Iron-specific phonon density of states in the superconductors LaFeAsO1-xFx and La1-xCaxFePO

We measured element-specific Fe-phonon densities of states (Fe-PDOSs) of LaFeAsO(1-x)F(x) (x=0,0.11) and La(1-x)Ca(x)FePO (x=0.13) by nuclear resonant inelastic scattering of synchrotron radiation. Since the bands crossing the Fermi edge are mainly Fe 3d 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 LaFeAsO(0.89)F(0.11) (T(c)=26 K) and that of nonsuperconductor LaFeAsO have similar structures both below T(c) (15 K) and above T(c) (298 K); therefore, fluorine doping does not have a notable effect on the Fe-PDOS. The measured Fe-PDOS of LaFeAsO(1-x)F(x) agrees well with Fe-PDOS spectrum determined from a previous first-principles calculation and shows structural resemblance with a calculated Eliashberg function alpha(2)F(omega), which indicates small electron-phonon coupling. Therefore, our results indicate that phonons are not the main contributors to the high-T(c) superconductivity of LaFeAsO(1-x)F(x).