Review Article

High-temperature superconductivity in iron pnictides and chalcogenides

  • Nature Reviews Materials 1, Article number: 16017 (2016)
  • doi:10.1038/natrevmats.2016.17
  • Download Citation
Published online:

Abstract

Superconductivity develops in metals upon the formation of a coherent macroscopic quantum state of electron pairs. Iron pnictides and chalcogenides are materials that have high superconducting transition temperatures. In this Review, we describe the advances in the field that have led to higher superconducting transition temperatures in iron-based superconductors and the wide range of materials that are used to form these superconductors. We summarize the essential aspects of the normal state and the mechanism for superconductivity. We emphasize the degree of electron–electron correlations and their manifestation in properties of the normal state. We examine the nature of magnetism, analyse its role in driving the electronic nematicity and discuss quantum criticality at the border of magnetism in the phase diagram. Finally, we review the amplitude and structure of the superconducting pairing, and survey the potential material settings for optimizing superconductivity.

  • Subscribe to Nature Reviews Materials for full access:

    $59

    Subscribe

Additional access options:

Already a subscriber?  Log in  now or  Register  for online access.

References

  1. 1.

    , , & Iron-based layered superconductor La[O1−xFx]FeAs (x = 0.05–0.12) with Tc = 26 K. J. Am. Chem. Soc. 130, 3296–3297 (2008).

  2. 2.

    & Possible high Tc superconductivity in the Ba–La–Cu–O system. Z. Phys. B. 64, 189–193 (1986).

  3. 3.

    et al. Superconductivity at 55 K in iron-based F-doped layered quaternary compound Sm[O1−xFx]FeAs. Chin. Phys. Lett. 25, 2215–2216 (2008).

  4. 4.

    et al. Interface-induced high-temperature superconductivity in single unit-cell FeSe films on SrTiO3. Chin. Phys. Lett. 29, 037402 (2012). Observation of superconductivity in the single-layer FeSe system that possesses the highest superconducting transition temperature in FeSCs.

  5. 5.

    et al. Phase diagram and electronic indication of high-temperature superconductivity at 65 K in single-layer FeSe films. Nat. Mater. 12, 605–610 (2013).

  6. 6.

    et al. Interfacial mode coupling as the origin of the enhancement of Tc in FeSe films on SrTiO3. Nature 515, 245–248 (2014).

  7. 7.

    et al. Onset of the Meissner effect at 65 K in FeSe thin film grown on Nb-doped SrTiO3 substrate. Sci. Bull. 60, 1301–1304 (2015).

  8. 8.

    et al. Superconductivity above 100 K in single-layer FeSe films on doped SrTiO3. Nat. Mater. 14, 285–289 (2015).

  9. 9.

    , & Is LaFeAsO1−xFx an electron-phonon superconductor?. Phys. Rev. Lett. 101, 026403 (2008).

  10. 10.

    , & Superconductivity at 38 K in the iron arsenide (Ba1−xKx)Fe2As2. Phys. Rev. Lett. 101, 107006 (2008).

  11. 11.

    et al. Superconductivity in the PbO-type structure α-FeSe. Proc. Natl Acad. Sci. USA 105, 14262–14264 (2008).

  12. 12.

    et al. Superconductivity in the iron selenide KxFe2Se2 (0 ≤ x ≤ 1.0). Phys. Rev. B 82, 180520 (2010).

  13. 13.

    et al. Re-emerging superconductivity at 48 kelvin in iron chalcogenides. Nature 483, 67–69 (2012).

  14. 14.

    et al. Electronic structure of the BaFe2As2 family of iron-pnictide superconductors. Phys. Rev. B 80, 024515 (2009).

  15. 15.

    et al. Band structure and Fermi surface of an extremely overdoped iron-based superconductor KFe2As2. Phys. Rev. Lett. 103, 047002 (2009).

  16. 16.

    Antiferromagnetic order and spin dynamics in iron-based superconductors. Rev. Mod. Phys. 87, 855–896 (2015).

  17. 17.

    , & Universality of the Mott–Ioffe–Regel limit in metals. Philos. Mag. 84, 2847–2864 (2004).

  18. 18.

    & Quantum criticality in the iron pnictides and chalcogenides. J. Phys. Condens. Matter 23, 223201 (2011).

  19. 19.

