The dynamic process of charge injection is investigated for an open conjugated polymer chain in contact with a metal electrode in the framework of a nonadiabatic approach. It is found that the injected charges form wave packets due to the strong electron-lattice interactions in the conjugated polymer. A wave packet may contain any quantity of charges up to two electronic units, which depends upon the injection condition. As the system is spin degenerate within the present model, a wave packet has no spin. Discrete energy levels will appear in the gap accompanying the wave packet, which results in potential asymmetry of the charge injection and ejection between the metal electrode and the polymer. The asymmetry can be used to explain the electrical hysteresis in organic devices. It is found that there exists an upper critical field or forming field, beyond which the injected charges could not form a wave packet. Under a strong driving electric field, it is also found that a wave packet will dissociate into an extended state. The dependence of the dissociating field on the charge quantity a wave packet contains is studied.

We theoretically investigate the ground-state properties of ferromagnetic metal/conjugated polymer interfaces. The work was partially motivated by recent experiments in which injection of spin-polarized electrons from ferromagnetic contacts into thin films of conjugated polymers was reported. We use a one-dimensional nondegenerate Su-Schrieffer-Heeger Hamiltonian to describe the conjugated polymer and one-dimensional tight-binding models to describe the ferromagnetic metal. We consider both a model for a conventional ferromagnetic metal, in which there are no explicit structural degrees of freedom, and a model for a halfmetallic ferromagnetic colossal magnetoresistance ~CMR! manganite that has explicit structural degrees of freedom. We investigate electron charge and spin transfer from the ferromagnetic metal to the organic polymer, and structural relaxation near the interface. We find that there can be spin density polarization in the polymer near the interface. The spin-density oscillates and decays into the polymer with a decay length of about six times the lattice constant of the polymer. We find an expansion of the end bonds of the CMR manganite segment and a contraction of the polymer bonds near the interface. By adjusting the relative chemical potential of the contact and the polymer, electrons can be transferred into the polymer from the magnetic layer through the interfacial coupling. We calculate the density of states (DOS) before and after coupling for cases in which electrons are transferred and are not transferred to the polymer. The DOS has important consequences for spin injection under electrical bias: polarized spin injection is possible when the Fermi level of the ferromagnet lies below the the bipolaron level of the polymer. However, if the Fermi level of the CMR manganite lies above the bipolaron level of the polymer, the transferred electrons form bipolarons, which have no spin, and there is no spin density in the bulk of the polymer.

The stabilities of solitons, polarons and bipolmns in divalent-doped frompolyacerylene have been shldied by considering the impurity Coulomb confinement as well as the eleetron-electron interactions in the framework of the tight-binding PeierlsHubbard model. It was found that transitions between bipolarons and solitons and between bipolarons and polarons will rake place under cert3in conditions. The calculated values of the energy msitions agree well with the experimental results in the dilute Ca2+-doped trans-polyaceiylene material. The stability of excitations with respect to the impurity confinement and the electron4edron interactions have been discussed.

Several boundary conditions have been proposed to stabilize the dimerized structures of finite-length Peierls–Hubbard chains. The effects of different boundary conditions on the lattice structures and the electronic energy spectra have been analysed. It was found that the natural or the fixed-end boundary conditions are more suitable to describe polymer chains with very short lengths than the periodic boundary condition. The competition of electron–electron interactions with electron–phonon interactions in finite-length chains has also been studied.

The effects of interchain coupling on vibration modes localized around a polaron in polyacetylene of finite length are investigated. It is shown that the eight localized modes found in an isolated finite chain are delocalized between chains due to the interchain coupling. New modes of lattice vibration emerge because of the delocalization of the charged polaron. The well-known Goldstone zero-frequency mode becomes pinned by the chain ends for sufficiently short chains. Our results can qualitatively account for the frequency shifts observed recently in Raman scattering and infrared absorption spectra.

In the framework of the tight-binding model, the effects of longitudinal as well as transverse coupling between polyacetylene chains with different lengths and the electron–electron interactions on excitations have been studied. A new spinless charged polaronic excitation (q=±e; s=0) was obtained due to the transverse interchain coupling, which is different from the general magnetic polaron (q=±e; s=1/2) obtained in the perfect one-dimensional model. Due to its end positioning, a polaron is more easily stimulated energetically in a longer chain than in a shorter one, and it can hop to the longer chains nearby via either transverse or longitudinal interchain coupling. In addition, the transverse coupling decreases the energy of creation of a polaron. Considering the strong electron–electron interactions and the end positioning of chains in actual materials, we concluded that the one-dimensional model of conducting polymers is fairly appropriate in certain circumstances.

Instead of spin electrons or holes, the carriers in polymers are supposed to be spin polarons or spinless bipolarons. Based on the spin diffusion theory and Ohm's law, we studied spin-polarized injection into organic polymers. It was obtained that only spin polarons are responsible for the current spin polarization in a ferromagnetic (FM)/organic system. Effects of the conductivity matching and the interfacial resistances on the current spin polarization were discussed.

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.

On the basis of a model similar to that of Su, Schrieffer, and Heeger, we investigate transitions be-tween bipolaron and polaron states In doped heterOCyCle polymers such as polypyrrole, polythlophene, and polyselenophene. The dopant impurity is assumed to be located between chains, and the interchain oupling is due to the electron hopping which is assumed to be confined to nearest-neighboring carbon groups around the counterion. The electron-electron interaction described by a Hubbard-type Hamil-tonian is also included in our calculation. It is shown that the system may be in a bipolaron state or a state of polaron pair of triplet triplet (singlet) nature. We find：that the interchain couping has litthstate of a polaron pair of spin triplet singlet nature We find that the interchain coupling has little influence on the stability of a bipolaron. Transitions from a bipolaron to a pair of polarons can be in-duced 0y a strong electron-electron Interactmn. WnlCn IS also responsible tor the energy dlfierence be-tween the singlet and triplet polarons states.

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.