Progress in Superconductivity

The Korean Superconductivity Society (한국초전도학회)
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Brief Review on Iron-based Superconductors: are There Clues for Unconventional Superconductivity?

  • Oh, Hyung-Ju (Department of Physics and IPAP, Yonsei University) ;
  • Moon, Ji-Soo (Department of Physics and IPAP, Yonsei University) ;
  • Shin, Dong-Han (Department of Physics and IPAP, Yonsei University) ;
  • Moon, Chang-Youn (Department of Chemistry, Pohang University of Science and Technology) ;
  • Choi, Hyoung-Joon (Department of Physics and IPAP, Yonsei University)
  • Received : 2011.12.01
  • Accepted : 2011.12.20
  • Published : 2011.12.31
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Abstract

Study of superconductivity in layered iron-based materials was initiated in 2006 by Hosono's group, and boosted in 2008 by the superconducting transition temperature, $T_c$, of 26 K in $LaFeAsO_{1-X}F_X$. Since then, enormous researches have been done on the materials, with $T_c$ reaching as high as 55 K. Here, we review briefly experimental and theoretical results on atomic and electronic structures and magnetic and superconducting properties of FeAs-based superconductors and related compounds. We seek for clues for unconventional superconductivity in the materials.

Keywords

References

  1. J. G. Bednorz and K. A. Muller, "Possible high Tc superconductivity in the Ba-La-Cu-O system", Z. Phys. B 64, 189-193 (1986).
  2. Y. Kamihara et al., "Iron-based layered superconductor: LaOFeP", J. Am. Chem. Soc. 128, 10012-10013 (2006).
  3. Y. Kamihara, T. Watanabe, M. Hirano, and H. Hosono, "Iron-based layered superconductor La[$O_{1-x}F_x$]FeAs (x = 0.05-0.12) with T_c$ = 26 K", J. Am. Chem. Soc. 130, 3296-3297 (2008).
  4. H. Ogino et al., "Superconductivity at 17 K in ($Fe_2P_2$)($Sr_4Sc_2O_6$): a new superconducting layered pnictide oxide with a thick perovskite oxide layer", Supercond. Sci. Technol. 22, 075008 (2009).
  5. X. Zhu et al., "Transition of stoichiometric $Sr_2VO_3FeAs$ to a superconducting state at 37.2 K", Phys. Rev. B 79, 220512(R) (2009).
  6. S. Sato et al., "Superconductivity in a new iron pnictide oxide $(Fe_2As_2)(Sr_4(Mg,Ti)_2O_6)$", Supercond. Sci. Technol. 23, 045001 (2010).
  7. K.-W. Lee and W. E. Pickett, "$Sr_2VO_3FeAs$: a nanolayered bimetallic iron pnictide superconductor", Euro. Phys. Lett. 89, 57008 (2010).
  8. J. Guo et al., "Superconductivity in the iron selenide $K_xFe_2Se_2$ ($0{\leq}x{\leq}1.0$)", Phys. Rev. B 82, 180520(R) (2010).
  9. Y. Zhang et al., "Nodeless superconducting gap in $A_xFe_2Se_2$ (A=K, Cs) revealed by angle-resolved photoemission spectroscopy", Nature Mater. 10, 273-277 (2011).
  10. D. Mou et al., "Distinct Fermi surface topology and nodeless superconducting gap in a $(Tl_{0.58}Rb_{0.42})Fe_{1.72}Se_2$ superconductor", Phys. Rev. Lett. 106, 107001 (2011).
  11. A. F. Wang et al., "Superconductivity at 32 K in single-crystalline $Rb_xFe_{2-y}Se_2$", Phys. Rev. B 83, 060512(R) (2011).
  12. H. Takahashi et al., "Superconductivity at 43 K in an iron-based layered compound $LaO_{1-x}F_xFeAs$", Nature (London) 453, 376-378 (2008).
