Structure and morphology: relation to thermopower properties of conductive polymers

Lattice and electronic structures of undoped and doped C 60 are investigated. The Su-Schrieffer-Heeger model is extended and solved numerically. For the undoped system, electronic level structures agree well with those of th tight-binding model [Hayden and Mele: Phys. Rev. B 36 (1987) 5010]. When one or two electrons are added or removed, the lattice deforms and two levels intrude into the energy gap like a polaron in polyacetylene. The dimerization becomes weaker along a meridian. Consequences of “polarons” for experimental studies are discussed.

Lattice and Electronic Structures of Undoped and Doped C 60 Molecules by the Extended Su-Schrieffer-Heeger Model

The structural and electronic properties of heavily doped trans-polyacetylene are studied using a total Hamiltonian consisting three terms: 1) the Su-Schrieffer-Heeger Hamiltonian, 2) an external potential term due to electrostatic interactions between electrons of the polymer chain and counterions and charges on neighboring chains, and 3) a term describing the intra-chain electron-electron interactions. The effects of weak disorder in the positions of the counterions are also studied. At all doping levels included in this study, the ground state configuration is a soliton lattice. The energy gap around the Fermi energy and the density of states at the Fermi energy are calculated. A low bandgap system is obatined in the case of weak disorder at doping levels above 8%. The electronic properties of the polaron lattice are also calculated. Previously reported experimental results and theoretical models for the metallic state of heavily doped trans-polyacetylene are reviewed and discussed in comparison with the results obtained in this study.

Electronic Properties of Heavily Doped Trans-Polyacetylene

: Radar absorbing materials require resistive or magnetic materials that will convert the electromagnetic energy into heat. The electrical properties of polypyrrole make it a suitable candidate for a radar absorbing material. There are several simple ways to make the polymer with controlled conductivity, and many strategies to process it into a robust material. Interesting materials include polypyrrole-paint, polypyrrole-polyvinylchloride injection moulded composites and polypyrrole coated fabrics and fibres. The highest imaginary component of the permittivity for a polypyrrole composite occurs at the percolation threshold. This is also the region where the permittivity changes rapidly with the amount polypyrrole in the composite and so a high degree of control is required for reproducible making the same formulation. At the conductivities required for microwave absorption, the dc conductivity dominates and the ac component is negligible. Polypyrrole stability is an issue for fabricating a radar absorber, and stability can be improved through a barrier protecting the polypyrrole from oxygen. In this paper many aspects of the formulation and characterisation of polypyrrole materials are considered with an emphasis on its suitability as a microwave absorber.

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Polypyrrole, Formation and Use

The metal-insulator transition in polyacetylene: variational study of the Peierls-Hubbard model

In order to substantiate the applicability of the coherent-potential approximation (CPA) to conducting polymers with randomly distributed impurities, the Su-Schrieffer-Heeger model is generalized by including the effects of the impurities and solved numerically. The lattice configurations, electronic levels, and wave functions are determined for each impurity distribution. The size of lattice dimerization and electronic density of states are determined and averaged over a large number of impurity distributions, which are selected independently and randomly. The study is confined to systems with one electron per site that we have studied previously using the CPA. Two types of impurities as well as three doping mechanisms are considered. It turns out that the magnitude of lattice dimerization agrees remarkably well with the CPA result for all impurity types and doping mechanisms under consideration. The density of states shows a characteristic tail at the edge of a band where the CPA predicts an impurity band at lower concentrations. Such a tail never occurs in regimes where the CPA does not predict such bands. Quasilocalization is found in the wave functions of the states at the band edges. It is closely associated with the characteristic structure of the density of states. The usefulness of the CPA is thus established for the Peierls system as far as gross qualitative properties are concerned.

Electronic states in doped conducting polymers: Numerical simulation of disordered systems.

Metal-insulator transition in doped conducting polymers: Disappearance of electronic gap with persisting bond alternation

Metal-insulator transition in doped conjugated polymers: Effects of long-ranged Coulomb potentials.

Etude des effets du desordre sur les etats electroniques du trans-polyacetylene dans le cas des impuretes de type liaison qui donnent naissance aux retro-diffusions d'electrons. Etude du modele generalise de Takayama-Lin-Liu-Maki dans l'approximation du potentiel coherent (CPA). On emet l'hypothese que la dimerisation du reseau est uniforme et representee par un parametre d'ordre Δ. La CPA fournit les equations pour les fonctions de Green des electrons. Resolution numerique de ces equations tandis que Δ est determinee de maniere a obtenir une energie minimale. On trouve trois solutions pour les faibles concentrations en impuretes dont une est stable, une autre metastable, et la troisieme instable. Pour la solution stable, qui reste valable pour des concentrations en impurete plus grandes, il n'y a pas formation de bande d'impuretes. Pour la solution metastable, la bande de conduction et la bande de valence chevauchent la region du gap indiquant la formation de bandes d'impuretes. Deduction de la dependance des parametres d'ordre avec la concentration, les gaps d'energie, et la concentration critique de la solution metastable

Electronic states in trans -polyacetylene with bond-type impurities in the coherent-potential approximation

The electronic level structure and the dimerization order parameter are studied in doped conjugated polymers when randomly distributed site-type impurities occupy different positions from those in previous investigations. The generalized Takayama-Lin-Liu-Maki model is analyzed with the help of the coherent-potential approximation using the assumption that the order parameter is uniform with one electron per site. The order parameter, the electronic level structure, and its phase diagram are obtained numerically

Kikuo Harigaya

Papers

Electronic states in doped conducting polymers: Numerical simulation of disordered systems.

Metal-insulator transition in doped conducting polymers: Disappearance of electronic gap with persisting bond alternation

Metal-insulator transition in doped conjugated polymers: Effects of long-ranged Coulomb potentials.

Electronic states in trans -polyacetylene with bond-type impurities in the coherent-potential approximation

Impurity distribution and electronic states in doped conjugated polymers in the coherent-potential approximation