Shallow energy levels in quasi-one-dimensional MX compounds have been found firstly by considering the two-band tight-binding model. These shallow levels are situated at the band edges of the CB and VB. The electronic states of the shallow levels are localized around solitons, polarons and bipolarons. It is expected that these shallow levels could be observed in the absorption spectrum of MX compounds. Finally, the effect of e-e interactions on the shallow levels was studied. It has been found that e–e interactions will decrease the mini-gap between the shallow levels and CB and increase the mini-gap between the shallow levels and VB.

The vibrational properties of strongly coupled one-dimensional electron-lattice systems are dis-tinct form those of weakly coupled ones. While the weakly coupled systems have only four localized modes around the soliton and there is overlap between acoustic and optical phonon branches, the strongly coupled systems can possess more localized modes, and no overlap exists. In terms of the coupling constant λ, the boundary beween strong and weak coupling is λe=0.2119. The number of localized modes depends on λ, and each localized mode can only exist for a certain region of λ. Then the whole rane [0.2/π] of λ can be divided into seven regions, each of which has a charac-teristic disposition of the localized modes.

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.

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 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.