Theoretical Study on Electron-Electron and Electron-phonon Interactions in Fullerene Compounds

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In alkali-metal-doped C_<60>, the electron-electron and electron-phonon interactions play very important roles. We have already found that the two interactions open the band gaps in A_2C_<60> and A_4C_<60> cooperatively and make these materials insulating. On the contrary, A_3C_<60>, which is a phase between A_2C_<60> and A_4C_<60>, remains metallic although the electron-electron and electron-phonon interactions should also be strong in this material. This has been unsolved so far. In the present study, we have studied the reason why A_3C_<60> remains metallic in spite of the strong electron-electron and electron-phonon interactions by using the same model that has been used for studying A_2C_<60> and A_4C_<60>. As a result, we have found that, in A_3C_<60>, the electron-electron and electron-phonon interactions compete with each other and, consequently, this material remains metallic. This competition results in the charge fluctuation. In paticular, we have found that the charge fluctuation is the origin of a shoulder observed in the photoemission spectra in A_3C_<60>. Furthermore, we have studied the present model by the quantum Monte Carlo method and found that the shoulder disappears at high temperature. This result is in good agreement with experimental temperature dependence of the photoemission spectra. The present results are very important for understanding low energy excitation in A_3C_<60>. Finally, we have studied the phase diagrams of A_xC_<60> (x=1,-,5) and found that there exist paramagnetic metal, superconductor, orbital-palarized metal, and spin- and orbital-polarized metal in A_1C_<60>, A_3C_<60>, and A_5C_<60> while there exist paramagnetic metal, superconductor, orbital-polarized insulator in A_2C_<60> and A_4C_<60>.