    Spin-density-wave antiferromagnetism in chromium. Rev. Mod. Phys. 60, 209–283 (1988).

  20. 20.

    & Strong correlations and magnetic frustration in the high Tc iron pnictides. Phys. Rev. Lett. 101, 076401 (2008). A theoretical study that emphasizes that the parent systems of the FeSCs display bad-metal behaviour and infers that their electron correlations are strong.

  21. 21.

    , , & Correlation effects in the iron pnictides. New J. Phys. 11, 045001 (2009).

  22. 22.

    et al. Electronic correlations in the iron pnictides. Nat. Phys. 5, 647–650 (2009). Experimental evidence from optical conductivity that iron arsenides possess strong electron correlations.

  23. 23.

    et al. Normal-state charge dynamics in doped BaFe2As2: roles of doping and necessary ingredients for superconductivity. Sci. Rep. 4, 5873 (2014).

  24. 24.

    et al. Electronic reconstruction through the structural and magnetic transitions in detwinned NaFeAs. New J. Phys. 14, 073019 (2012).

  25. 25.

    et al. Strong electron correlations in the normal state of the iron-based FeSe0.42Te0.58 superconductor observed by angle-resolved photoemission spectroscopy. Phys. Rev. Lett. 104, 097002 (2010).

  26. 26.

    et al. Observation of temperature-induced crossover to an orbital-selective Mott phase in AxFe2−ySe2 (A = K, Rb) superconductors. Phys. Rev. Lett. 110, 067003 (2013).

  27. 27.

    et al. Observation of universal strong orbital-dependent correlation effects in iron chalcogenides. Nat. Commun. 6, 7777 (2015). Angle-resolved photoemission measurements provide evidence for strong and orbital-selective electron correlations in iron chalcogenides.

  28. 28.

    et al. Band narrowing and Mott localization in iron oxychalcogenides La2O2Fe2O(Se, S)2. Phys. Rev. Lett. 104, 216405 (2010).

  29. 29.

    & Low-temperature nuclear and magnetic structures of La2O2Fe2OSe2 from x-ray and neutron diffraction measurements. Phys. Rev. B 81, 214433 (2010).

  30. 30.

    , & Metal-insulator transitions. Rev. Mod. Phys. 70, 1039–1263 (1998).

  31. 31.

    et al. Mott-Kondo insulator behavior in the iron oxychalcogenides. Phys. Rev. B 92, 155139 (2015).

  32. 32.

    et al. Fe-based superconductivity with Tc = 31 K bordering an antiferromagnetic insulator in (Tl, K)FexSe2. Europhys. Lett. 94, 27009 (2011).

  33. 33.

    et al. A novel large moment antiferromagnetic order in K0.8Fe1.6Se2 superconductor. Chin. Phys. Lett. 28, 086104 (2011).

  34. 34.

    , & Mott transition in modulated lattices and parent insulator of (K, Tl)yFexSe2 superconductors. Phys. Rev. Lett. 106, 186401 (2011).

  35. 35.

    , , & Theory for superconductivity in (Tl, K)FexSe2 as a doped Mott insulator. Europhys. Lett. 95, 17003 (2011).

  36. 36.

    et al. Mott localization in a pure stripe antiferromagnet Rb1−δFe1.5−σS2. Phys. Rev. B 92, 121101 (2015).

  37. 37.

    et al. Orbital-selective metal–insulator transition and gap formation above TC in superconducting Rb1−xFe2−ySe2. Nat. Commun. 5, 3202 (2014).

  38. 38.

    , , & Strong and nonmonotonic temperature dependence of Hall coefficient in superconducting KxFe2−ySe2 single crystals. Phys. Rev. B 89, 224515 (2014).

  39. 39.

    et al. Mott behaviour in KxFe2−ySe2 superconductors studied by pump-probe spectroscopy. Phys. Rev. B 89, 134515 (2014).

  40. 40.

    et al. Role of the 245 phase in alkaline iron selenide superconductors revealed by high-pressure studies. Phys. Rev. B 89, 094514 (2014).

  41. 41.

    & Orbital-selective Mott phase in multiorbital models for alkaline iron selenides K1−xFe2−ySe2. Phys. Rev. Lett. 110, 146402 (2013).

  42. 42.

