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The self-trapped exciton in polymers under the electric field has been studied using a tight-binding model including the electric field, symmetry-breaking term te and electron-lattice interaction simultaneously. It is discovered that the electric field will polarize the self-trapped exciton in polymers. The polarization depends on the electric field E and te. When te≤0.1eV，self-trapped biexciton in polymers shows the characteristic of the reverse polarization, i.e., electrons move along the direction of E; however, plus charges move along the minus direction of E. The cause of the polarization has been discussed.

Due to the one-dimensional characteristic, the excitation of electrons in polymers will be self-trapped, and two excitons can be combined to form one biexciton. This combination process is one of the most important channels leading to form the biexciton, the efficiency of is much higher than the two-photon process. This kind of combination is accompanied by lattice distortion. By employing the dynamic equation, the relaxation process of this combination is investigated in this paper, and it is found that the relaxation time is about 160fs. The effect of external electrical field on the combination is also studied, the results show that when the electric field E is greater than 0.5MV/cm, instead of the biexciton, two bipolarons are produced.

“A Full Potential Linearized Augmented Plane Wave” scheme is applied to the study of the electronic and magnetic features on a simple organic ferromagnet modified by an organic high-spin segment (CH2) to trans-polyacetylene. Some physical quantities, i.e., band structure, the total and partial spin densities of states, magnetic moment and splitting energy, are shown quantitatively in our report. An anti-ferromagnetic order is expected and a specific π orbital is announced to be important for the magnetic features in an organic ferromagnet.

The nesting of the Fermi surface in a two-dimensional (2D) system is not perfect, which is distinctly different from one-dimensional system. The nesting deviation can heavily suppress the instability of 2D system. In this paper, the effect of the nesting deviation on the Peierls transition is quantitatively studied, and the limit for the nesting deviation, beyond which the Peierls transition is destroyed, is determined. Such a behavior is favorable to get the superconductivity.

The effect of electron-electron interaction on the localization of soliton is studied. Because the conducting polymer possesses wide bandwidth, the Hubbard model is no longer a suitable approximation. It is needed to use a screened Coulomb potential to describe the e-e interaction. Our theory shows that the screening plays a key role in the effect of e-e interaction on the localization of soliton: in weak screening, the localization of soliton will be enhanced by the e-e interaction; in strong screening, it will be reduced. In the end, the physical reason of above-mentioned results is explained by analysing the relation between screening and electron correlation.

Does electron interaction enhance or reduce the creation energy of soliton? The results based on different theories conflict with each other. In this paper, the reason for such conflict has been revealed and a new method is proposed to calculate the creation energy of soliton (only intensive quantities are involved in calculation). Our result shows: the electron interaction enhances the creation energy of soliton, but the enhancement of the soliton is less than that of-e-h pair.

The effect of atomic disorder fluctuation on the distributions of bond lengths and electron densities for optically-excited C60 and C60 doped with alkali-metal are studied and we adopt the methods that the fluctuation is modeled by Gaussian-random parameters, and that the sample averaging is performed over a large number of independent atomic disorder configurations.The distributions of bond lengths and electron densities are shown to be the functions of the strength for the atomic dis order fluctuation,as well as the number and variety of extra charges.It is discovered:1 When the difference between the long and short bonds is 0.0044nm,the dimerization in C60 still remains when the atomic disorder fluctuation is 0—0.01nm.2 When the strength of the atomic disorder fluctuation is 0—0.00775nm,there exist polarons in C±60 and C--60; when the strength of the atomic disorder fluctuation is 0—0.00548nm,there also exist polarons in C－60,C3－60 and C±±60.

Exciton polaron states in C60 molecular are theoretically studied in the tight-binding approach supplemented by the extended Hubbard interaction. The binding energies of the lowest singlet and triplet excitons depend sensitively upon the electron correlation. Then the exciton polaron is formed due to the electron-lattice coupling. Relevent experiments in photo-luminescence spectrum of Cso can be explained qualitatively by the present model with moderate interaction strength Ｕ～５．０ｅＶ，Ｖ～２．０ｅＶ.

The third harmonic generation spectrum (THG) of trans-polyacetylene exhibits two peaks at 0.6eV and 0.9 eV respectively. None of the existing theories can give a quantitatively satis-factory interpretation. By considering properly the excitation-dependent damping, we can get the second peak at 0.9eV. The damping of excited states that we depict in this paper due to electron-phonon interaction of polymer varies with the excited states. According to this dam-ping, the theoretical results of THG agree with the observed spectrum very well.

We use the correlated basis functions method to explore the problem of the Pe-ierls instability due to the electron correlation in hydrogen chain. Our thory shows that the bond alternation is reduced from 0.37 (au) to 0.32 (au) by the electron correlation, but the Peierls instability survives under the electron-electron interaction.

In order to explain why the enhancement or reduction of dimerization of polymer is only determined by the range of electron interaction, the diagonal and off-diagoal matrix elements of screened Coulomb interaction are calculated, and it is shown that the off-diagonal matrix elements are getting more and more important when the interaction range decreases. The Competition between the off-diagonal and diagonal interactions causes the transition from enhancement of dimerization to reduction.