Optical study of electronic structures and phonons in alkali-metal-dopedC60

We have measured optical reflection spectra for the alkali-metal fullerides ${\mathrm{K}}_3$ ${\mathrm{C}}_{60}$, ${\mathrm{Rb}}_3$ ${\mathrm{C}}_{60}$, ${\mathrm{K}}_4$ ${\mathrm{C}}_{60}$, ${\mathrm{Rb}}_4$ ${\mathrm{C}}_{60}$, and ${\mathrm{Rb}}_6$ ${\mathrm{C}}_{60}$ grown by a cosublimation method in an energy region of 0.08–6 eV. A gap structure peaked at 0.5 eV was observed in ${\mathrm{K}}_4$ ${\mathrm{C}}_{60}$, indicating that this compound is an insulator. This result is in good agreement with other kinds of experiments, but contrary to a simple band theory. The metallic conductivity spectra of ${\mathrm{K}}_3$ ${\mathrm{C}}_{60}$ and ${\mathrm{Rb}}_3$ ${\mathrm{C}}_{60}$ were formed of a Drude part and midinfrared absorption. The energy of the latter contribution was close to the optical gap of ${\mathrm{K}}_4$ ${\mathrm{C}}_{60}$. The infrared-active molecular vibrational ${\mathit{T}}_{1\mathit{u}}$(4) mode shows an anomalous dependence on the alkali concentration x, and a large splitting (about 30 ${\mathrm{cm}}^{\mathrm{-}1}$) in the x=4 phase. These results suggest that the interaction between the doped electrons and the ${\mathit{T}}_{1\mathit{u}}$(4) vibration mode is extremely strong. The relation between the anomalous phonons and the midinfrared absorption in ${\mathrm{K}}_4$ ${\mathrm{C}}_{60}$ and ${\mathrm{K}}_3$ ${\mathrm{C}}_{60}$ is discussed.