Phonon Density of States of ${\mathrm{LaFeAsO}}_{1-x}{\mathrm{F}}_x$

We have studied the phonon density of states (PDOS) in ${\mathrm{LaFeAsO}}_{1-x}{\mathrm{F}}_x$ with inelastic neutron scattering methods. The PDOS of the parent compound ($x=0$) is very similar to the PDOS of samples optimally doped with fluorine to achieve the maximum $T_c$ ($x\sim 0.1$). Good agreement is found between the experimental PDOS and first-principles calculations with the exception of a small difference in Fe mode frequencies. The PDOS reported here is not consistent with conventional electron-phonon mediated superconductivity.

Figure 1

Neutron powder diffraction data: (a) LaFeAsO ($S1$) and (b) ${\mathrm{LaFeAsO}}_{0.89}{\mathrm{F}}_{0.11}$ ($S2$). In both panels, the first three lines are the sample phase, aluminum sample can, and FeAs. For (a), the fourth line is ${\mathrm{La}}_2{\mathrm{O}}_3$ while for (b), it is LaOF.

Figure 2

Inelastic neutron scattering intensity (color scale) of ${\mathrm{LaFeAsO}}_{0.89}{\mathrm{F}}_{0.11}$ ($S2$) at 10 K as a function of $Q$ and $E$. The scattering due to the empty sample holder has been removed.

Figure 3

Phonon density of states of ${\mathrm{LaFeAsO}}_{1-x}{\mathrm{F}}_x$ ($E_i=130\mathrm{meV}$). (a) Experimental PDOS at 35 and 300 K for ${\mathrm{LaFeAsO}}_{0.9}{\mathrm{F}}_{0.1}$ ($S4$). (b) Experimental PDOS for LaFeAsO ($S3$) and ${\mathrm{LaFeAsO}}_{0.9}{\mathrm{F}}_{0.1}$ ($S4$) at $T=300\mathrm{K}$. (c) First-principles calculations of Singh et al. [6]. The partial density of states have been normalized as described in the text.

Figure 4

The phonon density of states of ${\mathrm{LaFeAsO}}_{0.89}{\mathrm{F}}_{0.11}$ ($S2$). (a) Experimental PDOS with $E_i=100\mathrm{meV}$ at 10, 40, and 300 K. (b) High resolution data collected with $E_i=30\mathrm{meV}$ at 300 K compared with calculations of Singh et al. [6].