    , & Orbital-dependent effects of electron correlations in microscopic models for iron-based superconductors. Curr. Opin. Solid State Mater. Sci. 17, 65–71 (2013).

  43. 43.

    , , , & Orbital-selective Mott-insulator transition in Ca2−xSrxRuO4. Eur. Phys. J. B 25, 191–201 (2002).

  44. 44.

    & U(1) slave-spin theory and its application to Mott transition in a multi-orbital model for iron pnictides. Phys. Rev. B 86, 085104 (2012).

  45. 45.

    , & Selective Mottness as a key to iron superconductors. Phys. Rev. Lett. 112, 177001 (2014).

  46. 46.

    , & Orbital differentiation and the role of orbital ordering in the magnetic state of Fe superconductors. Phys. Rev. B 86, 174508 (2012).

  47. 47.

    et al. Observation of a novel orbital selective Mott transition in Ca1.8Sr0.2RuO4. Phys. Rev. Lett. 103, 097001 (2009).

  48. 48.

    , & Kinetic frustration and the nature of the magnetic and paramagnetic states in iron pnictides and iron chalcogenides. Nat. Mater. 10, 932–935 (2011). An article reporting calculations using dynamical mean field theory, illustrating that iron pnictides and chalcogenides provide a continuous range in the strength of electron correlations among the FeSCs.

  49. 49.

    , , & Superconductivity at the border of electron localization and itinerancy. Nat. Commun. 4, 2783 (2013).

  50. 50.

    , , , & Theory of electron nematic order in LaFeAsO. Phys. Rev. B 77, 224509 (2008).

  51. 51.

    , & Ising and spin orders in the iron-based superconductors. Phys. Rev. B 78, 020501(R) (2008).

  52. 52.

    , , & Electrodynamic response of incoherent metals: normal phase of iron pnictides. Phys. Rev. B 79, 024515 (2009).

  53. 53.

    , & Pairing symmetry in a two-orbital exchange coupling model of oxypnictides. Phys. Rev. Lett. 101, 206404 (2008).

  54. 54.

    , , & Properties of a two-orbital model for oxypnictide superconductors: magnetic order, B2g spin-singlet pairing channel, and its nodal structure. Phys. Rev. B 79, 134502 (2009).

  55. 55.

    , , & Strong coupling theory for superconducting iron pnictides. Phys. Rev. Lett. 102, 047006 (2009).

  56. 56.

    , & Fermiology, orbital order, orbital fluctuations, and Cooper pairing in iron-based superconductors. Phys. Rev. B 88, 100504(R) (2013).

  57. 57.

    , & A twisted ladder: relating the Fe superconductors to the high-Tc cuprates. New J. Phys. 11, 085007 (2009).

  58. 58.

    , Krü & Orbital ordering and unfrustrated (π, 0) magnetism from degenerate double exchange in the iron pnictides. Phys. Rev. B 82, 045125 (2010).

  59. 59.

    & Antiferroquadrupolar and Ising-nematic orders of a frustrated bilinear-biquadratic Heisenberg model and implications for the magnetism of FeSe. Phys. Rev. Lett. 115, 116401 (2015).

  60. 60.

    , & Nematicity and quantum paramagnetism in FeSe. Nat. Phys. 11, 959–963 (2015).

  61. 61.

    et al. Magnetic order close to superconductivity in the iron-based layered La(O1−xFx)FeAs systems. Nature 453, 899–902 (2008). Neutron scattering measurements demonstrate AFM and structural phase transitions in a parent compound of the FeSCs.

  62. 62.

    et al. Paramagnetic spin correlations in CaFe2As2 single crystals. Phys. Rev. B 81, 214407 (2010).

  63. 63.

    et al. Nematic spin fluid in the tetragonal phase of BaFe2As2. Phys. Rev. B 84, 054544 (2011).

  64. 64.

    et al. Itinerant spin excitations in SrFe2As2 measured by inelastic neutron scattering. Phys. Rev. B 83, 214519 (2011).

  65. 65.

    Origin of the 150-K anomaly in LaFeAsO: competing antiferromagnetic interactions, frustration, and a structural phase transition. Phys. Rev. Lett. 101, 057010 (2008).

  66. 66.

    , & Antiferromagnetic superexchange interactions in LaOFeAs. Phys. Rev. B 78, 224517 (2008).

