Low-energy spin excitations have been studied on polycrystalline LaFeAsO1-xFx samples by inelastic neutron scattering. The Q-integrated dynamical spin susceptibility χ″(ω) of the superconducting samples is found to be comparable to that of the magnetically ordered parent sample. On the other hand, χ″(ω) almost vanishes at x = 0.158, where the superconducting transition temperature Tc is suppressed to 7 K. In addition, χ″(ω) in optimally doped LaFeAsO0.918F0.082 with Tc = 29 K exhibits a spin resonance mode. The peak energy, Eres, when scaled by kBTc is similar to the value of about 4.7 reported in other high-Tc iron-based superconductors. This result suggests that there is intimate relationship between the dynamical spin susceptibility and high-Tc superconductivity in iron-based superconductors, and is consistent with a nesting condition between Fermi surfaces at the Γ and M points.
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