  We report neutron inelastic scattering measurements on the normal and
superconducting states of single-crystalline Cs0.8Fe1.9Se2. Consistent with
previous measurements on Rb(x)Fe(2-y)Se2, we observe two distinct spin
excitation signals: (i) spin-wave excitations characteristic of the block
antiferromagnetic order found in insulating A(x)Fe(2-y)Se2 compounds, and (ii)
a resonance-like magnetic peak localized in energy at 11 meV and at an in-plane
wave vector of (0.25, 0.5). The resonance peak increases below Tc = 27 K, and
has a similar absolute intensity to the resonance peaks observed in other
Fe-based superconductors. The existence of a magnetic resonance in the spectrum
of Rb(x)Fe(2-y)Se2 and now of Cs(x)Fe(2-y)Se2 suggests that this is a common
feature of superconductivity in this family. The low energy spin-wave
excitations in Cs0.8Fe1.9Se2 show no measurable response to superconductivity,
consistent with the notion of spatially separate magnetic and superconducting
phases.