  67. 67.

    , & Ising transition in frustrated Heisenberg models. Phys. Rev. Lett. 64, 88–91 (1990).

  68. 68.

    Lecture Notes on Electron Correlation and Magnetism Ch. 5 (World Scientific, 1999).

  69. 69.

    et al. Spin dynamics of a J1J2K model for the paramagnetic phase of iron pnictides. Phys. Rev. B 86, 085148 (2012).

  70. 70.

    , & Consistent model of magnetism in ferropnictides. Nat. Phys. 7, 485–489 (2011).

  71. 71.

    et al. Nature of magnetic excitations in superconducting BaFe1.9Ni0.1As2. Nat. Phys. 8, 376–381 (2012). Inelastic neutron scattering measurements demonstrate that the integrated spin spectral weight of iron pnictides is much larger than what is expected from particle–hole excitations near the Fermi energy.

  72. 72.

    et al. Competing orders and spin-density-wave instability in La(O1−xFx)FeAs. Europhys. Lett. 83, 27006 (2008).

  73. 73.

    , & Multiorbital spin susceptibility in a magnetically ordered state: orbital versus excitonic spin density wave scenario. Phys. Rev. B 83, 224503 (2011).

  74. 74.

    , , , & First-principles calculations of the electronic structure of tetragonal α-FeTe and α-FeSe crystals: evidence for a bicollinear antiferromagnetic order. Phys. Rev. Lett. 102, 177003 (2009).

  75. 75.

    Magnetic neutron scattering studies on the Fe-based superconductor system Fe1+yTe1−xSex. Ann. Phys. 358, 92–107 (2015).

  76. 76.

    , & The magnetic phase diagram of an extended J1J2 model on a modulated square lattice and its implications for the antiferromagnetic phase of KyFexSe2. Phys. Rev. B 84, 094451 (2011).

  77. 77.

    & Block spin ground state and three-dimensionality of (K, Tl)Fe1.6Se2. Phys. Rev. Lett. 107, 056401 (2011).

  78. 78.

    et al. Spin waves and magnetic exchange interactions in insulating Rb0.89Fe1.58Se2. Nat. Commun. 2, 580 (2011).

  79. 79.

    et al. Neutron scattering study of spin dynamics in superconducting (Tl, Rb)2Fe4Se5. Phys. Rev. B 87, 100501 (2013).

  80. 80.

    et al. Two spatially separated phases in semiconducting Rb0.8Fe1.5S2. Phys. Rev. B 90, 125148 (2014).

  81. 81.

    , , & Divergent nematic susceptibility in an iron arsenide superconductor. Science 337, 710–712 (2012). An experimental study of resistivity anisotropy in the presence of an uniaxial strain, demonstrating that the structural transition is driven by electronic nematicity.

  82. 82.

    , , & Ubiquitous signatures of nematic quantum criticality in optimally doped Fe-based superconductors. Preprint at (2015).

  83. 83.

    et al. Nematic susceptibility of hole-doped and electron-doped BaFe2As2 iron-based superconductors from shear modulus measurements. Phys. Rev. Lett. 112, 047001 (2015).

  84. 84.

    et al. Critical quadrupole fluctuations and collective modes in iron pnictide superconductors. Phys. Rev. B 93, 054515 (2016).

  85. 85.

    et al. Nematic fluctuations and the magneto-structural phase transition in Ba(Fe1−xCox)2As2. Preprint at (2015).

  86. 86.

    , , & Iron pnictides as a new setting for quantum criticality. Proc. Natl Acad. Sci. USA 106, 4118–4121 (2009). Theoretical proposal for quantum criticality in iron pnictides and for its realization by isoelectronic phosphorus for arsenic substitution in iron arsenides.

  87. 87.

    , & What drives nematic order in iron-based superconductors? Nat. Phys. 10, 97–104 (2014).

  88. 88.

    et al. Orbital order and spontaneous orthorhombicity in iron pnictides. Phys. Rev. B 82, 100504(R) (2010).

  89. 89.

    , & Ferro-orbital order and strong magnetic anisotropy in the parent compounds of iron-pnictide superconductors. Phys. Rev. Lett. 103, 267001 (2009).

  90. 90.

    , , & Spin-orbital frustrations and anomalous metallic state in iron-pnictide superconductors. Phys. Rev. B 79, 054504 (2009).

