(Last Update: January 22, 1996) 
1) K. Harigaya:
   Metal-insulator transition in C60-polymers,
   Phys. Rev. B 52 (1995) 7968-7971.
2) K. Harigaya:
   Metallic and insulating states in two-dimensional C60-polymers,
   Chem. Phys. Lett. 242 (1995) 585-591.
3) K. Harigaya, Y. Shimoi, and S. Abe:
   Exciton effects in soliton and bipolaron lattice states
   of doped electron-phonon Peierls systems,
   J. Phys.: Condens. Matter 7 (1995) 4061-4073.
4) K. Harigaya:
   Nonlinear optical response from excitons in soliton lattice systems
   of doped conjugated polymers,
   J. Phys.: Condens. Matter 7 (1995) 7529-7534.
K. Harigaya: Metal-insulator transition in C60-polymers
Variations in the band structures of C60-polymers are studied, when pi-conjugation conditions are changed. We discuss a metal-insulator transition, using a semiempirical model with the Su-Schrieffer-Heeger type electron-phonon interactions. We find that electronic structures change among direct-gap insulators and the metal, depending on the degree of pi-conjugations. High pressure experiments could observe such pressure-induced metal-insulator transitions. The transition from one insulator to another insulator is reentrant, and is specific to the C60-polymer system.
K. Harigaya: Metallic and insulating states in two-dimensional C60-polymers
Variations in the band structures of the two-dimensional C60-polymers are studied, when pi-conjugation conditions are changed. We investigate the rectangular and triangular polymers, in order to discuss metal-insulator transitions, using a semiempirical model with the Su-Schrieffer-Heeger type electron-phonon interactions. We find that electronic structures change among direct-gap insulators and the metal, depending on the degree of pi-conjugations in the rectangular polymer. The triangular polymer changes from the indirect gap insulator to the metal as the pi-conjugations increase. High pressure experiments could observe such pressure-induced metal-insulator transitions.
K. Harigaya, Y. Shimoi, and S. Abe: Exciton effects in soliton and bipolaron lattice states of doped electron-phonon Peierls systems
Exciton effects on soliton and bipolaron lattice states are investigated using an electron-lattice Peierls model with long-range Coulomb interactions. The Hartree-Fock (HF) approximation and the single- excitation configuration-interaction (single-CI) method are used to obtain optical absorption spectra. We discuss the following properties:
(1) The attraction between the excited electron and the remaining hole makes the excitation energy smaller when the correlations are taken into account by the single-CI. The oscillator strengths of the lower excited states become relatively larger than in the HF calculations.
(2) We look at variations of relative oscillator strengths of two or three kinds of excitons described by the single-CI. While the excess-electron concentration is small, the ratio of the oscillator strengths of the exciton with the lowest energy, which is calculated against the total electronic excitation oscillator strengths, increases almost linearly. The oscillator strengths accumulate at this exciton as the concentration increases.
K. Harigaya: Nonlinear optical response from excitons in soliton lattice systems of doped conjugated polymers
Exciton effects on conjugated polymers are investigated in soliton lattice states. We use the Su-Schrieffer-Heeger model with long-range Coulomb interactions. The Hartree-Fock (HF) approximation and the single-excitation configuration-interaction (single-CI) method are used to obtain optical absorption spectra. The third-harmonic generation (THG) at off-resonant frequencies is calculated as functions of the soliton concentration and the chain length of the polymer. The magnitude of the THG at the 10 percent doping increases by the factor about 10^2 from that of the neutral system. This is owing to the accumulation of the oscillator strengths at the lowest exciton with increasing the soliton concentration. The increase by the order two is common for several choices of Coulomb interaction strengths.
harigaya@etl.go.jp