Showing papers by "Eugene J. Mele published in 1986"

Correlation effects and excited states in conjugated polymers.

Abstract: We apply the Hubbard-Peierls Hamiltonian to study the low-lying excited states of several model conjugated polymers. The calculations employ a numerical renormalization-group method for chains of intermediate length, and a more approximate truncated configuration-interaction scheme to extrapolate the results to longer systems. In the range of on-site Coulomb repulsion strengths of physical interest, we find that a correlated even-parity singlet excited state competes with the odd-parity singlet excitation expected in the U=0 limit of the model, in agreement with previous studies on the Pariser-Parr-Pople model. The lowest electronic excitations in the model are identified as triplet excitations. Self-consistently-relaxed lattice configurations surrounding these electronic excitations are studied. In addition to the stable photoexcitations predicted in the U=0 limit of the theory, we find that the interelectronic repulsive potential leads to a rich spectrum of competing neutral structures, which are studied and characterized. Coulomb correlations provide a mechanism for production of long-lived spin-1/2 neutral excitations for degenerate ground-state systems, and spin-1 neutral excitations for nondegenerate ground-state systems.

Continuum model for vibrational excitations of conjugated polymers.

Abstract: The continuum model of the coupled electron-phonon system in conjugated polymers is extended to include the lattice kinetic energy and the coupling of several structural degrees of freedom. The model is used to study defect vibrational spectra for a variety of defects that may occur, e.g., solitons, polarons, and bipolarons, in conjugated polymer systems. In agreement with previous theoretical studies of polyacetylene, a class of infrared-active modes common to all mobile bond-alternation defects is obtained. In addition the study reveals a new class of bound vibrations localized at the defect center of a polaron or a bipolaron. These additional infrared-active modes can be used spectroscopically to identify defect types in conjugated polymers such as polyacetylene or polythiophene. Examples considered include doping and photoinduced defects (solitons and polarons) in both (CH${)}_{\mathrm{x}}$ and (CD${)}_{\mathrm{x}}$ as well as photoinduced defects (bipolarons) in polythiophene.