1131|bibcode = 2009JPCM...21z5701A| title = Superconductivity and magnetism in K-doped EuFe2As2| journal = Journal of Physics: Condensed Matter| volume = 21| issue = 26| article-number = 265701| year = 2009| last1 = Anupam| last2 = Paulose | first2 = P. L. | last3 = Jeevan | first3 = H. S. | last4 = Geibel | first4 = C. | last5 = Hossain | first5 = Z. |pmid = 21828476|s2cid = 18250230}}</ref> The maximum superconducting transition temperature found to date is 38 K in the Ba0.6K0.4Fe2As2.<ref name=Rotter2008>{{Cite journal | doi = 10.1103/PhysRevLett.101.107006|pmid = 18851249 |bibcode=2008PhRvL.101j7006R|arxiv = 0805.4630| title = Superconductivity at 38 K in the Iron Arsenide (Ba1−xKx)Fe2As2| journal = Physical Review Letters| volume = 101| issue = 10|article-number = 107006 | year = 2008| last1 = Rotter | first1 = M. | last2 = Tegel | first2 = M. | last3 = Johrendt | first3 = D. |s2cid = 25876149 }}</ref> The microscopic description of superconductivity in the 122s is yet unclear.<ref name=Pickett2009>{{cite journal
|title = Iron-based superconductors: Timing is crucial
|author = Pickett, Warren E.
|journal = Nature Physics
|volume = 5
|pages = 87–88
|year = 2009
|doi = 10.1038/nphys1192
|issue=2|bibcode = 2009NatPh...5...87P |url = https://zenodo.org/record/896844
}}</ref>

== Overview ==
Ever since the discovery of high-temperature (High Tc) superconductivity in the cuprate materials, scientists have worked tirelessly to understand the microscopic mechanisms responsible for its emergence. To this day, no theory can fully explain the [[Physics:High-temperature superconductivity|high-temperature superconductivity]] and unconventional (non-s-wave) pairing state found in these materials.<ref name=NatureSCReview2006>{{Cite journal | doi = 10.1038/nphys253|bibcode=2006NatPh...2..138.| title = Towards a complete theory of high TC | journal = Nature Physics| volume = 2| issue = 3| pages = 138–143| year = 2006| hdl = 1887/5125|s2cid=119554620| hdl-access = free}}</ref> However, the interest of the scientific community in understanding the pairing glue for unconventional superconductors—those in which the glue is electronic, i.e. cannot be attributed to the [[Software:Phonon|phonon]]-induced interactions between electrons responsible for conventional [[Physics:BCS theory|BCS theory]] s-wave superconductivity—has recently been expanded by the discovery of high temperature superconductivity (up to Tc = 55 K) in the doped [[Physics:Oxypnictide|oxypnictide]] (1111) superconductors with the chemical composition XOFeAs, where X = La, Ce, Pr, Nd, Sm, Gd, Tb, or Dy.<ref name=Takahashi2008>{{Cite journal | doi = 10.1038/nature06972| pmid = 18432191 |url=https://www.researchgate.net/publication/5421101| title = Superconductivity at 43 K in an iron-based layered compound LaO1−xFxFeAs| journal = Nature| volume = 453| issue = 7193| pages = 376–8| year = 2008| last1 = Takahashi | first1 = H. | last2 = Igawa | first2 = K. | last3 = Arii | first3 = K. | last4 = Kamihara | first4 = Y. | last5 = Hirano | first5 = M. | last6 = Hosono | first6 = H. |bibcode = 2008Natur.453..376T | s2cid = 498756 }}</ref><ref name=Huang2008>{{Cite journal | doi = 10.1103/PhysRevLett.101.257003|pmid = 19113744 |bibcode=2008PhRvL.101y7003H| title = Neutron-Diffraction Measurements of Magnetic Order and a Structural Transition in the Parent BaFe2As2 Compound of FeAs-Based High-Temperature Superconductors| journal = Physical Review Letters| volume = 101| issue = 25|article-number = 257003 | year = 2008| last1 = Huang | first1 = Q.| last2 = Qiu | first2 = Y.| last3 = Bao | first3 = W. | last4 = Green | first4 = M. A.| last5 = Lynn | first5 = J. W.| last6 = Gasparovic | first6 = Y. C.| last7 = Wu | first7 = T.| last8 = Wu | first8 = G.| last9 = Chen | first9 = X. H.
| arxiv = 0806.2776|s2cid = 119206773 }}</ref> The 122s contain the same iron-arsenide planes as the oxypnictides, but are much easier to synthesize in the form of large single crystals.

