Adiabatic and nonadiabatic electron–intramolecular-vibration couplings and superconductivity in fullerenes

We have studied adiabatic and nonadiabatic electron–intramolecular-vibration (e-MV) couplings in doped ${\mathrm{C}}_{60}$. We have shown that nonadiabatic e-MV couplings in the multiband system bring about an interband pair transfer interaction to give ${\mathit{T}}_{\mathit{c}}$ enhancement. We have estimated the adiabatic and the nonadiabatic e-MV coupling constants of the doped ${\mathrm{C}}_{60}$. The dimensionless adiabatic e-MV coupling constants with the three lowest ${\mathit{h}}_{\mathit{g}}$ modes are large (0.2–0.4) and they dominate the e-MV couplings in doped ${\mathrm{C}}_{60}$. The dimensionless nonadiabatic e-MV coupling constants of ${\mathit{h}}_{\mathit{g}}$ modes are estimated to be 0.02–0.12. They cannot be neglected in quantitative arguments. The nonadiabatic e-MV coupling may be important even in qualitative arguments of the mechanism of superconductivity if (i) the interband interaction is enhanced by many-body effects and/or (ii) the nonadiabatic e-MV coupling of ${\mathit{A}}_3$${\mathrm{C}}_{60}$ is larger by a factor of 2 or 3 than that of the dilute limit which we have estimated.