Soliton Charged

An alternative interpretation for the activated behavior of the photocurrent and the PIA-decrease with temperature was proposed by Townsend et al. [35], They assigned their experimental results to a thermally activated interchuin-hoppmg mechanism for bipolaron-like charged soliton pairs, the experiments of which were carried out on Durham /ran.v-polyacetylene. 

We have used the systems CnH +2 with n = 2,4,...,22, C H +2 with n = 3,5,...,21, and C H +2 with n = 4,6,...,22 to represent pure PA, positively charged solitons, and bipolarons respectively. SCF wavefunctions were calculated with a double-zeta quality basis set (denoted 6-3IG) and optimized geometries for all these systems were determined. In addition for the molecules with n up to 11 or 12 we calculated the vibrational spectrum, including infrared and Raman intensities. 

The trends in the spectra of the dications are not in as satisfactory an agreement with experiment as the trends observed in the cation spectra. We note, however, that CuHn may be a better representation of charged solitons than... 

The degeneracy of the ground state of polyacetylene influences its charge distribution. In fact, upon doping the charges, which in other polymers, such as the heterocyclics, would pair to form bipolarons, are here readily separated to form two positively charged solitons ... 

The main scheme is shown in Fig. 17. The photogenerated electron hole pairs transfer to the soliton-antisoliton pairs in 10 13s. Two kinks appeared in the polymer structure, which separates the degenerated regions. Due to the degeneration, two charged solitons may move without energy dissipation in the electric field and cause the photoconductivity. The size of the soliton was defined as 15 monomer links with the mass equal to the mass of the free electron. In the scheme in Fig. 17, the localized electron levels in the forbidden gap correspond to the free ( + ) and twice occupied ( — ) solitons. The theory shows the suppression of the interband transitions in the presence of the soliton. For cis-(CH)n the degeneration is absent, the soliton cannot be formed and photoconductivity practically does not exist. 

Fig. 17a, b. Band scheme (a) and chemical structure (b) of trans-(CH) . The polymer chain contains the charged soliton-antisoliton pair. In the middle of the forbidden gap there are levels connected with two solitons not occupied ( + ) and twice occupied ( —) [106]... 

Large third order susceptibilities have recently been observed for trans-polyacetylene, heteroaromatic polymers, and poly[p-phenylene vinylene] (4-11). Such nonlinear phenomena in electroactive polymers due to intense laser irradiation has been linked to the photogeneration of charged solitons on time scales of the order 10 s, and values of (3w-w+w+w)—4x 10 esu for... 

Fig. 4.2 A short segment of t rans -polyacetylene is shown with an abrupt (idealized) reversal of the bond alternation pattern (see text). Top- a neutral soliton with an unpaired spin and an energy state near the middle of the electron energy gap. Middle the addition of an electron results in the formation of a spinless negatively charged soliton. Bottom the extraction of an electron from the top results in the formation of a spinless positive soliton. The optical transitions associated with the charged solitons are indicated as arrows on the right.

Fig. 2.3. (a) and (b) Structure of the soliton S and antisoliton S (the filled circles represent the electrons still bound to the polymer chain), (c) normalized displacement in the C-H positions due to formation of the soliton, and (d) energy levels of the neutral and the charged solitons (the arrows show the electrons with spins up or spins down). The figure is adapted from different figures given in Ref. [14]. 

The photoinduced 0.5 eV band is strongest at low temperatures and starts decreasing in strength at 150 K. It becomes unobservable at 250 K. Photoconductivity is maximum at room temperature and becomes very weak below 200 K. This observation leads to the conclusion that the charged solitons are free to move only above 150 K. 

We now discuss the experimental evidence that the charged solitons have zero spins. Let Ns be the number of spins and Ach be the number of the charge carriers. If the charge carriers are Fermions, the ratio... 

Polaron — Polarons are charged quasiparticles with spin lf. This term has been introduced by physicists as one of the possible solutions to the equations of the relevant defect model of solids in order to describe an electron in a dielectric polarizing its environment (electron-phonon coupling), electrically situated below the conduction band, and transported together with its polarized environment. Polarons and -> bipolarons are the charge carriers in oxidized or reduced (doped) -> conducting polymers. A polaron is defined as a neutral and a charged -> soliton in the same... 

There are neutral solitons (radicals) with spin 1/2 and charged solitons (cations or anions) which carry no spin [ii]. The model calculations on the defect states induced upon -> doping provide a description of the electronic properties of the polymer semiconductors, and make the theoretical evaluation of the spectral and conductivity data possible. 

It is quite obvious from the above that disorder of any kind will reduce the CDW or BOW responses and the PLD fluctuations. Consequently, T% will decrease and will eventually disappear for large enough disorder [69]. Defects will also pin the Frolich mode and produce phase and amplitude modulations in the order parameter. Impurities may ionize (dopants) and produce charged solitons and discommensurations [70]. The commensurability pinning will decrease, affecting the soliton energy and width, as does the umklapp pinning. 

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] ... 

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