  13. X. H. Chen et al., "Superconductivity at 43 K in $SmFeAsO_{1-x}F_x$", Nature (London) 453, 761-762 (2008).
  14. G. F. Chen et al., "Superconductivity at 41 K and its competition with spin-density-wave instability in layered $CeO_{1-x}F_xFeAs$", Phys. Rev. Lett. 100, 247002 (2008).
  15. Z. A. Ren et al., "Superconductivity at 52 K in iron based F doped layered quaternary compound Pr[$O_{1-x}F_x$]FeAs", Mater. Res. Innov. 12, 105-106 (2008).
  16. K. Miyazawa et al., "Superconductivity above 50 K in $LnFeAsO_{1-y}$ (Ln=Nd, Sm, Gd, Tb, and Dy) synthesized by high-pressure technique", J. Phys. Soc. Jpn. 78, 034712 (2009).
  17. Z.-A. Ren et al., "Superconductivity at 55 K in iron-based F-doped layered quaternary compound Sm[$O_{1-x}F_x$]FeAs", Chin. Phys. Lett. 25, 2215-2216 (2008).
  18. M. Rotter, M. Tegel, and D. Johrendt, "Superconductivity at 38 K in the iron arsenide $(Ba_{1-x}K_x)Fe_2As_2$", Phys. Rev. Lett. 101, 107006 (2008).
  19. X. C. Wang et al., "The superconductivity at 18 K in LiFeAs system", Solid State Commun. 148, 538-540 (2008).
  20. R. Mittal et al., "Phonon dynamics in $Sr_{0.6}K_{0.4}Fe_2As_2$ and $Ca_{0.6}Na_{0.4}Fe_2As_2$ from neutron scattering and lattice-dynamical calculations", Phys. Rev. B 78, 224518 (2008).
  21. R. Cortes-Gil and S. J. Clarke, "Structure, magnetism, and superconductivity of the layered iron arsenides $Sr_{1-x}Na_xFe_2As_2$", Chem. Mater. 23, 1009-1016 (2011).
  22. F.-C. Hsu et al., "Superconductivity in the PbO-type structure ${\alpha}$-FeSe", Proc. Natl. Acad. Sci. U.S.A. 105, 14262-14264 (2008).
  23. S. Medvedev et al., "Electronic and magnetic phase diagram of ${\beta}-Fe_{1.01}Se$ with superconductivity at 36.7 K under pressure", Nature Mater. 8, 630-633 (2009).
  24. B. C. Sales et al., "Bulk superconductivity at 14 K in single crystals of $Fe_{1+y}Te_xSe_{1-x}$", Phys. Rev. B 79, 094521 (2009).
  25. R. Hu, E. S. Bozin, J. B. Warren, and C. Petrovic, "Superconductivity, magnetism, and stoichiometry of single crystals of $Fe_{1+y}(Te_{1-x}S_x)_z$", Phys. Rev. B 80, 214514 (2009).
  26. Y. Mizuguchi, F. Tomioka, S. Tsuda, T. Yamaguchi, and Y. Takano, "Superconductivity in S-substituted FeTe", Appl. Phys. Lett. 94, 012503 (2009).
  27. C. de la Cruz et al., "Magnetic order close to superconductivity in the iron-based layered $LaO_{1-x}F_xFeAs$ systems", Nature (London) 453, 899-902 (2008).
  28. N. Qureshi et al., "Crystal and magnetic structure of the oxypnictide superconductor $LaFeAsO_{1-x}F_x$: a neutron-diffraction study", Phys. Rev. B 82, 184521 (2010).
  29. J. Zhao et al., "Structural and magnetic phase diagram of $CeFeAsO_{1-x}F_x$ and its relation to high-temperature superconductivity", Nature Mater. 7, 953-959 (2008).
  30. M. Rotter, M. Pangerl, M. Tegel, and D. Johrendt, "Superconductivity and crystal structures of $(Ba_{1-x}K_x)Fe_2As_2$ (x=0-1)", Angew. Chem. Int. Ed. 47, 7949-7952 (2008).
