122 iron arsenide

Crystal structure of 122-type AEFe2Pn2 superconductors, AE = alkaline earth metal (Ca, Se, etc.), Pn = pnictide (As, P, etc.)[1]

The 122 iron arsenide unconventional superconductors are part of a new class of iron-based superconductors. They form in the tetragonal I4/mmm, ThCr2Si2 type, crystal structure. The shorthand name "122" comes from their stoichiometry; the 122s have the chemical formula AEFe2Pn2, where AE stands for alkaline earth metal (Ca, Ba, Sr or Eu) and Pn is pnictide (As, P, etc.).[1][2][3] These materials become superconducting under pressure and also upon doping.[4][5][6][7] The maximum superconducting transition temperature found to date is 38 K in the Ba0.6K0.4Fe2As2.[8] The microscopic description of superconductivity in the 122s is yet unclear.[9]

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 high-temperature superconductivity and unconventional (non-s-wave) pairing state found in these materials.[10] 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 phonon-induced interactions between electrons responsible for conventional BCS theory s-wave superconductivity—has recently been expanded by the discovery of high temperature superconductivity (up to Tc = 55 K) in the doped oxypnictide (1111) superconductors with the chemical composition XOFeAs, where X = La, Ce, Pr, Nd, Sm, Gd, Tb, or Dy.[11][12] 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.[5][6] 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 barium or 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+.[8] 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.[13]

Unlike the oxypnictides, large single crystals of the 122s can be easily synthesized by using the flux method.[14] The behavior of these materials is interesting by that superconductivity exists alongside antiferromagnetism.[9] Various studies including electrical resistivity, magnetic susceptibility, specific heat,[13][15] NMR,[16][17][18] neutron scattering,[2][12] X-ray diffraction, Mössbauer spectroscopy,[19] and quantum oscillations[20] 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 flux method.[14] 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 utilizing a different flux. The first method employs tin,[14] while the second uses the binary metallic compound FeAs (iron arsenide).[21]

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.[19] The planar geometry is reminiscent of the cuprate high-Tc superconductors in which the Cu-O layers are believed to support superconductivity.[22]

These materials undergo a first-order structural phase transition into the Fmmm orthorhombic structure below some characteristic temperature T0 that is compound specific.[3][15] NMR experiments on the CaFe2As2[16] show that there is a first-order antiferromagnetic magnetic phase transition at the same temperature; in contrast, the antiferromagnetic transition occurs at a lower temperature in the 1111s.[15] The high temperature magnetic state is paramagnetic, while the low temperature state is an antiferromagnetic state known as a spin-density-wave.[16]

Superconductivity

Superconductivity has been observed in the 122s up to a current maximum Tc of 38 K in Ba1−xKxFe2As2 with x ≈ 0.4.[19] Resistivity and magnetic susceptibility measurements have confirmed the bulk nature of the observed superconducting transition.[19] The onset of superconductivity is correlated with the loss of the spin-density-wave state.[9]

The Tc of 38 K in Ba1−xKxFe2As2 (x ≈ 0.4) superconductor shows the inverse iron isotope effect.[23]

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 unconventional superconductor" discovered 1978),[24] URu2Si2 (which is also a heavy fermion superconductor but is the focus of active present research due to the so-called "hidden-order phase" below 17.5 K),[25] and YbRh2Si2 (one of the prime examples of quantum criticality).[26]

