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 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.

Localized modes of lattice vibration in polythiophene are investigated numerically by considering a chain of finite length. th the periodic boundary condition, we find three even-parity and four odd-parity modes. One of the even modes due to the sulfur heteroatom in the heterocycle is believed to be re-sponsible for what is observed around 700 cm. in the Raman spectrum of polythiophene. With the free-end boundary condition, several modes from the highest optical branch become localized near the surface. These modes may be identified as the high-frequency modes observed in infrared-active vibra-tion and resonant. Raman. scattering absorption experiments.

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.