  31. N. Ni et al., "Effects of Co substitution on thermodynamic and transport properties and anisotropic $H_{c2}$ in $Ba(Fe_{1−x}Co_x)_2As_2$ single crystals", Phys. Rev. B 78, 214515 (2008).
  32. S. Jiang et al., "Superconductivity up to 30 K in the vicinity of the quantum critical point in $BaFe_2(As_{1−x}P_x)_2$", J. Phys.: Condens. Matter 21, 382203 (2009).
  33. J. Paglione and R. L. Greene, "High-temperature superconductivity in iron-based materials", Nature Phys. 6, 645-658 (2010).
  34. T. Park et al., "Pressure-induced superconductivity in $CaFe_2As_2$", J. Phys.: Condens. Matter 20, 322204 (2008).
  35. N. Doiron-Leyraud et al., "Quantum oscillations and the Fermi surface in an underdoped high-$T_c$ superconductor", Nature (London) 447, 565-568 (2007).
  36. S. Lebegue, "Electronic structure and properties of the Fermi surface of the superconductor LaOFeP", Phys. Rev. B 75, 035110 (2007).
  37. D. H. Lu et al., "Electronic structure of the iron-based superconductor LaOFeP", Nature (London) 455, 81-84 (2008).
  38. D. J. Singh and M.-H. Du, "Density functional study of $LaFeAsO_{1-x}F_x$: a low carrier density superconductor near itinerant magnetism", Phys. Rev. Lett. 100, 237003 (2008).
  39. D. H. Lu et al., "ARPES studies of the electronic structure of LaOFe(P,As)", Physica C 469, 452-458 (2009).
  40. G. Kotliar et al., "Electronic structure calculations with dynamical mean-field theory", Rev. Mod. Phys. 78, 865-951 (2006).
  41. S. L. Skornyakov, N. A. Skorikov, A. V. Lukoyanov, A. O. Shorikov, and V. I. Anisimov, "LDA+DMFT spectral functions and effective electron mass enhancement in the superconductor LaFePO", Phys. Rev. B 81, 174522 (2010).
  42. Z. P. Yin et al., "Electron-hole symmetry and magnetic coupling in antiferromagnetic LaFeAsO", Phys. Rev. Lett. 101, 047001 (2008).
  43. I. I. Mazin, M. D. Johannes, L. Boeri, K. Koepernik, and D. J. Singh, "Problems with reconciling density functional theory calculations with experiment in ferropnictides", Phys. Rev. B 78, 085104 (2008).
  44. V. Cvetkovic and Z. Tesanovic, "Multiband magnetism and superconductivity in Fe-based compounds", Europhys. Lett. 85, 37002 (2009).
  45. K.-W. Lee, "Electronic structure of superconducting NaFeAs", Progress in Superconductivity 10, 123-127 (2009).
  46. J. Dong et al., "Competing orders and spin-densitywave instability in La($O_{1−x}F_x$)FeAs", Europhys. Lett. 83, 27006 (2008).
  47. B. Lorenz et al., "Effect of pressure on the superconducting and spin-density-wave states of $SmFeAsO_{1−x}F_x$", Phys. Rev. B 78, 012505 (2008).
  48. H.-H. Klauss et al., "Commensurate spin density wave in LaFeAsO: a local probe study", Phys. Rev. Lett. 101, 077005 (2008).
  49. W. Z. Hu et al., "Origin of the spin density wave instability in $AFe_2As_2$ (A = Ba, Sr) as revealed by optical spectroscopy", Phys. Rev. Lett. 101, 257005 (2008).
  50. J. S. Kim et al., "Evidence for coexistence of superconductivity and magnetism in single crystals of Co-doped $SrFe_2As_2$", J. Phys.: Condens. Matter 21, 102203 (2009).
  51. S. Li et al., "First-order magnetic and structural phase transitions in $Fe_{1+y}S_exTe_{1−x}$", Phys. Rev. B 79, 054503 (2009).
