Formation of partial energy gap below the structural phase transition and the rare-earth element-substitution effect on infrared phonons in ReFeAsO (Re=La, Nd, and Sm)

Single crystals of LaFeAsO, NdFeAsO, and SmFeAsO have been prepared by means of a NaAs flux growth technique and studied by optical spectroscopy measurements. We show that the spectral features corresponding to the partial energy gaps in the spin-density-wave (SDW) state are present below the structural phase transition. This indicates that the electronic state below the structural phase transition is already very close to that in the SDW state. We also show that in-plane infrared phonon modes display systematic shifts toward high frequency upon rare-earth element Nd and Sm substitutions for La, suggesting a strong enhancement of the bonding strength. Furthermore, an asymmetric line shape of the in-plane phonon mode is observed, yielding evidence for the presence of electron-phonon coupling in Fe pnictides.