Katsufumi Hashimoto

A theoretical estimate for quasihomopolar singlet levels in long polyenes with a spin model hamiltonian

Abstract A spin model hamiltonian is employed to estimate the energy levels of quasihomopolar singlet states in long polyenes. To this end, the singlet excitation energies of the alternating spin chain are calculated as functions of γ and N using the Hartree-Fock method developed based on the spinless fermion formalism. The result indicates that the excitation energies decrease monotonically with increasing chain length in accord with the known experimental results. Using the parameters J and γ empirically determined from the data on short polyenes, an estimation is given for the energy levels of the lowest-lying 2 1 A g states as well as those of the second 1 B u quasihomopolar states in long polyenes. In particular, the 2 1 A g level in polyacetylene is predicted to be at 1.1 eV in good agreement with a recent photoluminescence result (1.19 eV).

Heisenberg spin Hamiltonian through a new Heitler-London approach: Test on the half-filled linear Hubbard model

Abstract The Heisenberg spin Hamiltonian constructed from a new Heitler-London method is tested on the half-filled linear Hubbard model. It is found that the Heisenberg Hamiltonian of the new Heitler-London method works well in a much wider parameter range than that of the usual perturbation method does.