Temperature-dependent Raman study of a CeFeAsO_0.9F_0.1 superconductor: crystal field excitations, phonons and their coupling

Abstract

We report temperature-dependent Raman spectra of CeFeAsO_0.9F_0.1 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 E_g 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 T_c, thus estimated to be ~10, suggests CeFeAsO_0.9F_0.1 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.