Effects of magnetic doping and temperature dependence of phonon dynamics in CaFe_1−xCo_xAsF compounds (x=0, 0.06, and 0.12)

We report detailed measurements of composition as well as temperature dependence of the phonon density of states in a new series of FeAs compounds with composition CaFe_1−xCo_xAsF (x=0,0.06,0.12). The electronic-structure calculations for these compounds show that bands near the Fermi level are mainly formed by Fe 3d states, which is quite different from other 122 and 1111 FeAs compounds where both Fe and As are believed to be related to superconductivity. The difference in electronic structure for fluorine-based compounds may cause phonon spectra to behave differently as a function of composition and temperature in comparison with our previous phonon studies on parent and superconducting MFe₂As₂ (M=Ba,Ca,Sr). The composition as well as temperature dependence of phonon spectra for CaFe_1−xCo_xAsF (x=0,0.06,0.12) compounds have been measured using time-of-flight IN4C and IN6 spectrometers at Institut Laue Langevin, France. The comparison of phonon spectra at 300 K in these compounds shows that acoustic phonon modes up to 12 meV harden in the doped compounds in comparison to the parent CaFeAsF. While intermediate-energy phonon modes from 15 to 25 meV are also found to shift toward high energies only in the 12% Co-doped CaFeAsF compound. The experimental results for CaFe_1−xCo_xAsF (x=0,0.06,0.12) are quite different from our previous phonon studies on parent and superconducting MFe₂As₂ (M=Ba,Ca,Sr) where low-energy acoustic phonon modes do not react with doping, while the phonon spectra in the intermediate range from 15 to 25 meV are found to soften in these compounds. We argue that stronger spin phonon interaction play an important role in the emergence of superconductivity in these compounds. The lattice dynamics of CaFe_1−xCo_xAsF (x=0,0.06,0.12) compounds is also investigated using the ab initio as well as shell-model phonon calculations. We show that the nature of the interaction between the Ca and the Fe-As layers in CaFeAsF compounds is quite different compared to our previous studies on CaFe₂As₂.