Iron-specific phonon density of states in the superconductors LaFeAsO _1-xF_x and La_1-xCa_xFePO

We measured element-specific Fe-phonon densities of states (Fe-PDOSs) of LaFeAsO1-x Fx (x=0,0.11) and La1-x Cax FePO (x=0.13) by nuclear resonant inelastic scattering of synchrotron radiation. Since the bands crossing the Fermi edge are mainly Fe3d bands, Fe is the key element for iron pnictide superconductivity, and therefore element-specific Fe-PDOS offers crucial information on the phonon contribution in iron pnictide superconductors. The measured Fe-PDOSs of superconductor LaFeAsO0.89 F0.11 (Tc =26 K) and that of nonsuperconductor LaFeAsO have similar structures both below Tc (15 K) and above Tc (298 K); therefore, fluorine doping does not have a notable effect on the Fe-PDOS. The measured Fe-PDOS of LaFeAsO1-x Fx agrees well with Fe-PDOS spectrum determined from a previous first-principles calculation and shows structural resemblance with a calculated Iliashberg function α2 F (ω), which indicates small electron-phonon coupling. Therefore, our results indicate that phonons are not the main contributors to the high- Tc superconductivity of LaFeAsO1-x Fx.