Studies On The Ground State And Structural Phase Transition In Organic Ferromagnetic Polymer

In this thesis,we have studied the electronic state,the configuration of spin density for two kinds of quasi-one-dimensional organic ferromagnets.Firstly,the properties of the ferrimagnetic ground state of a m-phenylene molecule chain are studied with the Hubbard model.Within HFA ( Hartree-Fock approximation ),the ferrimagnetic ground state is obtained.The result shows that if the on-site electron-electron repulsion U at the radical sites and U₀ at the phenylene ring sites are different,the energy gap may disappear and the ferrimagnetic ground state becomes unstable.The spin configuration exhibits that the ferrimagnetic ground state results from the antiferromagnetic correlations between the nearest neighbors.Due to cooperation and competition between the hopping integral and the on-site repulsion at different sites, the charge density and spin density can transfer between the radical sites and the phenylene ring sites. Secondly, we study the properties of an organic ferromagnetic polymer m-PDPC.By a theoretical model consisting of SSH ( Su-Schrieffer-Heeger ) Hamiltonian and a Hubbard term,the result exhibits that as the electron-phonon coupling A increases,there is a structure phase transition from a nondegenerate symmetric phase to a twofold degenerate symmetry-breaking phase.If A is greater than a critical value,which depends on the electron-electron U ,there are domain wall soliton describing the lattice deformation and spin envelopes describing spin localization.