  The recent observations of superconductivity at temperatures up to 55K in
compounds containing layers of iron arsenide 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 leading to theories in which magnetic fluctuations are
either responsible for or strongly coupled to the superconducting order
parameter. 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, and have even been observed in heavy fermion superconductors
with much lower transition temperatures. In this paper, we show neutron
scattering evidence of a resonant excitation in Ba0.6K0.4Fe2As2, which is a
superconductor below 38K, 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.
