Raman evidence for coupling of superconducting quasi-particles with a phonon and crystal field excitation in superconductor $Ce0.6Y0.4FeAsO0.8F0.2$

We report inelastic light scattering experiments on superconductor $Ce0.6Y0.4FeAsO0.8F0.2$ from 4K to 300K covering the superconducting transition temperature T_c ∼ 48.6K. A strong evidence of the superconductivity induced phonon renormalization for the $A1g$ phonon mode near 150cm⁻¹ associated with the Ce/Y vibrations is observed as reflected in the anomalous red-shift and decrease in the linewidth below T^c. Invoking the coupling of this mode with the superconducting gap, the superconducting gap (2Δ) at zero temperature is estimated to be ∼ 20meV i.e the ratio 2Δ(0)/k_BT_c is ∼ 5, suggesting $Ce0.6Y0.4FeAsO0.8F0.2$ to belong to the class of strong coupling superconductors. In addition, the mode near 430cm⁻¹ associated with $Ce3+$ crystal field excitation also shows anomalous increase in its linewidth below T_c suggesting strong coupling between crystal field excitation and the superconducting quasi-particles.