We measured element-specific Fe-phonon densities of states (Fe-PDOSs) of ${\text{LaFeAsO}}_{1-x}{\text{F}}_x$ $\left(x=0,0.11\right)$ and ${\text{La}}_{1-x}{\text{Ca}}_x\text{FePO}$ $\left(x=0.13\right)$ by nuclear resonant inelastic scattering of synchrotron radiation. Since the bands crossing the Fermi edge are mainly $\text{Fe}3d$ 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 ${\text{LaFeAsO}}_{0.89}{\text{F}}_{0.11}$ $\left(T_c=26\text{K}\right)$ 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 ${\text{LaFeAsO}}_{1-x}{\text{F}}_x$ agrees well with Fe-PDOS spectrum determined from a previous first-principles calculation and shows structural resemblance with a calculated Éliashberg function ${\alpha }^{2}F\left(\omega \right)$, which indicates small electron-phonon coupling. Therefore, our results indicate that phonons are not the main contributors to the high-$T_c$ superconductivity of ${\text{LaFeAsO}}_{1-x}{\text{F}}_x$.

• Received 22 August 2008

DOI:https://doi.org/10.1103/PhysRevB.78.174507