Localized Excitations in 1-D Peierls-Hubbard Models of Polyacetylene and MX Chains

Abstract

Recent interest in novel low-D materials — e.g., high-temperature superconducting copper oxides, “heavy-fermion” and charge-density wave systems, halogen-bridged transition-metal linear chain complexes (MX chains), and organic synthetic metals and superconductors — and their pressure, photoexcitation, and doping induced intragap states, has stimulated the theoretical study of competing electron-electron (e-e) and electron-phonon (e-p) interactions in reduced dimensions. For these new materials it is important in microscopic models to capture the essence of both e-p and e-e interactions and to represent faithfully their synergetic, or competing, effects. To this end, variants of the extended Peierls-Hubbard Hamiltonian (ePHH) have been widely used. In this article, intrinsic localized non-linear defects (solitons, polarons, and bipolarons) in 1- and 2-band versions of the 1-D ePHH are discussed and compared to experiments on the conducting polymer trana-polyacetylene, (CH)∞, and on the MX chain [Pt(en)2][Pt(en)2Cl2](ClO4)4 (en=N2C2H8), hereafter referred to as PtCl. Obtaining fits to many different observables with a single set of physically plausible parameters within a given model is essential for a true microscopic understanding of non-linear excitations in any specific material, and in general for an understanding of the whole class of novel low-D materials.