  We use inelastic neutron scattering to show that superconducting (SC)
rubidium iron selenide Rb0.82Fe1.68Se2 exhibits antiferromagnetic (AF) spin
excitations near the in-plane wave vector Q = (PI, 0) identical to that for
iron arsenide superconductors. Moreover, we find that these excitations change
from incommensurate to commensurate with increasing energy, and occur at the
expense of spin waves associated with the coexisting sqrt(5)\timessqrt(5) block
AF phase. Since angle resolved photoemission experiments reveal no evidence for
hole-like Fermi surface at Gamma(0, 0), our results suggest that the Q = (PI,
0) excitations in SC Rb0.82Fe1.68Se2 come from localized moments and may have a
similar origin as the hourglass-like spin excitations in copper oxide
superconductors.
