Energy-band structures have been calculated for ?SiR?x polyalkylsilene model compounds, where R indicates an H atom or a CH3 [methyl (Me)] group. A possible Peierls instability has also been investigated theoretically for these polymers. Polyalkylsilene with H-atom or Me side groups have overlapped conduction bands formed of skeleton bands and bands. This band overlapping suppresses the metal-insulator (M-I) transition due to the simple double-bond alternation found in ?CH?x. This band overlapping also causes three types of scattering processes: Two of them are intra--band and intra--band scatterings and the other is inter-band scattering. Inter-band scattering produces a charge-density wave (CDW) having a commensurate wave number of q=/a by dimerizing two skeleton unit cells. Intra--band scattering produces a CDW having an incommensurate wave number of q=2kF. However, this incommensurate CDW is equivalent to an incommensurate CDW with q=2kF produced by intra--band scattering, because the one dimensionality of overlapping band guarantees the relation of kF+kF=/a. The resulting polyalkylsilene tends to have the characteristics of an insulator due to intraband scattering rather than interband scattering. This is because of its orthogonality of overlapping and bands.
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