Inelastic neutron scattering study of the resonance mode in the optimally doped pnictide superconductor LaFeAsO0.92F0.08

An optimally doped iron-based superconductor LaFeAsO0.92F0.08 with T-c=29 K has been studied by inelastic powder neutron scattering. The magnetic excitation at Q=1.15 angstrom(-1) is enhanced below T-c, leading to a peak at E-res similar to 13 meV as the resonance mode, in addition to the formation of a gap at low energy below the crossover energy Delta(c) similar to 10 meV. The peak energy at Q=1.15 angstrom(-1) corresponds to 5.2k(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.