  91. 91.

    et al. Symmetry-breaking orbital anisotropy on detwinned Ba(Fe1−xCox)2As2 above the spin density wave transition. Proc. Natl Acad. Sci. USA 108, 6878–6883 (2011).

  92. 92.

    et al. Nematic spin correlations in the tetragonal state of uniaxial strained BaFe2−xNixAs2. Science 345, 657–660 (2014).

  93. 93.

    et al. Energy dependence of the spin excitation anisotropy in uniaxial-strained BaFe1.9Ni0.1As2. Phys. Rev. B 92, 180504(R) (2015).

  94. 94.

    et al. Tetragonal-to-orthorhombic structural phase transition at 90 K in the superconductor Fe1.01Se. Phys. Rev. Lett. 103, 057002 (2009).

  95. 95.

    et al. Electronic and magnetic phase diagram of β-Fe1.01Se with superconductivity at 36.7 K under pressure. Nat. Mater. 8, 630–633 (2009).

  96. 96.

    et al. Origin of the tetragonal-to-orthorhombic phase transition in FeSe: a combined thermodynamic and NMR study of nematicity. Phys. Rev. Lett. 114, 027001 (2015).

  97. 97.

    et al. Orbital-driven nematicity in FeSe. Nat. Mater. 14, 210–214 (2015).

  98. 98.

    et al. Effect of magnetic frustration on nematicity and superconductivity in iron chalcogenides. Nat. Phys. 11, 953–958 (2015).

  99. 99.

    , , , & Strong (π, 0) spin fluctuations in β-FeSe observed by neutron spectroscopy. Phys. Rev. B 91, 180501(R) (2015).

  100. 100.

    et al. Strong interplay between stripe spin fluctuations, nematicity and superconductivity in FeSe. Nat. Mater. 15, 159–163 (2016).

  101. 101.

    et al. Evolution from non-Fermi- to Fermi-liquid transport via isovalent doping in BaFe2(As1−xPx)2 superconductors. Phys. Rev. B 81, 184519 (2010). An experimental study that demonstrates the proposed quantum criticality in phosphorus-doped iron arsenides.

  102. 102.

    , , , & Fermi-liquid instabilities at magnetic quantum phase transitions. Rev. Mod. Phys. 79, 1015–1075 (2007).

  103. 103.

    et al. Transport near a quantum critical point in BaFe2(As1−xPx)2. Nat. Phys. 10, 194–197 (2014).

  104. 104.

    & Universal relationship of the resistivity and specific heat in heavy-Fermion compounds. Solid State Commun. 58, 507–509 (1986).

  105. 105.

    et al. Quasiparticle mass enhancement close to the quantum critical point in BaFe2(As1−xPx)2. Phys. Rev. Lett. 110, 257002 (2013).

  106. 106.

    , , , & Evidence of quantum criticality in the phase diagram of KxSr1−xFe2As2 from measurements of transport and thermoelectricity. Phys. Rev. B 79, 104504 (2009).

  107. 107.

    et al. Contrasting spin dynamics between underdoped and overdoped Ba(Fe1−xCox)2As2. Phys. Rev. Lett. 104, 037001 (2010).

  108. 108.

    et al. Structural quantum criticality and superconductivity in iron-based superconductor Ba(Fe1−xCox)2As2. J. Phys. Soc. Jpn 81, 024604 (2012).

  109. 109.

    et al. Effects of Co substitution on thermodynamic and transport properties and anisotropic Hc2 in Ba(Fe1−xCox)2As2 single crystals. Phys. Rev. B 78, 214515 (2008).

  110. 110.

    , , & Determination of the phase diagram of the electron-doped superconductor Ba(Fe1−xCox)2As2. Phys. Rev. B 79, 014506 (2009).

  111. 111.

    et al. Neutron scattering study of the interplay between structure and magnetism in Ba(Fe1−xCox)2As2. Phys. Rev. B 79, 144523 (2009).

  112. 112.

    et al. Anomalous suppression of the orthorhombic lattice distortion in superconducting Ba(Fe1−xCox)2As2 single crystals. Phys. Rev. Lett. 104, 057006 (2010).

  113. 113.

    et al. Quantum criticality in electron-doped BaFe2−xNixAs2. Nat. Commun. 4, 2265 (2013).

