Neutron scattering studies of spin excitations in superconducting Rb _0.82Fe _1.68Se ₂

We use inelastic neutron scattering to show that superconducting (SC) rubidium iron selenide Rb _0.82Fe _1.68Se ₂ exhibits antiferromagnetic (AF) spin excitations near the in-plane wave vector Q=(π,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 √5×√5 block AF phase. Since these spin excitations cannot come from Fermi surface nesting based on angle resolved photoemission experiments, our results indicate the presence of local moments in SC Rb _0.82Fe _1.68Se ₂ that may have a similar origin as the hourglass-like spin excitations in copper oxide superconductors.