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Theory of Electronic Structures and Lattice Distortions in Polyacetylene and Itinerant Peierls Systems. III: Coulomb Effects on Solitons and Effective Coulomb Potential in Trans-Polyacetylene

1983; Oxford University Press; Volume: 69; Issue: 2 Linguagem: Inglês

10.1143/ptp.69.373

1347-4081

Hideo Fukutome, Masaki Sasai,

Chemical and Physical Properties of Materials

We study effects of the π electron Coulomb interaction on solitons in trans-polyacetylene utilizing the PPP hamiltonian model developed in preceding papers of this series. Neutral soliton depends only on the on site and nearest neighbour parts of the effective Coulomb potential while charged solitons depend on its all parts. Owing to the Coulomb interaction, neutral soliton has a SDW like polarization cloud, while charged solitons have a CDW like one. The spreading of the polarization cloud is wider than that of the lattice kink in both neutral and charged solitons. The spreadings of a charged solitons are wider than those of a neutral soliton in both the polarization cloud and lattice kink. The polarization clouds are consistent with ESR, 13C NMR and XPS data. The formation energy of a zwitterionic pair of charged solitons, that gives the photoconductivity threshold, is very sensitively dependent on the strength and shape, especially on the screening of the long range part, of the effective Coulomb potential. To get the observed small photoconductivity threshold, the effective Coulomb potential in trans-polyacetylene must have a screened long range part and about 50% screened on site and nearest neighbour parts compared to the one in small conjugate molecules. Such screening of the effective Coulomb potential is essential to make possible splitting of the 1Bu exciton into a zwitterionic pair of charged solitons. The Coulomb interaction much affects the energies of the soliton levels. The neutral soliton level always lies below the gap center. The absolute and relative positions of the cationic and anionic soliton levels are much affected by the screening of the Coulomb interaction. Undoped and photoexcited trans-polyacetylenes have a low energy transition between soliton levels.