Inelastic neutron scattering study on the resonance mode in an optimally doped superconductor LaFeAsO${}_{0.92}$F${}_{0.08}$

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 $Q=1.15$ \AA${}^{-1}$ is enhanced below $T_c$, leading to a peak at $E_{res}\sim 13$ meV as the resonance mode, in addition to the formation of a gap at low energy below the crossover energy $\Delta_{c}\sim10 meV$\mathrm{.}Thepeakenergyat$Q=1.15$\mathring{A}$^{-1}$correspondsto$5.2 k_B T_c$ in good agreement with the other values of resonance mode observed in the various iron-based superconductors, even in the high-$T_c$ cuprates. 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.