Electronic Structures in C60-Polymers (review)
  Variations in the band structures of C60-polymers are studied, when conjugation conditions and the electron number are changed. We use a semiempirical model with the Su-Schrieffer-Heeger type electron-phonon interactions. In the neutral one-dimensional C60-polymer, electronic structures change among direct-gap insulators and the metal, depending on the degree of conjugations. High pressure experiments could observe such pressure-induced metal-insulator transitions. The C60-polymer doped with one electron per one molecule is always a metal. The energy difference between the highest-occupied state and the lowest-unoccupied state of the neutral system becomes smaller upon doping owing to the polaron effects. When the C60-polymer is doped with two electrons per one C60, the system is insulating. When the conjugation in the direction of the polymer chain is smaller, it is a direct-gap insulator. The energy gap becomes indirect when the conjugation is stronger. We also study the antiferromagnetic phase of AC60 by using a tight-binding model with long-range Coulomb interactions. The antiferromagnetism is well described by the model. The comparison with the photoemission studies shows that the new band around the Fermi energy of the AC60 phase can be explaind by the extremely large intrusion of an energy level into the gap of the neutral system. This indicates that the interaction effects among electrons are important in doped C60-polymers. 
Kikuo Harigaya (ETL, Japan)