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

While the high transition temperatures suggest that the conventional BCS phononmediated mechanism may not provide the main pairing mechanism in the recently discovered RFeAsO1−xFx (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−xFx with x=0.15 (Tc=26 K). Four excitation peaks were observed at 13.6±1.5, 24.2±0.8, 32.2±0.5, and 41.4±1.0 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−xFx 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.