Raman Evidence for Coupling of Superconducting Quasi-Particles with a Phonon and Crystal Field Excitation in Superconductor Ce_(0.6)Y_(0.4)FeAsO_(0.8)F_(0.2)

摘要

We report inelastic light scattering experiments on superconductor Ce_(0.6)Y_(0.4)FeAsO_(0.8)F_(0.2) from 4K to 300K covering the superconducting transition temperature T_c ~ 48.6K. A strong evidence of the superconductivity induced phonon renormalization for the A_(1g) 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 (2?) at zero temperature is estimated to be ~ 20meV i.e the ratio 2?(0)/k_BT_c is ~ 5, suggesting Ce_(0.6)Y_(0.4)FeAsO_(0.8)F0_2 to belong to the class of strong coupling superconductors. In addition, the mode near 430cm~(-1) associated with Ce~(3+) 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.