  Motivated by the premise that superconductivity in iron-based superconductors
is unconventional and mediated by spin fluctuations, an intense research effort
has been focused on characterizing the spin excitation spectrum in the
magnetically ordered parent phases of the Fe-pnictides2,3 and - chalcogenides4.
For these undoped materials it is well-established that the spin excitation
spectrum consists of a sharp, highly dispersive magnon spanning an energy range
of up to 200 meV (ref. 3). The fate of these high-energy magnetic modes upon
sizable doping is hitherto unresolved. Using resonant inelastic x-ray
scattering we show that optimally doped superconducting Ba0.6K0.4Fe2As2 retains
well defined, dispersive high-energy modes of magnetic origin. These paramagnon
modes are softer than, though as intense as, the magnon of undoped
antiferromagnetic BaFe2As2. The persistence of spin excitations well into the
superconducting phase suggests that, if spin fluctuations are responsible for
superconducting pairing, they originate from a distinctly correlated
spin-state. This connects Fe-pnictide superconductors to the high-Tc cuprates,
for which in spite of fundamental differences in the electronic structure,
similar paramagnon modes are present5.
