Lattice distortion and energy level structures in doped C_{60} and C_{70} studied with the extended Su-Schrieffer-Heeger model: Polaron excitations and optical absorption

Abstract

We extend the Su-Schrieffer-Heeger model of polyacetylene to C_{60} and C_{70} molecules, and solve numerically. The calculations of the undoped systems agree well with the known results. When the system (C_{60} or C_{70}) is doped with one or two electrons (or holes), the additional charges accumulate almost along an equatorial line of the molecule. The dimerization becomes weaker almost along the same line. Two energy levels intrude largely in the gap. The intrusion is larger in C_{70} than in C_{60}. Therefore, ``polarons'' are predicted in doped buckminster- fullerenes. We calculate optical absorption coefficient for C_{60} in order to look at how ``polarons'' will be observed. It is predicted that there appears a new peak at the lower energy than the intergap transition peaks. It is also found that C_{60} and C_{70} are related mutually with respect to electronical structures as well as lattice geometries. (to be published in Phys. Rev. B 45, June 15 issue)