Pseudo-gap and effective mass enhancement in alkali metal doped C60

We report a comparative study of the optical conductivity of C60, K3C60, and K4C60 at an energy region of 0.08-6eV. In the spectra of K4C60, a clear optical gap centered at 0.5eV was observed in accord with the insulating state of x=4 compounds. From the spectral analysis, a pseudo-gap was found in the mid-IR conductivity of K3C60, in addition to a Drude response with a reduction of spectral weight by a factor of 0.7. This weight reduction is consistent with the enhancement of the density of states at Fermi energy. Since the pseudo-gap in x=3 compounds is centered at 0.4–0.5eV similar to the gap in x=4 compounds, we speculate that there exist a common instability in the metallic state of A3C60 and A4C60. We propose a Jahn-Teller mechanism for the explanation of the gap-opening in these two compounds.