Polaron dynamics with impurities in conjugated polymers

We study the charge transport by polarons in conjugated polymers in the presence of impurities. The effects on the polaron motion due to the symmetry of the chain is considered. The polaron dynamics in doped conjugated polymers is numerically studied using the Su-Schrieffer-Heeger (SSH) model combined with the Pariser-Parr- Pople (PPP) model modified to include an external electric field and the Brazovskii-Kirova (BK) symmetry breaking interaction. The time dependent Hartree-Fock approximation is used. The electric field is used to accelerate the polaron in a polymer chain. Polaron trapped by the impurity and polaron-impurity collisions are considered. When the polaron-structure is around the impurity site a charge oscillation can be observed. Nevertheless, when there are collisions between the polaron and the impurity, the polaron-structure can pass, be trapped or be reflected. These effects are determined by the strength of the radical parameter and by the electric field intensity. The effects on the polaron are analyzed and an effective potential is determined. Therefore, the effective potential determines the polaron behavior for each case, as the polaron pass, or it is trapped or reflected by an impurity

Supersonic quasi-particles dynamics in organic semiconductors

AbstractWe investigate the dynamics of charge-carriers schemes on high external electric fields in organic semiconductors. Specifically, the critical field strength needed for the transition from subsonic to supersonic regime was determined for solitons and polarons in conjugated polymers, for both trans- and cis-symmetry. Also, the collision between quasi-particles and impurities, at supersonic regime, is investigated. For this purpose it is used a modified Su–Schrieffer–Heeger hamiltonian model in the scope of an Ehrenfest molecular dynamics. The results provide useful information regarding the behavior of the conductivity of conducting polymers measured for different electric fields and bearing different types of impurities

Charge carrier scattering in polymers : a new neutral coupled soliton channel

The dynamical scattering of two oppositely charged bipolarons in non-degenerate organic semiconducting lattices is numerically investigated in the framework of a one-dimensional tight-biding–Hubbard model that includes lattice relaxation. Our findings show that it is possible for the bipolaron pair to merge into a state composed of a confined soliton-antisoliton pair, which is characterized by the appearance of states within less than 0.1 eV from the Fermi level. This compound is in a narrow analogy to a meson confining a quark-antiquark pair. Interestingly, solitons are quasi-particles theoretically predicted to arise only in polymer lattices with degenerate ground state: in the general case of non-degenerate ground state polymers, isolated solitons are not allowed

O papel das associações de interesse privado no mercado cafeeiro brasileiro

A desregulamentação do mercado cafeeiro propiciou uma nova conjuntura institucional. Nesse cenário, buscou-se investir em estratégias de diferenciação e na criação de associações orientadas para implementá-las. É pertinente discutir o papel dessas organizações nesse contexto. O objetivo deste artigo é discutir a expansão do papel das associações de cafeicultores brasileiros, enfocando o suporte que tais organizações oferecem para que seus membros possam atuar com produtos diferenciados. Trata-se de um ensaio sobre o papel das associações de interesse privado sob o prisma da Economia dos Custos de Transação, das Redes e do Capital Social.Deregulation of the coffee market provided a new institutional scenario where investments in differentiation strategies were sought and private interest associations were formed to carry out these strategies. This situation was discussed together with the growing role of associations of Brazilian coffee producers which were focused on supporting members promoting differentiated products. A description of these associations was presented from the standpoint of transaction cost economics, networks and capital stock

Charge Transport Mechanism in Chevron--Graphene Nanoribbons

From the moment atomic precision control of the growth process of graphene was achieved, more elaborated carbon allotropes were proposed opening new channels for flat optoelectronics at the nanoscale. A special type of this material presenting a V-shape (or "kinked" pattern) was recently synthesized and named Chevron-graphene nanoribbons (C-GNRs). To realize the reach of C--GNRs in developing new applications, the formation, and transport of charge carriers in their lattices should be primarily understood. Here, we investigate the static and dynamical properties of quasiparticles in C-GNRs. We study the effects of electron-phonon coupling and doping on the system. We also determine the kind of charge carriers present in C--GNR. It is observed that a phase transition occurs between a delocalized regime of conduction and regimes mediated by charge carriers. Such a phase transition is highly dependent on the doping concentration. Remarkably, crucial differences from the transport in standard graphene nanoribbons are identified. These factors are noted to have a profound impact on the mobility on the system which, in turn, should decisively impact the performance of electronic devices based on C-GNRs.Comment: 12 pages, and 09 figure