Degradation of Superconductivity and Spin Fluctuations by Electron Overdoping in LaFeAsO1-xFx

Low-energy spin fluctuations are studied for the electron-doped Fe-based superconductor LaFeAsO1 F-x(x) by inelastic neutron scattering up to an energy transfer of omega = 15 meV using polycrystalline samples. Superconducting samples (x = 0.057, T-c = 25 K and x = 0.082, T-c = 29 K) show a dynamical spin susceptibility chi ''(omega) comparable to that of the parent samples. However, chi ''(omega) almost vanishes in the x = 0.158 sample, in which the superconductivity is highly suppressed. These results are compatible with the theoretical suggestions that spin fluctuation plays an important role in superconductivity.