The electrical transport of DNA is closely related to the density of itinerant π electrons because of the strong electron-lattice interaction. The resistivities of two typical DNA molecules [poly(dG)-poly(dC) and λ-DNA] with varied densities of itinerant π electrons are calculated. It is found that the dependence of the resistivity on the density of itinerantπelectrons is symmetrical about the half-filling state of itinerant π electrons in poly(dG)-poly(dC). At the half-filling state, the Peierls phase transition takes place and poly(dG)-poly(dC) has a large resistivity. When the density of itinerant π electrons departs far from the half-filling state, the resistivity of poly(dG)-poly(9dC) becomes small. Forλ-DNA, there is no Peierls phase transition due to the aperiodicity of its base pair arrangement. The resistivity of poly(dG)-poly(dC) decreases with increasing length of the molecular chain, but the resistivity of λ-DNA increases with increasing length. The conducting mechanisms for poly(dG)-poly(dC) and a few λ-DNA molecules with varied densities of itinerantπelectrons are analyzed.

Single-photon excitation in a charged π-conjugated oligomer is studied theoretically. An apparent reverse polarization is obtained through single-photon excitation, which is different from that obtained through a double-photon excitation. The polarizability is calculated and it is found that a maximum reverse polarization will appear at a suitable conjugation length. In addition, we indicate that the reverse polarization is a nonlinear behavior with the induced electric field. Effects of nondegenerate confinement and interchain interactions on the reverse polarization are also discussed.

The effect of atomic disorder or chain length on the stability of photoinduced polarization inversion has been studied. The atomic disorder was simulated by square-random or Gaussian-random model. It was found that for the square-random distribution case, photoinduced polarization inversion remains steadily when atomic disorder is less than 1 8a-a is the lattice constant of polyacetylene, the reversion polarization disappears and gets into normal polarized state when the disorder is stronger than over 1 8a. The reason of a normal polarization resulted from the strong lattice disorder was discussed. The relationship between the reverse polarization of biexciton state and the confinement constant te as well as the variation of chain length was also studied.

Spin-polarized current through a ferromagnetic/organic interface was studied theoretically. This study was stimulated by recent experiments on La1−xSrxMnO3 /Alq3/Co and La1−xSrxMnO3/sexithienyl/La1−xSrxMnO3 structures. According to the spin-diffusion equation, the current spin polarization was obtained by assuming that spin polarons as well as spinless bipolarons to be the carriers in the organic semiconductor. It was found that polarons are the effective spin carriers of a spin-polarized current, even though they constitute only a fraction of all carriers. Effects of interfacial electrochemical-potential difference and conductivity matching on spin polarization of the current are discussed.

Effect of sizes and dimensions on the spin density wave (SDW) and charge density wave (CDW) in organic ferromagnets was studied. It was found that the stable ferromagnetic state obtained in a closed ring with periodic boundary conditions would become unstable in an open chain due to the effect of chain ends. There was a competition between the spin-spin (s-s) couplings and the electron-electron (e-e) interactions for the formation of SDW and CDW in the main chain. Interchain couplings would strengthen SDW. It was suggested that the onedimensional model should be improved to understand the organic magnetism well.