Dimerization gap

The anions located in centrosymmetrical cavities lie slightly above or below the molecular planes. This structure results in a dimerization of the intermolecular distance (overlap) with a concomitant splitting of the HOMO conduction band into a filled lower band separated from a half-filled upper (holelike) band by a gap at 2k% called the dimerization gap which is shown in Fig. 6 at the point X of the new Brillouin zone. However, on account of the transverse dispersion, this dimerization gap does not lead to a genuine gap in the density of states as shown from the extended-Hiickel band calculation (Fig. 7). The only claim which can be made is that these conductors have a commensurate band filling (3/4) coming from the 2 1 stoichiometry with a tendency towards half... 

Fig. 2. Intraband contribution to the oscillator strength of a quarter-filled dimerized chain vs. the dimerization gap A. f, is the oscillator strength for a regular chain.

In the noninteracting limit, the band gap is directly proportional to the dimerization gap. In fact, this prediction is violated in traris-polyacteylene, indicating the importance of electron-electron interactions - as we describe in Chapter 7. 

Figure 7. The dimerization gap for a regular (O) and disordered ( ) soliton lattices as a function of doping level.

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