Polaron excitations in doped C 60 and C 70

Abstract

We propose a Su-Schrieffer-Heeger type electron-phonon model for fullerenes: C 60 and C 70. The model is solved by a numerical iteration method with applying the adiabatic approximation to phonons. When the system (C 60 or C 70) is doped with one or two electrons (or holes), the additional charges accumulate along almost an equatorial line of the molecule. The dimerization becomes weaker along almost the same line. Two energy levels, the occupied state and the empty state, intrude largely in the gap. The intrusion is larger in C 70 than in C 60. These are “polarons” in doped fullerenes. It is also found that C 60 and C 70 are related mutually with respect to electronical structures as well as lattice geometries. We also calculate optical properties of C 60. In optical absorption of the doped C 60, there is a new peak at lower energy than those of the intergap transition peaks. The third harmonic generation of the neutral C 60 in low frequencies agrees with experiments. Relations of polarons with recent experiments are pointed out.