Temperature-dependent Raman study of a CeFeAsO0.9F0.1 superconductor: crystal field excitations, phonons and their coupling

We report temperature-dependent Raman spectra of CeFeAsO0.9F0.1 from 4 to 300 K in the spectral range of 60--1800 cm - 1 and interpret them using estimates of phonon frequencies obtained from first-principles density functional calculations. We find evidence for strong coupling between the phonons and crystal field excitations; in particular the Ce3 + crystal field excitation at 432 cm - 1 couples strongly with the Eg oxygen vibration at 389 cm - 1. Below the superconducting transition temperature, the phonon mode near 280 cm - 1 shows softening, signaling its coupling with the superconducting gap. The ratio of the superconducting gap to Tc, thus estimated to be ~ 10, suggests CeFeAsO0.9F0.1 to be a strong coupling superconductor. In addition, two high frequency modes observed at 1342 and 1600 cm - 1 are attributed to electronic Raman scattering from the (x2--y2) to xz /yz d-orbitals of Fe.