Quantum Monte Carlo study of the one-dimensional ionic Hubbard model

Quantum Monte Carlo methods are used to study a quantum phase transition in a one-dimensional Hubbard model with on-site interaction U and a staggered ionic potential (Delta). Using recently formulated methods, the electronic polarization and localization are determined directly from the correlated ground state wave function and compared to results of previous work using exact diagonalization and Hartree-Fock. We find a transition from a symmetric band insulator to a broken-symmetry bond ordered (BO) phase as the ratio of U/Delta is increased, with a metallic point at the transition. Since it is known that at Delta = 0 the usual Hubbard model is a Mott insulator (MI) with no long-range order, we have searched for a second transition to this state by (i) increasing U at fixed Delta and (ii) decreasing Delta at fixed U. We find no transition from the BO to MI state, and we propose that the symmetric MI state exists only at Delta = 0.