Showing papers by "Sven Stafström published in 1991"

Electronic Properties of Heavily Doped Trans-Polyacetylene

Abstract: The structural and electronic properties of heavily doped trans-polyacetylene are studied using a total Hamiltonian consisting three terms: 1) the Su-Schrieffer-Heeger Hamiltonian, 2) an external potential term due to electrostatic interactions between electrons of the polymer chain and counterions and charges on neighboring chains, and 3) a term describing the intra-chain electron-electron interactions. The effects of weak disorder in the positions of the counterions are also studied. At all doping levels included in this study, the ground state configuration is a soliton lattice. The energy gap around the Fermi energy and the density of states at the Fermi energy are calculated. A low bandgap system is obatined in the case of weak disorder at doping levels above 8%. The electronic properties of the polaron lattice are also calculated. Previously reported experimental results and theoretical models for the metallic state of heavily doped trans-polyacetylene are reviewed and discussed in comparison with the results obtained in this study.

Soliton lattice in highly doped trans -polyacetylene

Abstract: The geometrical and electronic properties of highly doped trans-polyacetylene are studied by use of a total Hamiltonian consisting of the Su-Schrieffer-Heeger Hamiltonian, describing the electron hopping along the polymer chain, an extended Hubbard term due to the intrachain electron-electron interaction, and an external Coulomb potential arising from interchain electrostatic interactions and interactions with counterions. The three-dimensional structure of the system including polymer chains and dopant ions is that obtained from x-ray crystallography studies of sodium-doped trans-polyacetylene. At all doping levels (y\ensuremath{\le}12 at. %), and for all chain lengths (100N224) included in this study, the ground-state configuration is a soliton lattice, commensurate with the periodicity of the external potential. A self-consistent treatment of the soliton charge distribution entering the external potential is essential in order to calculate the electronic properties of the system correctly. For a regular distribution of the counterions, the evolution of the single-particle gap in the electronic spectrum exhibits a sharp decrease in the gap at doping levels between y\ensuremath{\sim}8 and 10 at. %. Increasing the doping level further does not lower the gap significantly. This result indicates the possibility of having a purely electronic phase transition within the soliton-lattice configuration, in agreement with experimental data. The size of the energy gap at high doping levels is, however, in the nonmetallic regime. When disorder in the positions of the counterions is introduced, the electronic states at the soliton and conduction-band edges tail off and close the gap around the Fermi level above \ensuremath{\sim}10 at.% doping. This phenomenon occurs already at very weak (intrinsic) disorder for which the counterions are displaced by maximum one-half of a carbon-carbon bond length.

Electronic properties of highly doped trans-polyacetylene.

Abstract: The evolution of the density of states at the Fermi energy for a single chain of highly doped trans-polyacetylene is studied as a function of the dopant concentration. The studies are focused on the metallic state of the polymer, which is obtained for dopant concentrations above y\ensuremath{\sim}6%. Two types of lattice structures are considered: the soliton lattice and the polaron lattice. An effective potential due to the dopant counterions is included. The calculations are restricted to perfectly ordered samples. It is shown that the soliton lattice exhibits metallic properties only at very high doping levels. The polaron lattice is shown to exhibit a finite density of states at the Fermi energy, which in the metallic regime compares favorably with the experimental result both in magnitude and as concerns the evolution with increasing dopant concentration.