Charge dynamics in the Mott insulating phase of the ionic Hubbard model

I extend to charge and bond operators the transformation that maps the ionic Hubbard model at half filling onto an effective spin Hamiltonian. Using the transformed operators I calculate the amplitude of the charge-density wave in different dimensions D. In one dimension, the charge-charge correlations at large distance d decay as d^-3ln^-3/2d, in spite of the finite charge gap, due to remaining charge-spin coupling. Bond-bond correlations decay as (-1)ᵈd^-1ln^-3/2d as in the usual Hubbard model.