Defect-induced formation of gap states in homogeneously dimerized polyacetylene chains.

Defect-induced states in the band gap of a dimerized static trans-polyacetylene chain are examined within the Su-Schrieffer-Heeger model. We consider models for ``on-site'' and ``bond'' impurities located at the same site and for a location at different lattice sites. It is found that gap states are only absent in a restricted parameter range, if at a disturbed strong (weak) bond the change of the transfer integral ${\mathrm{\ensuremath{\tau}}}_{\mathit{n}}$ exceeds (remains under) a threshold value. For model impurities at different sites, the quantitative parameter range, where gap states are suppressed, strongly depends on an even- or odd-valued site distance. In addition, this range tends to decrease with increasing site distance.