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While the high transition temperatures suggest that the conventional BCS phononmediated mechanism may not provide the main pairing mechanism in the recently discovered RFeAsO1\u2212xFx (1111-type) superconductors, there is, as yet, no consensus, despite extensive experimental and theoretical study. We report here the results of an inelastic neutron scattering investigation of an overdoped polycrystalline sample of LaFeAsO1\u2212xFx with x = 0.15 (Tc = 26 K). Four excitation peaks were observed at 13.6\ub11.5, 24.2\ub10.8, 32.2\ub10.5, and 41.4\ub11.0 meV. They were identified as phonon modes based on their wavevector and temperature dependence. The peak positions agree well with first-principles calculations of phonon density of states as well as experimental data on both the insulating parent and optimally doped LaFeAsO1\u2212xFx compounds. No evidence for the presence of a resonance mode was found. We found that the phonon density of states of the x = 0.15 sample remains unchanged below Tc and is similar to samples with other fluorine concentrations. This suggests that a standard electron-phonon pairing mechanism cannot explain the high transition temperatures observed in these materials.Peer reviewed: YesNRC publication: Ye

Phonon density of states and the search for a resonance mode in LaFeAsO0.85F0.15(Tc= 26 K)

Phonon density of states and the search for a resonance mode in LaFeAsO_0.85F_0.15(T_c= 26 K)