  52. F. Ma, W. Ji, J. Hu, Z.-Y. Lu, and T. Xiang, "First-principles calculations of the electronic structure of tetragonal ${\alpha}$-FeTe and ${\alpha}$-FeSe crystals: evidence for a bicollinear antiferromagnetic order", Phys. Rev. Lett. 102, 177003 (2009).
  53. T. Yildirim, "Origin of the 150-K anomaly in LaFeAsO: competing antiferromagnetic interactions, frustration, and a structural phase transition", Phys. Rev. Lett. 101, 057010 (2008).
  54. Q. Si and E. Abrahams, "Strong correlations and magnetic frustration in the high $T_c$ iron pnictides", Phys. Rev. Lett. 101, 076401 (2008).
  55. L. X. Yang et al., "Electronic structure and unusual exchange splitting in the spin-density-wave state of the $BaFe_2As_2$ parent compound of iron-based superconductors", Phys. Rev. Lett. 102, 107002 (2009).
  56. C.-Y. Moon, S. Y. Park, and H. J. Choi, "Dominant role of local-moment interactions in the magnetic ordering of iron pnictide superconductors: a comparative study of arsenides and antimonides from first principles", Phys. Rev. B 80, 054522 (2009).
  57. J. Wu, P. Phillips, and A. H. Castro Neto, "Theory of the magnetic moment in iron pnictides", Phys. Rev. Lett. 101, 126401 (2008).
  58. S.-P. Kou, T. Li, and Z.-Y. Weng, "Coexistence of itinerant electrons and local moments in iron-based superconductors", Europhys. Lett. 88, 17010 (2009).
  59. J. Wu and P. Phillips, "Magnon-mediated pairing and isotope effect in iron-based superconductors", J. Phys.: Condens. Matter 23, 094203 (2011).
  60. E. Dengler et al., "Strong reduction of the Korringa relaxation in the spin-density wave regime of $EuFe_2As_2$ observed by electron spin resonance", Phys. Rev. B 81, 024406 (2010).
  61. G. D. Samolyuk and V. P. Antropov, "Character of magnetic instabilities in $CaFe_2As_2$", Phys. Rev. B 79, 052505 (2009).
  62. S. J. Moon et al., "Dual character of magnetism in $EuFe_2As_2$: optical spectroscopic and densityfunctional calculation study", Phys. Rev. B 81, 205114 (2010).
  63. H. Oh, D. Shin, and H. J. Choi, "Orbital-selective magnetism in FeAs-based superconductors", arXiv: 1012.2224.
  64. H. Gretarsson et al., "Revealing the dual nature of magnetism in iron pnictides and iron chalcogenides using x-ray emission spectroscopy", Phys. Rev. B 84, 100509(R) (2011).
  65. A. Subedi, L. Zhang, D. J. Singh, and M. H. Du, "Density functional study of FeS, FeSe, and FeTe: electronic structure, magnetism, phonons, and superconductivity", Phys. Rev. B 78, 134514 (2008).
  66. M. J. Han and S. Y. Savrasov, "Doping driven (${\pi}$, 0) nesting and magnetic properties of $Fe_{1+x}Te$ superconductors", Phys. Rev. Lett. 103, 067001 (2009).
  67. Y. Xia et al., "Fermi surface topology and low-lying quasiparticle dynamics of parent $Fe_{1+x}Te/Se$ superconductor", Phys. Rev. Lett. 103, 037002 (2009).
  68. K. Nakayama et al., "Angle-resolved photoemission spectroscopy of the iron-chalcogenide superconductor $Fe_{1.03}Te_{0.7}Se_{0.3}$: strong coupling behavior and the universality of interband scattering", Phys. Rev. Lett. 105, 197001 (2010).
  69. C.-Y. Moon and H. J. Choi, "Chalcogen-height dependent magnetic interactions and magnetic order switching in $FeSe_xTe_{1-x}$", Phys. Rev. Lett. 104, 057003 (2010).
