Raman spectra in iron-based quaternary CeO${}_{1-x}$F${}_x$FeAs and LaO${}_{1-x}$F${}_x$FeAs

Abstract

Raman spectra have been measured on iron-based quaternary CeO${}_{1-x}$F${}_x$FeAs and LaO${}_{1-x}$F${}_x$FeAs with varying fluorine doping at room temperatures. A group analysis has been made to clarify the optical modes. Based on the first principle calculations, the observed phonon modes can be assigned accordingly. In LaO${}_{1-x}$F${}_x$FeAs, 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${}_{1-x}$F${}_x$FeAs, 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. 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 process. Nevertheless it is also possible that they are related to magnetic fluctuations or interband transitions of d orbitals considering their energies.

12 pages, 3 figures, 2 tables