OPTICAL STUDY OF ELECTRONIC-STRUCTURES AND PHONONS IN ALKALI-METAL-DOPED C-60

We have measured optical reflection spectra for the alkali-metal fullerides K3C60, Rb3C60, K4C60, Rb4C60, and Rb6C60 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 K4C60, 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 K3C60 and Rb3C60 were formed of a Drude part and midinfrared absorption. The energy of the latter contribution was close to the optical gap of K4C60. The infrared-active molecular vibrational T1u(4) mode shows an anomalous dependence on the alkali concentration x, and a large splitting (about 30 cm-1) in the x=4 phase. These results suggest that the interaction between the doped electrons and the T1u(4) vibration mode is extremely strong. The relation between the anomalous phonons and the midinfrared absorption in K4C60 and K3C60 is discussed. © 1995 The American Physical Society.