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

Abstract

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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Figure 1: The crystal structure of Ba0.6K0.4Fe2As2.
Figure 2: Resonant spin excitation in Ba0.6K0.4Fe2As2.
Figure 3: Energy dependence of the resonant spin excitation.
Figure 4: Temperature dependence of the resonant spin excitation.

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Acknowledgements

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.

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Correspondence to R. Osborn.

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Christianson, A., Goremychkin, E., Osborn, R. et al. Unconventional superconductivity in Ba0.6K0.4Fe2As2 from inelastic neutron scattering. Nature 456, 930–932 (2008). https://doi.org/10.1038/nature07625

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