Electron-vibration coupling constants in positively charged fullerene

Abstract

Recent experiments have shown that C₆₀ 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 C₆₀ molecule. The couplings predict a D₅ distortion, and an H_u vibronic ground state for C₆₀ ⁺. They are also used to generate the dimensionless coupling constant λ which controls the superconductivity and the phonon contribution to the electrical resistivity in the crystalline phase. We find that λ is 1.4 times larger in positively charged C₆₀ 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.