  We have studied the low-energy spin-excitation spectrum of the
single-crystalline Rb2Fe4Se5 superconductor (Tc = 32 K) by means of inelastic
neutron scattering. In the superconducting state, we observe a magnetic
resonant mode centered at an energy of 14 meV and at the (0.5 0.25 0.5) wave
vector (unfolded Fe-sublattice notation), which differs from the ones
characterizing magnetic resonant modes in other iron-based superconductors. Our
finding suggests that the 245-iron-selenides are unconventional superconductors
with a sign-changing order parameter, in which bulk superconductivity coexists
with the sqrt(5) x sqrt(5) magnetic superstructure. The estimated ratios of the
resonance energy to Tc and the superconducting gap indicate moderate pairing
strength in this compound, similar to that in optimally doped 1111- and
122-pnictides.