There are two different ways in which superconductivity was achieved in the 122s. One method is the application of pressure to the undoped parent compounds.<ref name=Park /><ref name=Alireza2008 /> The second is the introduction of other elements (dopants) into the crystal structure in very specific ratios. There are two doping schemes: The first type of doping involves the introduction of holes into the [[Chemistry:Barium|barium]] or [[Chemistry:Strontium|strontium]] varieties; hole doping refers to the substitution of one ion for another with fewer electrons. Superconducting transition temperatures as high as 38 K have been reported upon substitution of the 40% of the Ba2+ or Sr2+ ions with K+.<ref name=Rotter2008 /> The second doping method is to directly dope the iron-arsenide layer by replacing iron with cobalt. Superconducting transition temperatures up to ~20 K have been observed in this case.<ref name=Leithe-Jasper2008>{{Cite journal | doi = 10.1103/PhysRevLett.101.207004|pmid = 19113371|bibcode=2008PhRvL.101t7004L|arxiv = 0807.2223| title = Superconducting State in SrFe2−xCoxAs2 by Internal Doping of the Iron Arsenide Layers| journal = Physical Review Letters| volume = 101| issue = 20|article-number = 207004| year = 2008| last1 = Leithe-Jasper | first1 = A.| last2 = Schnelle | first2 = W.| last3 = Geibel | first3 = C.| last4 = Rosner | first4 = H.|s2cid = 8645332}}</ref>

Unlike the oxypnictides, large single crystals of the 122s can be easily synthesized by using the [[Physics:Flux method|flux method]].<ref name=Canfield2009>{{Cite journal|arxiv = 0901.4672
|title = Structural, magnetic and superconducting phase transitions in CaFe2As2 under ambient and applied pressure
|journal = Physica C: Superconductivity
|volume = 469
|issue = 9–12
|page = 404
|author = Canfield, P. C.
|year = 2009
|last2 = Bud'ko
|last3 = Ni
|last4 = Kreyssig
|last5 = Goldman
|last6 = McQueeney
|last7 = Torikachvili
|last8 = Argyriou
|last9 = Luke
|last10 = Yu
|doi = 10.1016/j.physc.2009.03.033
|bibcode = 2009PhyC..469..404C |s2cid = 119281157
}}</ref> The behavior of these materials is interesting by that superconductivity exists alongside [[Physics:Antiferromagnetism|antiferromagnetism]].<ref name=Pickett2009 /> Various studies including electrical resistivity, [[Physics:Magnetic susceptibility|magnetic susceptibility]], [[Physics:Specific heat|specific heat]],<ref name=Leithe-Jasper2008 /><ref name=Krellner22008>{{Cite journal | doi = 10.1103/PhysRevB.78.100504|arxiv = 0806.1043 |bibcode = 2008PhRvB..78j0504K| title = Magnetic and structural transitions in layered iron arsenide systems: AFe2As2 versus RFeAsO| journal = Physical Review B| volume = 78| issue = 10|article-number = 100504 | year = 2008| last1 = Krellner | first1 = C.| last2 = Caroca-Canales | first2 = N.| last3 = Jesche | first3 = A.| last4 = Rosner | first4 = H.| last5 = Ormeci | first5 = A.| last6 = Geibel | first6 = C.|s2cid = 119288375 }}</ref> [[Physics:Nuclear magnetic resonance|NMR]],<ref name=Baek>{{Cite journal | doi = 10.1103/PhysRevB.79.052504|arxiv = 0808.0744 |bibcode = 2009PhRvB..79e2504B| title = First-order magnetic transition in single-crystalline CaFe2As2 detected by 75As nuclear magnetic resonance| journal = Physical Review B| volume = 79| issue = 5|article-number = 052504 | year = 2009| last1 = Baek | first1 = S. -H. | last2 = Curro | first2 = N. J.| last3 = Klimczuk | first3 = T.| last4 = Bauer | first4 = E. D.| last5 = Ronning | first5 = F.| last6 = Thompson | first6 = J. D.|s2cid = 119004910 }}</ref><ref name=Baek22009>{{Cite journal | doi = 10.1103/PhysRevLett.102.227601|pmid=19658902|bibcode=2009PhRvL.102v7601B| title = NMR Investigation of Superconductivity and Antiferromagnetism in CaFe2As2 under Pressure| journal = Physical Review Letters| volume = 102| issue = 22|article-number=227601| year = 2009| last1 = Baek | first1 = S. -H. | last2 = Lee | first2 = H.| last3 = Brown | first3 = S. E.| last4 = Curro | first4 = N. J.| last5 = Bauer | first5 = E. D.| last6 = Ronning | first6 = F.| last7 = Park | first7 = T.| last8 = Thompson | first8 = J. D.|arxiv = 0903.2011 |s2cid=18061290}}</ref><ref name=Curro2009>{{Cite journal | doi = 10.1088/1367-2630/11/7/075004| title = Low-energy spin dynamics in the antiferromagnetic phase of CaFe2As2| journal = New Journal of Physics| volume = 11| issue = 7| article-number = 075004| year = 2009| last1 = Curro | first1 = N. J. | last2 = Dioguardi | first2 = A. P. | last3 = Aproberts-Warren | first3 = N. | last4 = Shockley | first4 = A. C. | last5 = Klavins | first5 = P. | bibcode = 2009NJPh...11g5004C|arxiv = 0902.4492 | s2cid = 14216790}}</ref> neutron scattering,<ref name=Krey /><ref name=Huang2008 /> [[Physics:X-ray diffraction|X-ray diffraction]], [[Mössbauer spectroscopy]],<ref name=Rotter22008>{{Cite journal | doi = 10.1103/PhysRevB.78.020503|url=https://www.researchgate.net/publication/202152901| title = Spin-density-wave anomaly at 140 K in the ternary iron arsenide BaFe2As2| journal = Physical Review B| volume = 78| issue = 2|article-number=020503| year = 2008| last1 = Rotter | first1 = M. | last2 = Tegel | first2 = M. | last3 = Johrendt | first3 = D. | last4 = Schellenberg | first4 = I. | last5 = Hermes | first5 = W. | last6 = Pöttgen | first6 = R. |bibcode=2008PhRvB..78b0503R|arxiv = 0805.4021 |s2cid=118379843}}</ref> and [[Physics:Quantum oscillations|quantum oscillations]]<ref name=Harrison2009>{{Cite journal | doi = 10.1088/0953-8984/21/32/322202|url=https://www.researchgate.net/publication/51239140| title = Quantum oscillations in antiferromagnetic CaFe2As2 on the brink of superconductivity| journal = Journal of Physics: Condensed Matter| volume = 21| issue = 32| article-number = 322202| year = 2009| last1 = Harrison | first1 = N. | last2 = McDonald | first2 = R. D. | last3 = Mielke | first3 = C. H. | last4 = Bauer | first4 = E. D. | last5 = Ronning | first5 = F. | last6 = Thompson | first6 = J. D. |pmid=21693960|bibcode=2009JPCM...21F2202H| arxiv = 0902.1481|s2cid=12345149}}</ref> have been performed for the undoped parent compounds, as well as the superconducting versions.

