Spin excitations in iron oxypnictide superconductor system

The spin excitations in the LaFeAsO _1-xF _x system have been studied by inelastic neutron scattering measurements with powder samples. In the parent compound, the spin excitation was observed as a magnetic rod at (π, π, l) in the downfolded tetragonal Brillouin zone. It persisted even in the tetragonal phase at T = 280 K. In the superconducting compounds with x = 0.057 and 0.082, the spin excitations developed below the superconducting transition temperatures up to the same order of magnitude as that in the parent compound, and they were peaked at about 11 meV. The spin excitation, however, almost disappeared in an overdoped compound with x = 0.158, which exhibited only low temperature superconductivity below 7 K. These results indicate that the present superconductivity intimately correlates with the spin fluctuation possibly in relation to the nesting condition between the Fermi surfaces at Γ and M points.