Theoretical Study on Low-Energy Excitations in Alkali-Metal-Deped C_<60>

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Alkali-metal-doped C60 is a material whose transition temperature of superconductivity is extraordinarily high among molecular crystals. Although the origin of superconductivity in this material is still controversial, we have claimed that the origin is the competition between the electron-electron interaction and the electron phonon interaction, where the dynamic Jahn-Teller effect plays an essential role for the latter interaction to overcome the former one. In the present study, we try to explain experimental facts from our point of view. We first investigate the anomalous behavior observed in the photoemission spectra in alkali-metal-doped C60 by using the dynamical mean field approximation. Consequently, we find that A4C60, where A is alkali metal, is a nonmagnetic insulator due to the cooperation between the electron-electron interaction and the electron-phonon interaction. The reason for the nonmagnetic nature is that the electron-phonon interaction is much stronger than the Hund coupling originated in the electron-electron interaction. Furthermore, a peak structure observed in the infrared reflection spectra of alkali metal-doped C60 is found to be originated in the exciton in this material. In particular, it is found that, in alkali-metal-doped C60, the spin-singlet exciton is energetically lower than the spin-triplet exciton in contrast with usual materials. We conclude that all the characteristic properties are due to the Jahn-Teller effect