Inelastic neutron scattering study of the resonance mode in the optimally doped pnictide superconductor LaFeAsO_0.92F_0.08

An optimally doped iron-based superconductor LaFeAsO_0.92 F _0.08 with T_c =29 K has been studied by inelastic powder neutron scattering. The magnetic excitation at Q=1.15 Å^-1 is enhanced below T_c, leading to a peak at E_res ∼13 meV as the resonance mode, in addition to the formation of a gap at low energy below the crossover energy Δ_c ∼10 meV. The peak energy at Q=1.15 Å^-1 corresponds to 5.2 k_B T_c in good agreement with the other values of resonance mode observed in iron-based superconductors. Although the phonon density of states has a peak at the same energy as the resonance mode in the present superconductor, the Q dependence is consistent with the resonance being of predominately magnetic origin.