References

  1. 1 2 Hosono, H.; Tanabe, K.; Takayama-Muromachi, E.; Kageyama, H.; Yamanaka, S.; Kumakura, H.; Nohara, M.; Hiramatsu, H.; Fujitsu, S. (2015). "Exploration of new superconductors and functional materials, and fabrication of superconducting tapes and wires of iron pnictides". Science and Technology of Advanced Materials. 16 (3): 033503. arXiv:1505.02240Freely accessible. Bibcode:2015STAdM..16c3503H. doi:10.1088/1468-6996/16/3/033503. PMC 5099821Freely accessible. PMID 27877784.
  2. 1 2 Kreyssig, A.; Green, M. A.; Lee, Y.; Samolyuk, G. D.; Zajdel, P.; Lynn, J. W.; Bud’ko, S. L.; Torikachvili, M. S.; Ni, N.; Nandi, S.; Leão, J. B.; Poulton, S. J.; Argyriou, D. N.; Harmon, B. N.; McQueeney, R. J.; Canfield, P. C.; Goldman, A. I. (2008). "Pressure-induced volume-collapsed tetragonal phase of CaFe2As2 as seen via neutron scattering". Physical Review B. 78 (18): 184517. arXiv:0807.3032Freely accessible. Bibcode:2008PhRvB..78r4517K. doi:10.1103/PhysRevB.78.184517.
  3. 1 2 Tegel, M.; Rotter, M.; Weiß, V.; Schappacher, F. M.; Pöttgen, R.; Johrendt, D. (2008). "Structural and magnetic phase transitions in the ternary iron arsenides SrFe2As2 and EuFe2As2". Journal of Physics: Condensed Matter. 20 (45): 452201. arXiv:0806.4782Freely accessible. Bibcode:2008JPCM...20S2201T. doi:10.1088/0953-8984/20/45/452201.
  4. ↑ Shirage, P. M.; Miyazawa, K.; Kito, H.; Eisaki, H.; Iyo, A. (2008). "Superconductivity at 26 K in (Ca1-xNax)Fe2As2". Applied Physics Express. 1: 081702. Bibcode:2008APExp...1h1702M. doi:10.1143/APEX.1.081702.
  5. 1 2 Park, T.; Park, E.; Lee, H.; Klimczuk, T.; Bauer, E. D.; Ronning, F.; Thompson, J. D. (2008). "Pressure-induced superconductivity in CaFe2As2". Journal of Physics: Condensed Matter. 20 (32): 322204. Bibcode:2008JPCM...20F2204P. doi:10.1088/0953-8984/20/32/322204.
  6. 1 2 Alireza, P. L.; Ko, Y. T. C.; Gillett, J.; Petrone, C. M.; Cole, J. M.; Lonzarich, G. G.; Sebastian, S. E. (2009). "Superconductivity up to 29 K in SrFe2As2 and BaFe2As2 at high pressures". Journal of Physics: Condensed Matter. 21: 012208. arXiv:0807.1896Freely accessible. Bibcode:2009JPCM...21a2208A. doi:10.1088/0953-8984/21/1/012208.
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  10. ↑ "Towards a complete theory of high TC". Nature Physics. 2 (3): 138. 2006. Bibcode:2006NatPh...2..138.. doi:10.1038/nphys253.
  11. ↑ Takahashi, H.; Igawa, K.; Arii, K.; Kamihara, Y.; Hirano, M.; Hosono, H. (2008). "Superconductivity at 43 K in an iron-based layered compound LaO1-xFxFeAs". Nature. 453 (7193): 376–8. Bibcode:2008Natur.453..376T. doi:10.1038/nature06972. PMID 18432191.
  12. 1 2 Huang, Q.; Qiu, Y.; Bao, W.; Green, M. A.; Lynn, J. W.; Gasparovic, Y. C.; Wu, T.; Wu, G.; Chen, X. H. (2008). "Neutron-Diffraction Measurements of Magnetic Order and a Structural Transition in the Parent BaFe2As2 Compound of FeAs-Based High-Temperature Superconductors". Physical Review Letters. 101 (25): 257003. Bibcode:2008PhRvL.101y7003H. doi:10.1103/PhysRevLett.101.257003. PMID 19113744.
  13. 1 2 Leithe-Jasper, A.; Schnelle, W.; Geibel, C.; Rosner, H. (2008). "Superconducting State in SrFe2-xCoxAs2 by Internal Doping of the Iron Arsenide Layers". Physical Review Letters. 101 (20): 207004. arXiv:0807.2223Freely accessible. Bibcode:2008PhRvL.101t7004L. doi:10.1103/PhysRevLett.101.207004. PMID 19113371.