== Synthesis ==
One of the important qualities of the 122s is their ease of synthesis; it is possible to grow large single crystals, up to ~5×5×0.5 mm, using the [[Physics:Flux method|flux method]].<ref name="Canfield2009" /> In a nutshell, the flux method uses some solvent in which the starting materials for a chemical reaction are able to dissolve and eventually crystallize into the desired compound. Two standard methods show up in the literature, each using a different flux. The first method employs tin,<ref name=Canfield2009 /> while the second uses the binary metallic compound FeAs (iron arsenide).<ref name=Luo2008>{{Cite journal | doi = 10.1088/0953-2048/21/12/125014|arxiv = 0807.0759 |bibcode = 2008SuScT..21l5014L| title = Growth and characterization of A1−xKxFe2As2 (A = Ba, Sr) single crystals with x = 0–0.4| journal = Superconductor Science and Technology| volume = 21| issue = 12| article-number = 125014| year = 2008| last1 = Luo | first1 = H. | last2 = Wang | first2 = Z. | last3 = Yang | first3 = H. | last4 = Cheng | first4 = P. | last5 = Zhu | first5 = X. | last6 = Wen | first6 = H. H. |s2cid = 2746754 }}</ref>

== Structural and magnetic phase transition ==
The 122s form in the I4/mmm tetragonal structure. For example, the tetragonal unit cell of SrFe2As2, at room temperature, has lattice parameters a = b = 3.9243 Å and c = 12.3644 Å.<ref name=Rotter22008 /> The planar geometry is reminiscent of the cuprate high-Tc superconductors in which the Cu-O layers are believed to support superconductivity.<ref name=Sadovskii2008>{{cite journal
|title = High-temperature superconductivity in iron-based layered iron compounds
|author = Sadovskii, Mikhail V.
|journal = Physics-Uspekhi
|volume = 51
|pages = 1201–1227
|year = 2008
|doi = 10.1070/PU2008v051n12ABEH006820
|issue = 12|bibcode = 2008PhyU...51.1201S |arxiv = 0812.0302|s2cid = 59490013
}}</ref>

