Magnetic excitation spectrum in LaFeAsOF system

Shamoto, Shinichi  ; Wakimoto, Shuichi  ; Ishikado, Motoyuki; Kodama, Katsuaki   ; Kajimoto, Ryoichi  ; Arai, Masatoshi; Matsuda, Masaaki; Kakurai, Kazuhisa; Esaka, Fumitaka  ; Iyo, Akira*; Miyazawa, Kiichi*; Shirage, P. M.*; Kito, Hijiri*; Eisaki, Hiroshi*; Kim, S.-W.*; Hosono, Hideo*; Guidi, T.*; Bewley, R.*; Bennington, S. M.*

One of the electron-doped Fe pnictide superconductor systems, LaFeAsOF, has been studied by neutron powder inelastic scattering measurements at MERLIN of ISIS and TAS-1, 2 of JRR-3. As for the parent compound, the inelastic scattering intensity along the Q-axis at T = 140 K exhibits a prominent asymmetric peak ascribed to a magnetic rod, suggesting the two-dimensionality of the spin density wave above the magnetic transition temperature. It persists even in the tetragonal phase up to room temperature. Superconducting samples (x = 0.057, T = 25 K and x = 0.082, T = 29 K) also exhibit the peaks at the same Q positions but at low temperatures. The observed dynamical spin susceptibilities have a maximum at about 11 meV at 4 K. The relationship to superconductivity is discussed together with the recent result on an overdoped sample with x = 0.158.