(PI,0) antiferromagnetic spin excitations in superconducting Rb0.82Fe1.68Se2

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.