Exact Solution of an Extended Hubbard Model with Electron-Lattice Interaction for an Organic Ferromagnetic Polymer

Using an exact diagonalization method, we study an extended Hubbard model with an electron-lattice interaction for an organic ferromagnetic chain with radical coupling. The result shows that the ferromagnetic ground state originates from the antiferromagnetic correlation between adjoining sites, which is enhanced by the on-site e-e repulsion. The intersite e-e repulsion induces the imhomogeneous distribution of the charge density. The dimerization is decreased by the e-e interaction and the radical coupling. The electron-lattice interaction and the radical coupling can transfer the spin density and charge density between the main chain and the radicals.