Solitons neutral

The most interesting aspect of solitons is their charge-spin relation [ 11. Chaiged solitons are spinless, while neutral solitons have spin 1/2. This property provided a natural explanation for the fact that in doped /rrm.v-polyacctylenc the electrical... 

A sequence may form and eventually meet a B sequence, as shown, but in doing so, a free radical, called a soliton, is produced. The soliton is a relatively stable electron with an unpaired spin and is located in a nonbonding state in the energy gap, midway between the conduction and valence bands. It is the presence of these neutral solitons which gives frany-polyacetylene the characteristics of an intrinsic semiconductor with conductivities of 10 to 10 (f2 cm) ... 

The soliton conductivity model for rrans-(CH) was put forward by Kivelson [115]. It was shown that at low temperature phonon assisted electron hopping between soliton-bound states may be the dominant conduction process in a lightly doped one - dimensional Peierls system such as polyacetylene. The presence of disorder, as represented by a spatially random distribution of charged dopant molecules causes the hopping conduction pathway to be essentially three dimensional. At the photoexitation stage, mainly neutral solitons have to be formed. These solitons maintain the soliton bands. The transport processes have to be hopping ones with a highly expressed dispersive... 

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.

When two trans-polyacetylene chains with different phases are put together, an obvious disturbance occurs in the standard conjugation pattern. The bond alternation defect that appears is known as a neutral soliton (Fig. 1.7). This kind of quasi-particle has an unpaired electron but is electrically neutral and is isoenergetically mobile along the polymer chain in both directions. This soliton gives rise to a state in the middle of the otherwise empty energy gap that can be occupied by zero, one or two electrons (Fig. 1.8). 

Measurements of diffusion of neutral solitons have been made using the electron spin resonance (ESR) and the nuclear magnetic resonance (NMR). An unambiguous interpre-... 

A soliton is a giant solitary wave produced in canals by a cancellation of nonlinear and dispersive effects. The connection between aqueous solitons and tsunamis ("harbor waves") is not definitively established. In "doped" conducting polyacetylene, a neutral soliton is a collective excitation of a polyacetylene oligomer that has amplitude for several adjacent sites [57],... 

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. 

The effect of this is that the charged defects are independent of one another and can form domain walls that separate two phases of opposite orientation and identical energy. These are called solitons and can sometimes be neutral. Solitons produced in polyacetylene are believed to be delocalised over about 12 CH units with the maximum charge density next to the dopant counterion. The bonds closer to the defect show less amount of bond alternation than the bonds away from the centre. 

Figure 16 Photothermal deflection spectrum of a NH3 -compensated trans-VA. Dots and dashed line show the absorption edge recorded on two films, 4 p.m and 0.1 xm, respectively. Dotted line shows the residual absorption of an uncompensated sample. Solid line sketches the expected neutral soliton absorption in the absence of electron correlation. (From Ref. 130.)...

Figure 5 Bond alternation defect, or neutral soliton in /nww-polyacetylene.

The major contribution to the EPR signal at room temperature is from Curie-like spins, which are associated with localized free radicals (trapped neutral solitons). The high activation energy, AEj, and broad line width of PMQl shows the effect of short conjugated sequences on trapped spins. 

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