The recent observations of superconductivity at temperatures up to 55K in compounds containing layers of iron arsenide [1,2,3,4] have revealed a new class of high temperature superconductors that show striking similarities to the more familiar cuprates. In both series of compounds, the onset of superconductivity is associated with the suppression of magnetic order by doping holes and/or electrons into the band [5] leading to theories in which magnetic fluctuations are either responsible for or strongly coupled to the superconducting order parameter [6]. In the cuprates, theories of magnetic pairing have been invoked to explain the observation of a resonant magnetic excitation that scales in energy with the superconducting energy gap and is suppressed above the superconducting transition temperature, Tc. Such resonant excitations have been shown by inelastic neutron scattering to be a universal feature of the cuprate superconductors [7], and have even been observed in heavy fermion superconductors with much lower transition temperatures [8,9,10]. In this paper, we show neutron scattering evidence of a resonant excitation in Ba0.6K0.4Fe2As2, which is a superconductor below 38 K [4], at the momentum transfer associated with magnetic order in the undoped compound, BaFe2As2, and at an energy transfer that is consistent with scaling in other strongly correlated electron superconductors. As in the cuprates, the peak disappears at Tc providing the first experimental confirmation of a strong coupling of the magnetic fluctuation spectrum to the superconducting order parameter in the new iron arsenide superconductors.Unconventional superconductivity has been the subject of considerable theoretical and experimental interest since the discovery of superconductivity in CeCu 2 Si 2 and other heavy fermion compounds [11], an interest that was only intensified by the discovery of cuprate superconductors with transition temperatures in excess of 100 K [6]. Although significant progress has been made, the origin of unconventional superconductivity is still not understood. The observation of a magnetic resonance in the spin excitation spectrum which appears concurrently with the onset of superconductivity in both the high T c cuprates [12,13,14,15,16] and the heavy fermion superconductors [8,9,10] offers the tantalizing possibility of a unifying theme for unconventional superconductivity that spans a diverse range of superconducting materials. Recently, a new family of superconductors containing layers of Fe 2 As 2 has been discovered with T c s in excess of 50 K stimulating considerable experimental and theoretical activity [1,2,3]. Although there is mounting evidence that the superconductivity in this new family is also unconventional [17], there is as yet no consensus concerning the mechanism giving rise to superconductivity or even the superconducting pairing symmetry. In this letter, we describe neutron scattering data that confirm for the first time the existence of a resonant spin excitation below T c in the iron arsenide ma...

Inelastic neutron scattering measurements on single crystals of superconducting BaFe1.84Co0.16As2 reveal a magnetic excitation located at wavevectors (1/2 1/2 L) in tetragonal notation. On cooling below TC, a clear resonance peak is observed at this wavevector with an energy of 8.6(0.5) meV, corresponding to 4.5(0.3) kBTC . This is in good agreement with the canonical value of 5 kBTC observed in the cuprates. The spectrum shows strong dispersion in the tetragonal plane but very weak dispersion along the c-axis, indicating that the magnetic fluctuations are two-dimensional in nature. This is in sharp contrast to the anisotropic three dimensional spin excitations seen in the undoped parent compounds.PACS numbers: 78.70.Nx, 74.20.Mn Understanding the physics of superconductivity in high-T c cuprates and other unconventional superconductors remains a central unresolved problem at the forefront of condensed matter physics. One widespread school of thought maintains that magnetic fluctuations are intimately involved in the pairing mechanism. This view is supported by a growing number of neutron scattering investigations showing the appearance of a magnetic excitation coincident with the onset of superconductivity [1,2,3,4,5,6,7,8]. The spectrum shows a resonance at a wavevector related to the antiferromagnetic order in the non-superconducting parent compounds. The apparent resonance energy scales with T C for different cuprate materials exhibiting a wide range of superconducting transition temperatures [9], providing tantalizing evidence for a common mechanism related to magnetic fluctuations.The discovery of a new family of Fe-based high temperature superconductors with T C as high as 55 K [10,11,12,13,14,15,16] presents an exciting opportunity to examine the relationship of spin excitations to the superconducting condensate in unconventional superconductors. The new materials are composed of Fe containing planes (FeAs or FeSe). Both theory and experiment indicate that simple electron-phonon coupling cannot describe superconductivity in these materials [17,18]. Furthermore, the superconducting state exists in close proximity to magnetism as the parent compounds exhibit spin-density wave order [19,20]. These observations have been put forth as evidence that the superconductivity in the Fe-based materials is unconventional. The presence of the Fe planes suggests quasi-two-dimensionality, as observed in the cuprates. However, neutron scattering investigations of the spin waves in the undoped parent compounds SrFe 2 As 2 [21], BaFe 2 As 2 [22], and CaFe 2 As 2 [23], indicate anisotropic exchange that cannot be classified as two dimensional. Band structure calculations [24,25] indicate that doping should enhance the twodimensionality of the Fermi surface, favoring superconductivity [25]. Directly probing the magnetic fluctuations in superconducting Fe-based systems is crucial for further progress.Recent measurements on a polycrystalline sample of Ba 0.6 K 0.4 Fe 2 As 2 found a spin excitation that appears at the onset...