Effect of pressure, temperature, fluorine doping, and rare earth elements on the phonon density of states of LFeAsO studied by nuclear inelastic scattering

We have performed systematic studies of the lattice dynamics in LFeAsO (L=La, Ce, Pr, Nd, Sm) in the parent and in the ∼10% F-doped compounds as a function of pressure and temperature. We have found that the modifications in the partial Fe density of phonon states are mainly governed by the Fe-As bond length. The change of this bond length explains the change of the Fe density of phonon states above 25 meV. We further observe anomalies in the behavior of the phonon mode near 16 meV. In the parent phase, this mode softens anomalously upon cooling through the structural phase transition. Upon F doping, this mode hardens indicating a strong electron-phonon coupling. This suggests that the corresponding phonons play an important role in the competition between superconductivity and magnetism in these materials.