Effective Hamiltonian for a half-filled tetramerized ionic-Hubbard chain
  We derive an effective spin Hamiltonian for the one-dimensional half-filled tetramerized ionic-Hubbard model in the limit of strong on-site repulsion. We show that the effective Hamiltonian which describes the low-energy spin sector of the model is a spin S=1/2 Heisenberg Hamiltonian with alternating nearest-neighbour exchange. 
M. Menteshashvili, N. Chachava and G. I. Japaridze