Raman spectra in iron-based quaternary CeO1-xFxFeAs and LaO1-xFxFeAs

Raman spectra have been measured on iron-based quaternary CeO_1-xF_xFeAs and LaO_1-xF_xFeAs with different amounts of fluorine doping at room temperatures. A group analysis has been made to clarify the optical modes. Based on first-principle calculations, the observed phonon modes can be assigned accordingly. In LaO_1-xF_xFeAs, the E_g and A_1g modes related to the vibrations of La are suppressed with increasing F doping. However, F doping only has a small effect on the E_g and A_1g modes of Fe and As. The Raman modes of La and As are absent in rare-earth substituted CeO_1-xF_xFeAs, and the E_g mode of oxygen, corresponding to the in-plane vibration of oxygen, moves to around 450 cm^-1 and shows a very sharp peak. The electronic scattering background is low, and electron-phonon coupling is not evident for the observed phonon modes. Three features are found above 500 cm^-1, which may be associated with multi-phonon processes. Nevertheless, it is also possible that they are related to magnetic fluctuations or interband transitions of d orbitals, considering their energies.