Letter | Published:

Unconventional superconductivity in Ba0.6K0.4Fe2As2 from inelastic neutron scattering

Nature volume 456, pages 930932 (18 December 2008) | Download Citation


A new family of superconductors containing layers of iron arsenide1,2,3 has attracted considerable interest because of their high transition temperatures (Tc), some of which are >50 K, and because of similarities with the high-Tc copper oxide superconductors. In both the iron arsenides and the copper oxides, superconductivity arises when an antiferromagnetically ordered phase has been suppressed by chemical doping4. A universal feature of the copper oxide superconductors is the existence of a resonant magnetic excitation, localized in both energy and wavevector, within the superconducting phase5,6,7,8,9. This resonance, which has also been observed in several heavy-fermion superconductors10,11,12, is predicted to occur when the sign of the superconducting energy gap takes opposite values on different parts of the Fermi surface13, an unusual gap symmetry which implies that the electron pairing interaction is repulsive at short range14. Angle-resolved photoelectron spectroscopy shows no evidence of gap anisotropy in the iron arsenides, but such measurements are insensitive to the phase of the gap on separate parts of the Fermi surface15. Here we report inelastic neutron scattering observations of a magnetic resonance below Tc in Ba0.6K0.4Fe2As2, a phase-sensitive measurement demonstrating that the superconducting energy gap has unconventional symmetry in the iron arsenide superconductors.

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  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.

    et al. Superconductivity at 43 K in an iron-based layered compound LaO1-xFxFeAs. Nature 453, 376–378 (2008)

  3. 3.

    et al. Superconductivity and phase diagram in iron-based arsenic-oxides ReFeAsO1-δ (Re = rare-earth metal) without fluorine doping. Europhys. Lett. 83, 17002 (2008)

  4. 4.

    et al. Magnetic order close to superconductivity in the iron-based layered LaO1-xFxFeAs systems. Nature 453, 899–902 (2008)

  5. 5.

    et al. Neutron scattering study of the YBa2Cu3O6+x system. Physica C 185, 86–92 (1991)

  6. 6.

    , , , & Polarized neutron determination of the magnetic excitations in YBa2Cu3O7. Phys. Rev. Lett. 70, 3490–3493 (1993)

  7. 7.

    et al. Neutron scattering from magnetic excitations in Bi2Sr2CaCu2O8+δ. Nature 398, 588–591 (1999)

  8. 8.

    , , , & Resonance as a measure of pairing correlations in the high-Tc superconductor YBa2Cu3O6.6. Nature 406, 965–968 (2000)

  9. 9.

    et al. Magnetic resonant mode in the single-layer high-temperature superconductor Tl2Ba2CuO6+δ. Science 295, 1045–1047 (2002)

  10. 10.

    et al. Strong coupling between local moments and superconducting ‘heavy’ electrons in UPd2Al3. Nature 410, 340–343 (2001)

  11. 11.

    , , , & Spin resonance in the d-wave superconductor CeCoIn5. Phys. Rev. Lett. 100, 087001 (2008)

  12. 12.

    et al. Magnetism and superconductivity in the heavy-fermion compound CeCu2Si2 studied by neutron scattering. Physica B (Amsterdam) 403, 973–976 (2008)

  13. 13.

    , , & Theory of magnetic excitons in the heavy-fermion superconductor UPd2Al3. Phys. Rev. B 75, 024503 (2007)

  14. 14.

    , , & Unconventional superconductivity with a sign reversal in the order parameter of LaFeAsO1-xFx. Phys. Rev. Lett. 101, 057003 (2008)

  15. 15.

    et al. Observation of Fermi-surface–dependent nodeless superconducting gaps in Ba0.6K0.4Fe2As2. Europhys. Lett. 83, 47001 (2008)

  16. 16.

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

  17. 17.

    et al. Superconducting Fe-based compounds (A1-xSrx)Fe2As2 with A = K and Cs with transition temperatures up to 37 K. Phys. Rev. Lett. 101, 107007 (2008)

  18. 18.

    et al. Pressure-induced superconductivity in CaFe2As2. J. Phys. Condens. Matter 20, 322204 (2008)

  19. 19.

    et al. Magnetic order in BaFe2As2, the parent compound of the FeAs based superconductors in a new structural family. Preprint at 〈〉 (2008)

  20. 20.

    et al. MERLIN, a new high count-rate spectrometer at ISIS. Physica B (Amsterdam) 385–386, 1029–1031 (2006)

  21. 21.

    et al. K-doping dependence of the Fermi surface of the iron-arsenic Ba1-xKxFe2As2 superconductor using angle-resolved photoemission spectroscopy. Phys. Rev. Lett. 101, 177005 (2008)

  22. 22.

    et al. Band structure and electronic signature of the spin density waves in bilayer iron-oxypnictide BaFe2As2. Preprint at 〈〉 (2008)

  23. 23.

    & Theory of magnetic excitations in iron-based layered superconductors. Phys. Rev. B 78, 140509(R) (2008)

  24. 24.

    & Theory of neutron scattering as a probe of the superconducting gap in the iron pnictides. Phys. Rev. B 78, 020514(R) (2008)

  25. 25.

    , , & Two gaps make a high-temperature superconductor? Rep. Prog. Phys. 71, 062501 (2008)

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We acknowledge discussions with M. Norman and C. Stock. This work was supported by the Division of Materials Sciences and Engineering Division and the Scientific User Facilities Division of the Office of Basic Energy Sciences, US Department of Energy Office of Science.

Author information


  1. Neutron Scattering Science Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, USA

    • A. D. Christianson
    •  & M. D. Lumsden
  2. Materials Science Division, Argonne National Laboratory, Argonne, Illinois 60439-4845, USA

    • E. A. Goremychkin
    • , R. Osborn
    • , S. Rosenkranz
    • , C. D. Malliakas
    • , I. S. Todorov
    • , H. Claus
    • , D. Y. Chung
    •  & M. G. Kanatzidis
  3. ISIS Pulsed Neutron and Muon Facility, Rutherford Appleton Laboratory, Chilton, Didcot OX11 0QX, UK

    • E. A. Goremychkin
    • , R. I. Bewley
    •  & T. Guidi
  4. Department of Chemistry, Northwestern University, Evanston, Illinois 60208-3113, USA

    • C. D. Malliakas
    •  & M. G. Kanatzidis


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Corresponding author

Correspondence to R. Osborn.

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    Supplementary Information

    This file contains Supplementary Methods, Supplementary Results Supplementary Figures 1-2 with Legends, Supplementary Table 1 and Supplementary Data.

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