Effects of intersite Coulomb interaction on ferromagnetism and dimerization in nanographite ribbons

Using the Peierls-extended Hubbard model and Hartree-Fock approximation, the stability and dimerization of the ferromagnetic ground state of a nanographite ribbon are investigated. For a given on-site Coulomb interaction U, if the intersite Coulomb interaction V is larger than a critical value V-c, the ferromagnetic ground state is unstable and a transition from the spin-density-wave state to the charge-density-wave state occurs. On increasing the width of the ribbon, the critical value V-c changes from V-c approximate to 0.5U to V-c approximate to 0.25U, which are similar values to those in the one- and two-dimensional extended Hubbard model, respectively. The on-site repulsion usually suppresses the dimerization while the intersite repulsion can turn the dimerization on and off.