Dynamics of photogenerated soliton pairs in polyacetylene

The dynamics of a photogenerated soliton pair in polyacetylene are numerically studied using the Su-Schrieffer-Heeger model extended to include short-ranged Coulomb interactions. The unrestricted Hartree-Fock approximation is used. An electric field is introduced to the system so that the solitons approach each other and eventually collide together. The states after the collision are quite different for the cases with a charged soliton pair and with a neutral soliton pair. In the case of the charged soliton pair, the solitons repel each other and reflect to opposite directions. The charge exchange between the solitons takes place only when the initial velocity exceeds a certain threshold value. However, the exchange is not complete. In the case of neutral soliton pair, the solitons attract each other and make a dynamic bound state. This is the breather, which was found first in the system by Bishop et al. The present results tell us that photogenerated charged soliton pairs are highly stable, while the neutral soliton pairs are unstable.     

Comparative study on shirakawa polyacetylene and naarmann-theophilou-type polyacetylene: neutral soliton dynamics

A spin dynamics study of the neutral soliton is reported at room temperature to compare Shirakawa (S-PA) and Naarmann-Theophilou (NT-PA) type polyacetylenes. Intrachain (D_⊥) and interchain (D_⊥) diffusion rates, and a residual ESR width are discussed. A qualitative difference in dimensionality of spin dynamics, as reported by Wang et al. (Phys. Rev. B, 44 (1991) 12 070), was not observed at all. On the other hand, a weak correlation was found between D_⊥ and the residual ESR linewidth that is caused by trapping of the neutral soliton. It is concluded that the difference of conductivity in both polyacetylenes is dominated by the higher density and the higher degree of orientation in the NT-PA film than the S-PA film.     

A soliton in polyacetylene: Optical and dynamical activities

Doping of a polyacetylene chain is numerically simulated. The Su, Schrieffer, and Heeger model is used for the chain. Two types of doping mechanisms are examined. One is the doping by “site-type” impurities. They randomly modify the electronic energy levels at each site. Other is by “bond-type” impurities. The electronic transfers between two adjacent sites are randomly modified. For each random configuration of impurities, we determine metastable bond configurations, electron concentrations, dynamical conductivity, and infrared active phonon modes. It is found that the doping gives rise to solitons which are pinned by the impurities. Dynamical conductivity shows that the charged solitons, produced by the doping, are playing important roles in the infrared absorption.     

Displacement vectors of localized vibrational modes around a charged soliton in Trans-polyacetylene

The localized vibrational modes of finite polyacetylene with a charged soliton are calculated taking full degrees of freedom into consideration. The displacement vectors of each localized vibrational mode are obtained and shown in this report. The vectors are discussed in connection with mode mixing. The localized modes are successfully classified into branch modes of four modes: translational, amplitude oscillation, third localized and staggered. Displacement patterns of localized modes indicate that the coupling between the two types of C-C stretching branch modes is weak, but there is a strong coupling between one of the C-C stretching modes and the C-H bending mode. Calculated frequencies and infrared intensities are found to be in good agreement with the infrared spectrum. Furthermore, it has been found that a force constant matrix obtained from a short cluster model is not suitable for calculation in the case of a long polyacetylene chain.     

Calculation of localized vibrational modes around a soliton in trans-polyacetylene using the molecular orbital method

Vibrational frequencies of polyacetylene with a charged soliton are calculated for finite chain lengths using the molecular orbital method. A translational mode, an amplitude oscillation mode, a third localized mode and a staggered mode are obtained as localized modes. It is found that the frequency of the translational mode is inversely proportional to the square of the chain length, and approaches zero when the length becomes infinite. This can be explained if the soliton is regarded as a quantum particle in a box. A type of electron-hole symmetry holds, even with the Coulomb effect, between occupied orbitals of the cation and unoccupied orbitals of the anion and between unoccupied orbitals of the former and occupied orbitals of the latter.     

