Electron-vibration coupling constants in positively charged fullerene

Recent experiments have shown that C-60 can be positively field doped. In that state, fullerene exhibits a higher resistivity and a higher superconducting temperature than the corresponding negatively doped state. A strong intramolecular hole-phonon coupling, connected with the Jahn-Teller effect of the isolated positive ion, is expected to be important for both properties, but the actual coupling strengths are so far unknown. Based on density functional calculations, we determine the linear couplings of the two a(g), six g(g) and eight h(g) vibrational modes to the H-u highest occupied molecular orbital level of the C-60 molecule. The couplings predict a D-5 distortion, and an H-u vibronic ground state for C-60(+). They are also used to generate the dimensionless coupling constant lambda which controls the superconductivity and the phonon contribution to the electrical resistivity in the crystalline phase. We find that lambda is 1.4 times larger in positively charged C-60 than in the negatively doped case. These results are discussed in the context of the available transport data and superconducting temperatures. The role of higher orbital degeneracy in superconductivity is also addressed.