Solitons defect-like

The electronic band structure of a neutral polyacetylene is characterized by an empty band gap, like in other intrinsic semiconductors. Defect sites (solitons, polarons, bipolarons) can be regarded as electronic states within the band gap. The conduction in low-doped poly acetylene is attributed mainly to the transport of solitons within and between chains, as described by the intersoliton-hopping model (IHM) . Polarons and bipolarons are important charge carriers at higher doping levels and with polymers other than polyacetylene. 

Such an effect is not due to a trivial Joule heating of the samples. From a detailed experimental study based mainly on electrical [68], optical [70], and x-ray [71] measurements, this effect has been attributed to the motion of charged soliton-like defects existing in the TCNQ chains. A soliton in a dimerized chain is expected to have the following general form [47] ... 

In the case of conjugated conducting polymers, which were previously mentioned, the ET theory applies since the conductivity is not a simple band type conductivity but, depending on the doping level, merely an intrachain and/or interchain motion of charged soliton-like or polaron-like defects. We can sketch this motion as indicated in Fig. 36.26. 

The main results are that a reduction in particle size at one position of the array increases the potential at this point which may lead, at least, to localization, i. e. the single excess electron in the array might be trapped. At a packing defect, which affects the inter-particle capacitance at one point and acts like an inhomogenity, the soliton will interact with its mirror-image soliton (or anti-soliton) and will therefore be attracted. Concerning the practical use of this method, it was emphasized that the total reflection amplitude obtained from these calculations is directly related to the Landauer resistance,and reflects the electrical characteristics of such multijunction arrays. 

In polyparaphenylene, a soliton wave is not possible because the two phases, quinoid and aromatic, are not of the same energy, which excludes free motion. A double defect is possible though, a bipolaron. Such a defect represents a section of the quinoid stmcture (in the aromatic-like chain), at the end of which we have two unpaired electrons. The electrons, when paired with extra electrons from donor dopants or when removed by acceptor dopants, form a double ion (bipolaron), which may contribute to electric conductance. 

I ig. 25. Simulation of solitary pulses. A defect with slightly higher oxygen sticking coefficient on the reconstructed surface is located around x = 40 /j,m. The same defect can explain wave splitting, soliton-like behavior and partial annihilation, (c) differs from (b) only in a stronger asymmetry of the initial conditions after [115]. 

We can expect that in these highly conducting polymers electrons are much less localized and that they will reflect certain features of inorganic metals. But the chain-like structure of the conjugated polymers will give rise to a very anisotropic behaviour and some aspects of one-dimensionality will show up, such as the metal-insulator transition (Peierls-transition / /). The most exciting speculation is probably that on the existence of mobile conjugation defects, which can be described as non-linear excitations and are often referred to as solitons /5/ (Fig. U). These defects share many properties with non-linear excitations in other fields of physics and offer an interdisciplinary connection all the way from chemistry to elementary particle physics and field theory /6/. 

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