  70. C. Lester et al., "Neutron scattering study of the interplay between structure and magnetism in $Ba(Fe_{1−x}Co_x)_2As_2$", Phys. Rev. B 79, 144523 (2009).
  71. D. K. Pratt et al., "Coexistence of competing antiferromagnetic and superconducting phases in the underdoped $Ba(Fe_{0.953}Co_{0.047})_2As_2$ compound using X-ray and neutron scattering techniques", Phys. Rev. Lett. 103, 087001 (2009).
  72. J. Koo et al., "Magnetic and structural phase transitions of $EuFe_2As_2$ studied via neutron and resonant x-ray scattering", J. Phys. Soc. Jpn. 79, 114708 (2010).
  73. C. Fang, H. Yao, W.-F. Tsai, J. Hu, and S. A. Kivelson, "Theory of electron nematic order in LaFeAsO", Phys. Rev. B 77, 224509 (2008).
  74. T. Yildirim, "Strong coupling of the Fe-spin state and the As-As hybridization in iron-pnictide superconductors from first-principle calculations", Phys. Rev. Lett. 102, 037003 (2009).
  75. C. Xu, M. Muller, and S. Sachdev, "Ising and spin orders in the iron-based superconductors", Phys. Rev. B 78, 020501(R) (2008).
  76. M. D. Johannes and I. I. Mazin, "Microscopic origin of magnetism and magnetic interactions in ferropnictides", Phys. Rev. B 79, 220510 (2009).
  77. F. Krüger, S. Kumar, J. Zaanen, and J. van den Brink, "Spin-orbital frustrations and anomalous metallic state in iron-pnictide superconductors", Phys. Rev. B 79, 054504 (2009).
  78. C.-C. Chen, B. Moritz, J. van den Brink, T. P. Devereaux, and R. R. P. Singh, "Finite-temperature spin dynamics and phase transitions in spin-orbital models", Phys. Rev. B 80, 180418(R) (2009).
  79. W. Lv, J. Wu, and P. Phillips, "Orbital ordering induces structural phase transition and the resistivity anomaly in iron pnictides", Phys. Rev. B 80, 224506 (2009).
  80. C.-C. Lee, W.-G. Yin, and W. Ku, "Ferro-orbital order and strong magnetic anisotropy in the parent compounds of iron-pnictide superconductors", Phys. Rev. Lett. 103, 267001 (2009).
  81. M. A. Tanatar et al., "Direct imaging of the structural domains in the iron pnictides $AFe_2As_2$ (A = Ca, Sr, Ba)", Phys. Rev. B 79, 180508(R) (2009).
  82. T.-M. Chuang et al., "Nematic electronic structure in the "parent" state of the iron-based superconductor $Ca(Fe_{1-x}Co_x)_2As_2$", Science 327, 181-184 (2010).
  83. J.-H. Chu et al., "In-plane resistivity anisotropy in an underdoped iron arsenide superconductor", Science 329, 824-826 (2010).
  84. P. Richard et al., "Observation of dirac cone electronic dispersion in $BaFe_2As_2$", Phys. Rev. Lett. 104, 137001 (2010).
  85. M. Yi et al., "Symmetry-breaking orbital anisotropy observed for detwinned $Ba(Fe_{1-x}Co_x)_2As_2$ above the spin density wave transition", Proc. Natl. Acad. Sci. U.S.A. 108, 6878-6883 (2011).
  86. Y. Kim et al., "Electronic structure of detwinned $BaFe_2As_2$ from photoemission and first principles", Phys. Rev. B 83, 064509 (2011).
  87. M. Yi et al., "Unconventional electronic reconstruction in undoped $(Ba,Sr)Fe_2As_2$ across the spin density wave transition", Phys. Rev. B 80, 174510 (2009).
  88. Q. Huang et al., "Neutron-diffraction measurements of magnetic order and a structural transition in the parent $BaFe2As_2$ compound of FeAs-based high-temperature superconductors", Phys. Rev. Lett. 101, 257003 (2008).
