Optical study of electronic states and phonons in alkali metal doped C/sub 60/

Summary form only given. We report optical reflectivity spectra of alkali metal doped fullerenes (A/sub x/C/sub 60/, x=3,4,6) at an energy region of 0.08-5.5eV. All samples synthesized by co-sublimation method were found to be in single phase by measurements of Raman spectra and x-ray diffraction patterns. Depending on alkali concentration x, a drastic change in optical spectra was observed. While A/sub 3/C/sub 60/ shows a Drude-like response, the spectra of x=4 and 6 compounds were characteristic of semiconductors. Comparing the the spectra of K/sub 3/C/sub 60/ and Rb/sub 3/C/sub 60/, we conclude that the optical response is understood in the framework of band picture. In contrast, the semiconducting behavior in K/sub 4/C/sub 60/ with the lowest absorption peak of about 0.4eV indicates a breakdown of the band picture. The infrared active molecular vibration mode t/sub 1u/(4) shows a nonlinear softening as a function of x. In A/sub 4/C/sub 60/, this mode also shows an anomalously large splitting (about 30cm/sup -1/) and broadening. These results suggest that the t/sub 1u/(4) mode is strongly coupled with the doped electrons.