  114. 114.

    et al. Avoided quantum criticality and magnetoelastic coupling in BaFe2−xNixAs2. Phys. Rev. Lett. 110, 257001 (2013).

  115. 115.

    et al. Short-range cluster spin glass near optimal superconductivity in BaFe2−xNixAs2. Phys. Rev. B 90, 024509 (2014).

  116. 116.

    -Ph. et al. Universal heat conduction in the iron arsenide superconductor KFe2As2: evidence of a d-wave state. Phys. Rev. Lett. 109, 087001 (2012).

  117. 117.

    et al. Octet-line node structure of superconducting order parameter in KFe2As2. Science 337, 1314–1317 (2012).

  118. 118.

    et al. Nodal gap in iron-based superconductor CsFe2As2 probed by quasiparticle heat transport. Phys. Rev. B 87, 144502 (2013).

  119. 119.

    et al. Heat transport in RbFe2As2 single crystals: evidence for nodal superconducting gap. Phys. Rev. B 91, 024502 (2015).

  120. 120.

    et al. Evidence of strong correlations and coherence–incoherence crossover in the iron pnictide superconductor KFe2As2. Phys. Rev. Lett. 111, 027002 (2013).

  121. 121.

    et al. Calorimetric study of single-crystal CsFe2As2. Phys. Rev. B 87, 214509 (2013).

  122. 122.

    et al. Quantum criticality in AFe2As2 with A = K, Rb, and Cs suppresses superconductivity. Preprint at (2015).

  123. 123.

    et al. Observation of Fermi-surface-dependent nodeless superconducting gaps in Ba0.6K0.4Fe2As2. Europhys. Lett. 83, 47001 (2008). Measurement of the quasi-particle excitation energy gap, which provides evidence that the superconducting pairing function in iron arsenides has no nodes.

  124. 124.

    , & Gap symmetry and structure of Fe-based superconductors. Rep. Prog. Phys. 74, 124508 (2011).

  125. 125.

    , & Superconductivity in multi-orbital tJ1J2 model and its implications for iron pnictides. Europhys. Lett. 91, 37006 (2010).

  126. 126.

    , , & Near-degeneracy of several pairing channels in multiorbital models for the Fe pnictides. New J. Phys. 11, 025016 (2009).

  127. 127.

    et al. Unconventional pairing originating from the disconnected Fermi surfaces of superconducting LaFeAsO1−xFx. Phys. Rev. Lett. 101, 087004 (2008).

  128. 128.

    , , , & Functional renormalization-group study of the pairing symmetry and pairing mechanism of the FeAs-based high-temperature superconductor. Phys. Rev. Lett. 102, 047005 (2009).

  129. 129.

    , & Orbital-selective superconductivity, gap anisotropy, and spin resonance excitations in a multiorbital tJ1J2 model for iron pnictides. Phys. Rev. B 89, 024509 (2014).

  130. 130.

    et al. Anisotropic but nodeless superconducting gap in the presence of spin-density wave in iron-pnictide superconductor NaFe1−xCoxAs. Phys. Rev. X 3, 011020 (2013).

  131. 131.

    et al. Double spin resonances and gap anisotropy in superconducting underdoped NaFe0.985Co0.015As. Phys. Rev. Lett. 111, 207002 (2013).

  132. 132.

    et al. Neutron spin resonance as a probe of superconducting gap anisotropy in partially detwinned electron underdoped NaFe0.985Co0.015As. Phys. Rev. B 91, 104520 (2015).

  133. 133.

    , & Orbital selectivity and emergent superconducting state from quasi-degenerate s- and d-wave pairing channels in iron-based superconductors. Preprint at (2015).

  134. 134.

    et al. Evidence for an s-wave superconducting gap in KxFe2−ySe2 from angle-resolved photoemission. Phys. Rev. B 85, 220504 (2012).

  135. 135.

    et al. Distinct Fermi surface topology and nodeless superconducting gap in a (Tl0.58Rb0.42)Fe1.72Se2 superconductor. Phys. Rev. Lett. 106, 107001 (2011). An angle-resolved photoemission experiment confirming that the superconducting pairing amplitude in iron chalcogenides without Fermi-surface nesting is comparable to that in iron pnictides.

