Atsushi Yamashiro

Spin- and charge-polarized states in nanographene ribbons with zigzag edges

Effects of the nearest-neighbor Coulomb interaction on nanographene ribbons with zigzag edges are investigated using the extended Hubbard model within the unrestricted Hartree-Fock approximation. The nearest Coulomb interaction stabilizes an electronic state with the opposite electric charges separated and localized along both edges, resulting in a finite electric dipole moment pointing from one edge to the other. This charge-polarized state competes with the peculiar spin-polarized state caused by the on-site Coulomb interaction and is stabilized by an external electric field.

Novel electronic states in graphene ribbons-competing spin and charge orders

In a nanographene ring with zigzag edges, the spin-polarized state and the charge-polarized state are stabilized by the on-site and the nearest-neighbor Coulomb repulsions, U and V, respectively, within the extended Hubbard model under the mean-field approximation. In a Mobius strip of the nanographene with a zigzag edge, U stabilizes two magnetic states, the domain wall state and the helical state. Both states have ferrimagnetic spins localized along the zigzag edge while the former connects the opposite ferrimagnetic orders resulting in a magnetic frustration forced by the topology and the latter rotates the ferrimagnetic spins uniformly to circumvent the frustration. The helical state is lower in energy than the domain wall state. On the other hand, V stabilizes another domain wall state connecting the opposite charge orders.

Electronic and lattice structures in potassium doped stage-1 polyacetylene

Summary form only given. Interchain transfers (ICT) via dopants and interchain Coulomb (ICC) and dopants Coulomb (DC) potentials in a Thomas Fermi approximation are calculated in a K doped stage-1 (CH)/sub x/.. Owing to the large sizes of the valence orbitals of K, dopants give a dense net of ICTs . We made Hatree Fock calculations to see their effects on the electronic and lattice structures of stage-1 (CH)/sub x/ using a resonable intrachain Hamiltonian. The ICTs little affect intrachain states but much change the orbital energy spectrum though only a little affect it near the Fermi level. The DC converts the BOW charged soliton lattice into a CDW bipolaron lattice. The ICC enlarges the gap and makes the phases of the bipolaron lattices ordered. The calculation with these three interactions resulted in highly K doped (CH)/sub x/ with a gap and lattice distortions similar to those of charged soliton lattices. Electron correlation may be important to get a metallic (CH)/ssub x/. Effects of incommensurability will also be disscussed.

A conduction mechanism in the non-magnetic regime of doped polyacetylene

We develop a model for the intrachain conduction mechanism in the non-magnetic regime of doped polyacetylene. We assign as current carriers superdefects, interstitial solitons and soliton holes, in a pinned charged soliton lattice. The internal strain energy and the Coulomb interactions in the system are taken into account by a phase Hamiltonian. Variational calculations show that the model gives a rather large activation energy in the conductivity beyond a doping concentration that is due to the internal strain energy and inconsistent with experiments