  89. Z. P. Yin, K. Haule and G. Kotliar, "Magnetism and charge dynamics in iron pnictides", Nature Phys. 7, 294-297 (2011).
  90. M. Nakajima et al., "Evolution of the optical spectrum with doping in $Ba(Fe_{1−x}Co_x)_2As_2$", Phys. Rev. B 81, 104528 (2010).
  91. F. Hunte et al., "Two-band superconductivity in $LaFeAsO_{0.89}F_{0.11}$ at very high magnetic fields", Nature (London) 453, 903-905 (2008).
  92. A. Gurevich, "Enhancement of the upper critical field by nonmagnetic impurities in dirty two-gap superconductors", Phys. Rev. B 67, 184515 (2003).
  93. A. Gurevich, "Limits of the upper critical field in dirty two-gap superconductors", Physica C 456, 160-169 (2007).
  94. T. Y. Chen, Z. Tesanovic, R. H. Liu, X. H. Chen, and C. L. Chien, "A BCS-like gap in the superconductor $SmFeAsO_{0.85}F_{0.15}$", Nature (London) 453, 1224- 1227 (2008).
  95. H. Ding et al., "Observation of Fermi-surface- dependent nodeless superconducting gaps in $Ba_{0.6}K_{0.4}Fe_2As_2$", Europhys. Lett. 83, 47001 (2008).
  96. L. Zhao et al., "Multiple nodeless superconducting gaps in $(Ba_{0.6}K_{0.4})Fe_2As_2$ superconductor from angle-resolved photoemission spectroscopy", Chin. Phys. Lett. 25, 4402-4405 (2008).
  97. C. Liu et al., "K-doping dependence of the Fermi surface of the iron-arsenic $Ba_{1-x}K_xFe_2As_2$ superconductor using angle-resolved photoemission spectroscopy", Phys. Rev. Lett. 101, 177005 (2008).
  98. Y. Zhang et al., "Unusual doping dependence of the electronic structure and coexistence of spin-density-wave and superconductor phases in single crystalline $Sr_{1-x}K_xFe_2As_2$", Phys. Rev. Lett. 102, 127003 (2009).
  99. H.-S. Lee et al., "Effects of two gaps and paramagnetic pair breaking on the upper critical field of $SmFeAsO_{0.85}$ and $SmFeAsO_{0.8}F_{0.2}$ single crystals", Phys. Rev. B 80, 144512 (2009).
  100. R. S. Gonnelli et al., "Point-contact Andreev-reflection spectroscopy in $ReFeAsO_{1-x}F_x$ (Re = La, Sm): possible evidence for two nodeless gaps", Physica C 469, 512-520 (2009).
  101. J. Park et al., "Observation of two-gap superconductivity in $SrFe_{1.85}Co_{0.15}As_2$ single crystals by scanning tunneling microscopy and spectroscopy", New J. Phys. 13, 033005 (2011).
  102. L. Boeri, O. V. Dolgov, and A. A. Golubov, "Is $LaFeAsO_{1-x}F_x$ an electron-phonon superconductor?", Phys. Rev. Lett. 101, 026403 (2008).
  103. R. H. Liu et al., "A large iron isotope effect in $SmFeAsO_{1-x}F_x$ and $Ba_{1-x}K_xFe_2As_2$", Nature (London) 459, 64-67 (2009).
  104. Y. Bang, "Isotope effect and the role of phonons in the iron-based superconductors", Phys. Rev. B 79, 092503 (2009).
  105. H.-Y. Choi, J. H. Yun, Y. Bang, and H. C. Lee, "Model for the inverse isotope effect of FeAs-based superconductors in the ${\pi}$-phase-shifted pairing state", Phys. Rev. B 80, 052505 (2009).
  106. I. I. Mazin, D. J. Singh, M. D. Johannes, and M. H. Du, "Unconventional superconductivity with a sign reversal in the order parameter of LaFeAsO_{1-x}F_x$", Phys. Rev. Lett. 101, 057003 (2008).
