Temperature dependence and resonance effects in Raman scattering of   phonons in NdFeAsO$_{x}$F$_{1-x}$ single crystals
  We report plane-polarized Raman scattering spectra of iron oxypnictide superconductor NdFeAsO$_{1-x}$F$_x$ single crystals with varying fluorine $x$ content. The spectra exhibit sharp and symmetrical phonon lines with a weak dependence on fluorine doping $x$. The temperature dependence does not show any phonon anomaly at the superconducting transition. The Fe related phonon intensity shows a strong resonant enhancement below 2 eV. We associate the resonant enhancement to the presence of an interband transition around 2 eV observed in optical conductivity. Our results point to a rather weak coupling between Raman-active phonons and electronic excitations in iron oxypnictides superconductors. 
Y. Gallais, A. Sacuto, M. Cazayous, P. Cheng, L. Fang and H.H. Wen