  136. 136.

    et al. Strong nodeless pairing on separate electron Fermi surface sheets in (Tl, K)Fe1.78Se2 probed by ARPES. Europhys. Lett. 93, 57001 (2011).

  137. 137.

    et al. Observation of an isotropic superconducting gap at the Brillouin zone centre of Tl0.63K0.37Fe1.78Se2. Europhys. Lett. 99, 67001 (2012).

  138. 138.

    et al. Magnetic resonant mode in the low-energy spin-excitation spectrum of superconducting Rb2Fe4Se5 single crystals. Phys. Rev. Lett. 107, 177005 (2011).

  139. 139.

    et al. Reciprocal-space structure and dispersion of the magnetic resonant mode in the superconducting phase of RbxFe2−ySe2 single crystals. Phys. Rev. B 85, 140511(R) (2012).

  140. 140.

    et al. A unifying phase diagram with correlation-driven superconductor-to-insulator transition for the 122* series of iron chalcogenides. Phys. Rev. B 93, 054516 (2016).

  141. 141.

    et al. Electron correlation-tuned superconductivity in Rb0.8Fe2(Se1−zSz)2. Phys. Rev. Lett. 115, 256403 (2015).

  142. 142.

    et al. Observation of strong electron pairing on bands without Fermi surfaces in LiFe1−xCoxAs. Nat. Commun. 6, 6056 (2015).

  143. 143.

    , , & Effective exchange interactions for bad metals and implications for iron-based superconductors. Preprint at (2014).

  144. 144.

    & The discovery of superconductivity. Phys. Today 63, 38–43 (2010).

  145. 145.

    , & Microscopic theory of superconductivity. Phys. Rev. 106, 162–164 (1957).

  146. 146.

    & Iron-based superconductors: current status of materials and pairing mechanism. Physica C 514, 399–422 (2015).

  147. 147.

    Iron pnictide superconductors: electrons on the verge. Nat. Phys. 5, 629–630 (2009).

  148. 148.

    et al. Pressure-induced antiferromagnetic transition and phase diagram in FeSe. J. Phys. Soc. Jpn 84, 063701 (2015).

  149. 149.

    et al. Coexistence of superconductivity and magnetism in FeSe1−x under pressure. Phys. Rev. B 85, 064517 (2012).

Download references

Acknowledgements

The authors thank J. Analytis, M. Bendele, P. C. Dai, W. Ding, L. Harriger, X. Lu and P. Nikolic for their input. They have benefited from collaborations and/or discussions with J. Dai, P. C. Dai, W. Ding, P. Goswami, K. Grube, D. H. Lu, A. H. Nevidomskyy, E. Nica, P. Nikolic, Z.-X. Shen, H. von Löhneysen, Z. Wang, M. Yi, and J.-X. Zhu. This work was supported in part by the NSF (grant number DMR-1309531) and the Robert A. Welch Foundation (grant number C-1411) (Q.S.), and by the National Science Foundation of China (grant number 11374361) and the Fundamental Research Funds for the Central Universities and the Research Funds of Renmin University of China (R.Y.). They acknowledge the support provided in part by the NSF (grant number. NSF PHY11-25915) at KITP, UCSB, for our participation in the autumn 2014 programme on “Magnetism, Bad Metals and Superconductivity: Iron Pnictides and Beyond”. Q.S. and E.A. acknowledge the hospitality of the Aspen Center for Physics (NSF grant number 1066293).

Author information

Affiliations

  1. Department of Physics and Astronomy, Rice University, Houston, Texas 77005, USA.

    • Qimiao Si
  2. Department of Physics and Beijing Key Laboratory of Opto-electronic Functional Materials and Micro-nano Devices, Renmin University, Beijing 100872, China.

    • Rong Yu
  3. Department of Physics and Astronomy, Collaborative Innovation Center of Advanced Microstructures, Shanghai Jiaotong University, Shanghai 200240, China.

    • Rong Yu
  4. Department of Physics and Astronomy, University of California Los Angeles, Los Angeles, California 90095, USA.

    • Elihu Abrahams

Authors

  1. Search for Qimiao Si in:

  2. Search for Rong Yu in:

  3. Search for Elihu Abrahams in:

Competing interests

The authors declare no competing interests.

Corresponding authors

Correspondence to Qimiao Si or Rong Yu or Elihu Abrahams.