These materials undergo a first-order structural [[Phase transition|phase transition]] into the Fmmm [[Chemistry:Orthorhombic|orthorhombic]] structure below some characteristic temperature T0 that is compound specific.<ref name=Tegel /><ref name="Krellner22008" /> NMR experiments on the CaFe2As2<ref name=Baek /> show that there is a first-order [[Physics:Antiferromagnetic|antiferromagnetic]] magnetic phase transition at the same temperature; in contrast, the antiferromagnetic transition occurs at a lower temperature in the 1111s.<ref name=Krellner22008 /> The high temperature magnetic state is [[Physics:Paramagnetic|paramagnetic]], while the low temperature state is an antiferromagnetic state known as a [[Physics:Spin density wave|spin-density-wave]].<ref name=Baek />

== Superconductivity ==
Superconductivity has been observed in the 122s up to a current maximum Tc of 38 K in Ba1−xKxFe2As2 with x ≈ 0.4.<ref name="Rotter22008" /> Resistivity and magnetic susceptibility measurements have confirmed the bulk nature of the observed superconducting transition.<ref name="Rotter22008" /> The onset of superconductivity is correlated with the loss of the spin-density-wave state.<ref name=Pickett2009 />

The Tc of 38 K in Ba1−xKxFe2As2 (x ≈ 0.4) superconductor shows the inverse iron isotope effect.<ref>{{Cite journal | doi = 10.1103/PhysRevLett.103.257003|pmid = 20366277|bibcode=2009PhRvL.103y7003S|arxiv = 0903.3515| title = Inverse Iron Isotope Effect on the Transition Temperature of the (Ba,K)Fe2As2 Superconductor| journal = Physical Review Letters| volume = 103| issue = 25|article-number = 257003| year = 2009| last1 = Shirage | first1 = P. M. | last2 = Kihou | first2 = K. | last3 = Miyazawa | first3 = K. | last4 = Lee | first4 = C. H. | last5 = Kito | first5 = H. | last6 = Eisaki | first6 = H. | last7 = Yanagisawa | first7 = T. | last8 = Tanaka | first8 = Y. | last9 = Iyo | first9 = A.
|s2cid = 44684501}}</ref>

== Other compounds with 122 structure ==
In addition to the iron arsenides, the 122 crystal structure plays an important role for other material systems as well. Three famous examples from the field of heavy fermions are CeCu2Si2 (the "first [[Physics:Unconventional superconductor|unconventional superconductor]]" discovered 1978),<ref>{{cite journal
|doi=10.1103/PhysRevLett.43.1892
|title=Superconductivity in the Presence of Strong Pauli Paramagnetism: CeCu2Si2
|year=1979|journal=Physical Review Letters|volume=43|page=1892
|last1=Steglich|first1=F.|first2=J.|last2=Aarts|last3=Bredl|first3=C.D.|last4=Lieke|first4=W.
|last5=Meschede|first5=D.|last6=Franz|first6=W.|last7=Schäfer|first7=H.|bibcode=1979PhRvL..43.1892S|issue=25|hdl=1887/81461|s2cid=123497750
|hdl-access=free}}</ref>
URu2Si2 (which is also a [[Physics:Heavy fermion superconductor|heavy fermion superconductor]] but is the focus of active present research due to the so-called "hidden-order phase" below 17.5 K),<ref>{{cite journal
|doi=10.1103/PhysRevLett.55.2727
|title=Superconducting and Magnetic Transitions in the Heavy-Fermion System URu2Si2
|year=1985|journal=Physical Review Letters|volume=55|pages= 2727–2730
|last1=Palstra|first1=T. T. M.|last2=Menovsky|first2=A. A.|last3=van den Berg|first3=J.|last4=Dirkmaat|first4=A. J.
|last5=Kes|first5=P. H.|last6=Nieuwenhuys|first6=G. J.|last7=Mydosh|first7=J.A.|issue=24|pmid=10032222|bibcode=1985PhRvL..55.2727P
|url=https://pure.rug.nl/ws/files/14558353/1985PhysRevLettPalstra.pdf
}}</ref>
and [[Chemistry:Ytterbium dirhodium disilicide|YbRh2Si2]] (one of the prime examples of quantum criticality).<ref>{{cite journal|doi=10.1038/nphys892
|title=Quantum criticality in heavy-fermion metals|year=2008|journal=Nature Physics|volume=4|issue=3|page=186
|last1=Gegenwart|first1=P.|last2=Si|first2=Q.|last3=Steglich|first3=F.|bibcode=2008NatPh...4..186G|arxiv = 0712.2045 |s2cid=119086996}}</ref>

== References ==
{{reflist}}

[[Category:Superconductors]]
[[Category:Correlated electrons]]
[[Category:High-temperature superconductors]]
[[Category:Iron compounds]]
[[Category:Arsenides]]

{{Sourceattribution|122 iron arsenide}}

Physics:122 iron arsenide - Revision history

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