Interplay of disorder and electron-phonon coupling in models of polyacetylene

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Abstract
We present, within the Su-Schrieffer-Heeger model, analytical and numerical calculations of the effects of two types of model impurities, namely bond and site, on the statics and dynamics of the lattice structure, and adiabatically associated one-electron spectrum, of trans-polyacetylene. For both types of impurities two or more localized electronic levels are produced, which may be in the intragap region or beyond the π-band edges (ultraband). Linear-response calculations of the induced lattice defects are in good agreement with the numerical calculations. A numerical study of kink-impurity dynamics shows that a kink may be reflected, transmitted, or trapped by the impurity depending on both the impurity strength and the topology of the impurity-induced lattice defect. A kink trapped by a site impurity of sufficient strength is found to have no midgap level but is supported by a level beyond the edge of the π band. We also find that the trapping of a kink by a site impurity may result in the production of a potentially long-lived polaron. Such strong electron-phonon renormalization effects on the electronic structure in the presence of disorder are also found for bond impurities, where ultraband levels may form at the expense of more localized intragap levels. Photoexcitation experiments in the defected system produce the full range of nonlinear excitations: kinks, breathers, polarons, excitons, and trapped kinks. We calculate the optical absorption spectra in typical experiments and identify extrinsic intragap absorption.

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