Synthesis of highly-oriented polyacetylene film in a liquid crystal solvent

The synthesis of oriented polyacetylene films is investigated by using a nematic liquid crystal as polymerization solvent. Highly-oriented thin films are obtained under a flow of catalyst solution in which a nematic liquid crystal is used instead of common solvents such as hexane and toluene. Nematic liquid crystals that could be used as solvents for the Ziegler-Natta catalysts are limited due to their reactive substituents with the active species and/or the components of the catalyst. Among the available nematic liquid crystals, phenylcyclohexane type is stable in the presence of Ziegler-Natta catalysts. Since fibril alignment in the oriented films is found to be along the flow direction of the catalyst solution, it appears that the direction of fibril growth coincides with that of the liquid crystal orientation, which is caused by an alignment effect of the liquid crystal molecules under a flow of the solution.This new and simple method allows quite large highly-oriented films to be synthesized, which makes it possible to investigate various kinds of anisotropic properties of polyacetylene.     

THF insertion in n-doped polyacetylene

The residual coinsertion of THF in polyacetylene (PA), n-doped by THF solutions of organo lithium, sodium and potassium, has been quantitatively analysed after cryogenic pumping (at −80 °C and −196 °C) of samples kept at room temperature.

It is found that the amount of coinserted THF per CH increases with the doping level y up to a maximum of y = 8–10% and then decreases. This maximum increases when the size of the cation decreases, from K to Li. With Na and K, and for any doping level, the THF content is higher in cis-doped than in trans-doped samples, while with Li no difference is observed between the two isomers. The influence of solvation on properties is discussed.

A synergic effect was observed during solvation by a THF/toluene mixture.     

Localization in a polyacetylene chain with sp3 carbon defects

The electronic structure of a model for a polyacetylene chain with sp³ carbon defects is studied by the tight-binding molecular orbital theory. There are two types of localization of wavefunctions, on-defect type and in-chain-fragment type. The localized states in the band wings consist of the former type whereas those inside the band the latter type. Existence of sp³ carbon defects induces the localization of in-chain-fragment type around the Fermi level and, in other words, reduces the conjugation length of a chain, which is consistent with general idea hitherto ambiguously considered.     

Ammonium doping of polyacetylene by a chemical method

The different methods of chemically inserting organic cations in polyacetylene are discussed. The most efficient among them was exploited quantitatively to insert tetraalkyl ammonium ions (R₄N⁺) of well defined size. The influence of the size of (R₄N)⁺ ions on the maximum doping levels, room temperature conductivities and ESR band widths of the doped products was demonstrated.     

Autoresonance control of a magnetization soliton

A method of controlling amplitude of a soliton of magnetization in an easy-axis magnet based on the phenomenon of autoresonance is proposed. Equations have been obtained that describe the temporal evolution of the soliton parameters under the effect of external variable fields. Configurations of pumping fields and conditions for the soliton-phase locking necessary for controlling field configurations have been found.     

Long-term conductivity decrease in polyacetylene samples

Investigations of the conductivity decrease of polyacetylene samples doped with iodine, AsF₅ and FeCl₃, respectively, and stored under various conditions (laboratory atmosphere, dry and humid air, vacuum, argon, argon at ?30 °C) are reported. In laboratory atmosphere the conductivity drops by several orders of magnitude within a few weeks in samples prepared by the conventional Shirakawa technique. In argon atmosphere at ?30 °C, after an initial drop by a factor of about two within a few days, the samples are very stable. With the exception of iodine doping, highly-doped samples are less stable in laboratory atmosphere or humid air than moderately-doped ones. Samples prepared by a new method from a polyacetylene suspension turn out to be much more stable than Shirakawa samples.     