  107. M. M. Korshunov and I. Eremin, "Theory of magnetic excitations in iron-based layered superconductors", Phys. Rev. B 78, 140509(R) (2008).
  108. F. Wang, H. Zhai, Y. Ran, A. Vishwanath, and D.-H. Lee, "Functional renormalization-group study of the pairing symmetry and pairing mechanism of the FeAs-based high-temperature superconductor", Phys. Rev. Lett. 102, 047005 (2009).
  109. K. Kuroki et al., "Unconventional pairing originating from the disconnected Fermi surfaces of superconducting $LaFeAsO_{1-x}F_x$", Phys. Rev. Lett. 101, 087004 (2008).
  110. Q. Han, Y. Chen, and Z. D. Wang, "A generic twoband model for unconventional superconductivity and spin-density-wave order in electron- and hole-doped iron-based superconductors", Europhys. Lett. 82, 37007 (2008).
  111. P. A. Lee and X.-G. Wen, "Spin-triplet p-wave pairing in a three-orbital model for iron pnictide superconductors", Phys. Rev. B 78, 144517 (2008).
  112. X. Dai, Z. Fang, Y. Zhou, and F.-C. Zhang, "Even parity, orbital singlet, and spin triplet pairing for superconducting $LaFeAsO_{1-x}F_x$", Phys. Rev. Lett. 101, 057008 (2008).
  113. Y. Bang and H.-Y. Choi, "Possible pairing states of the Fe-based superconductors", Phys. Rev. B 78, 134523 (2008).
  114. Y. Bang, H.-Y. Choi, and H. Won, "Impurity effects on the ${\pm}s-wave$ state of the iron-based superconductors", Phys. Rev. B 79, 054529 (2009).
  115. Y. Bang, "Superfluid density of the ${\pm}s-wave$ state for the iron-based superconductors", Europhys. Lett. 86, 47001 (2009).
  116. Y. Bang, "Volovik effect in the ${\pm}s-wave$ state for the iron-based superconductors", Phys. Rev. Lett. 104, 217001 (2010).
  117. K. Hashimoto et al., "Microwave penetration depth and quasiparticle conductivity of $PrFeAsO_{1-y}$ single crystals: evidence for a full-gap superconductor", Phys. Rev. Lett. 102, 017002 (2009).
  118. T. Kondo et al., "Momentum dependence of the superconducting gap in $NdFeAsO_{0.9}F_{0.1}$ single crystals measured by angle resolved photoemission spectroscopy", Phys. Rev. Lett. 101, 147003 (2008).
  119. T. Hanaguri, S. Niitaka, K. Kuroki, and H. Takagi, "Unconventional s-wave superconductivity in Fe(Se,Te)", Science 328, 474-476 (2010).
  120. K. Horigane, H. Hiraka, and K. Ohoyama, "Relationship between structure and superconductivity in $FeSe_{1-x}Te_x$", J. Phys. Soc. Jpn. 78, 074718 (2009).
  121. H. Ogino et al., "Superconductivity at 17 K in $(Fe_2P_2)(Sr_4Sc_2O_6)$: a new superconducting layered pnictide oxide with a thick perovskite oxide layer", Supercond. Sci. Technol. 22, 075008 (2009).
  122. H. Okabe, N. Takeshita, K. Horigane, T. Muranaka, and J. Akimitsu, "Pressure-induced high-$T_c$ superconducting phase in FeSe: correlation between anion height and $T_c$", Phys. Rev. B 81, 205119 (2010).
  123. W. Bao et al., "Tunable (${\delta}{\pi},\,{\delta}{\pi}$)-type antiferromagnetic order in ${\alpha}$-Fe(Te,Se) superconductors", Phys. Rev. Lett. 102, 247001 (2009).
  124. J. S. Kim et al., "Electron-hole asymmetry in Coand Mn-doped $SrFe_2As_2$", Phys. Rev. B 82, 024510 (2010).