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  16. 1 2 3 Baek, S. -H.; Curro, N. J.; Klimczuk, T.; Bauer, E. D.; Ronning, F.; Thompson, J. D. (2009). "First-order magnetic transition in single-crystalline CaFe2As2 detected by 75As nuclear magnetic resonance". Physical Review B. 79 (5): 052504. arXiv:0808.0744Freely accessible. Bibcode:2009PhRvB..79e2504B. doi:10.1103/PhysRevB.79.052504.
  17. ↑ Baek, S. -H.; Lee, H.; Brown, S. E.; Curro, N. J.; Bauer, E. D.; Ronning, F.; Park, T.; Thompson, J. D. (2009). "NMR Investigation of Superconductivity and Antiferromagnetism in CaFe2As2 under Pressure". Physical Review Letters. 102 (22): 227601. arXiv:0903.2011Freely accessible. Bibcode:2009PhRvL.102v7601B. doi:10.1103/PhysRevLett.102.227601. PMID 19658902.
  18. ↑ Curro, N. J.; Dioguardi, A. P.; Aproberts-Warren, N.; Shockley, A. C.; Klavins, P. (2009). "Low-energy spin dynamics in the antiferromagnetic phase of CaFe2As2". New Journal of Physics. 11 (7): 075004. arXiv:0902.4492Freely accessible. Bibcode:2009NJPh...11g5004C. doi:10.1088/1367-2630/11/7/075004.
  19. 1 2 3 4 Rotter, M.; Tegel, M.; Johrendt, D.; Schellenberg, I.; Hermes, W.; Pöttgen, R. (2008). "Spin-density-wave anomaly at 140 K in the ternary iron arsenide BaFe2As2". Physical Review B. 78 (2): 020503. arXiv:0805.4021Freely accessible. Bibcode:2008PhRvB..78b0503R. doi:10.1103/PhysRevB.78.020503.
  20. ↑ Harrison, N.; McDonald, R. D.; Mielke, C. H.; Bauer, E. D.; Ronning, F.; Thompson, J. D. (2009). "Quantum oscillations in antiferromagnetic CaFe2As2 on the brink of superconductivity". Journal of Physics: Condensed Matter. 21 (32): 322202. Bibcode:2009JPCM...21F2202H. doi:10.1088/0953-8984/21/32/322202.
  21. ↑ Luo, H.; Wang, Z.; Yang, H.; Cheng, P.; Zhu, X.; Wen, H. H. (2008). "Growth and characterization of A1−xKxFe2As2 (A = Ba, Sr) single crystals with x = 0–0.4". Superconductor Science and Technology. 21 (12): 125014. arXiv:0807.0759Freely accessible. Bibcode:2008SuScT..21l5014L. doi:10.1088/0953-2048/21/12/125014.
  22. ↑ Sadovskii, Mikhail V. (2008). "High-temperature superconductivity in iron-based layered iron compounds". Physics-Uspekhi. 51 (12): 1201–1227. Bibcode:2008PhyU...51.1201S. doi:10.1070/PU2008v051n12ABEH006820.
  23. ↑ Shirage, P. M.; Kihou, K.; Miyazawa, K.; Lee, C. H.; Kito, H.; Eisaki, H.; Yanagisawa, T.; Tanaka, Y.; Iyo, A. (2009). "Inverse Iron Isotope Effect on the Transition Temperature of the (Ba,K)Fe2As2 Superconductor". Physical Review Letters. 103 (25): 257003. arXiv:0903.3515Freely accessible. Bibcode:2009PhRvL.103y7003S. doi:10.1103/PhysRevLett.103.257003. PMID 20366277.
  24. ↑ Steglich, F.; Aarts, J.; Bredl, C.D.; Lieke, W.; Meschede, D.; Franz, W.; Schäfer, H. (1979). "Superconductivity in the Presence of Strong Pauli Paramagnetism: CeCu2Si2". Physical Review Letters. 43 (25): 1892. Bibcode:1979PhRvL..43.1892S. doi:10.1103/PhysRevLett.43.1892.
  25. ↑ Palstra, T. T. M.; Menovsky, A. A.; van den Berg, J.; Dirkmaat, A. J.; Kes, P. H.; Nieuwenhuys, G. J.; Mydosh, J.A. (1985). "Superconducting and Magnetic Transitions in the Heavy-Fermion System URu2Si2". Physical Review Letters. 55 (24): 2727–2730. Bibcode:1985PhRvL..55.2727P. doi:10.1103/PhysRevLett.55.2727. PMID 10032222.
  26. ↑ Gegenwart, P.; Si, Q.; Steglich, F. (2008). "Quantum criticality in heavy-fermion metals". Nature Physics. 4 (3): 186. arXiv:0712.2045Freely accessible. Bibcode:2008NatPh...4..186G. doi:10.1038/nphys892.
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