Anderson localization in a polyacetylene chain with sp3 carbon defects

The electronic structure of a model for a polyacetylene chain with a random distribution of sp³ carbon defects is studied by the tight-binding molecular orbital theory. The dependence of the Anderson transition near the band centre (levels around the Fermi level) and in the band wings (levels near the lowest occupied and the highest unoccupied levels) on the number of sp³ carbons introduced is examined. Even one sp³ carbon in a chain leads to localization at the band edge. Near the centre of the band, both the extended and the localized states co-exist densely even if the number of sp³ carbons increases. It is concluded that conduction π electrons delocalize over a chain at a finite temperature as a result of the mixing of the states near the Fermi level.     

Physics of confined solitons in polyacetylene

The confinement potential for solitons in polyacetylene due to the interchain π-electron hopping is calculated. The confinement potential localizes neutral solitons within 35 to 70 lattice constants from the edge of the chain, which is consistent with a recent triple magnetic resonance experiment. The photogenerated charged soliton pair forms bound states due to the confinement potential. At low temperatures, the bound pair with large quantum number cascades radiatively down to the ground state, giving rise to the luminescence.     

Conductivity and magnetoconductance in iodine-doped polyacetylene

Disorder-induced localization plays a major role in the metal-insulator (M-I) transition of doped conducting polymers. Thus the quantification of disorder is important to understand the transport properties of conducting polymers. A plot of the logarithmic derivative of conductivity (W= (Δ In σ/Δ In T)) versus temperature is a simple and sensitive technique to characterize the extent of disorder, and to identify the metallic (M), critical (C) and insulating (I) regimes. A typical example is shown in the case of I-(CH)_x samples. Another important parameter that determines the M-I transition in conducting polymers is the inter-chain interaction. Pressure and magnetic field can tune the relative position of the mobility edge with respect to the Fermi level. The localization-interaction model is observed to be satisfactory in the metallic regime. A power-law behavior of the temperature dependence of conductivity is observed in the critical regime. The positive temperature coefficient of resistivity (TCR), at low temperatures, in oriented I-(CH)_x samples is sensitive to the direction of field with respect to the chain axis. The anisotropic magnetoconductance (MC) indicates the subtle interplay of weak localization and e-e interaction contributions, and the positive contribution to MC is less when the field is parallel to the chain axis. This indicates that the anisotropic MC probes the microscopic anisotropy in charge transport parameters. The temperature dependences of conductivity and magnetoresistance are important tools to study the M-I transition due to disorder, inter-chain interaction, etc., in conducting polymers.     

Breathers in polyacetylene and their optical absorption

A semiclassical formalism for the optical absorption in a coupled electron-phonon system is derived and applied to breather modes in trans-polyacetylene. We show that such breathers can account for both the photoinduced absorption peak observed at 1.35eV and the groundstate intragap absorption tail.     

Raman scattering and infrared phonons in polyacetylene

The Raman active normal modes of polyacetylene are described in terms of amplitude modes associated with the Peierls gap. A narrow distribution in the effective electron-phonon coupling constant accounts for the spectrum and its dependence on laser frequency. The resonant Raman condition determines a functional dependence of the gap on the coupling constant which is consistent with the Peierls relation. All infrared phonons induced by either doping or photogeneration are determined by the Raman data and a pinning parameter.     

Nonlinear optical response in polyacetylene and polydiacetylenes

We review and discuss recent developments concerning the nonlinear optical responses of polyacetylene and polydiacetylenes, and the application of such materials for photonic switching.     

Time dependent relaxation of photoexcited states in polyacetylene

We present the results obtained by numerical simulation of the time behaviour, in the long time regime, of the transient photoinduced bleaching. The polymeric chain is assumed to be formed by conjugated segments of different length, whose distribution is derived by the resonant Raman scattering spectra interpretation. The kinetics of the photoinduced carriers is simulated through a random walk over a sequence of conjugated segments, separated by temperature dependent barriers with randomly distributed heights. The numerical simulation is able to determine a power law behaviour t^−α as observed experimentally in the picosecond regime (50–1000 ps) with α decreasing as function of temperature.