Lattice and magnetic instabilities near the neutral-ionic phase transition of the one-dimensional extended Hubbard model with alternating potentials in the thermodynamic limit

The effects of dimerization causing the alternation of transfer integrals on the neutral-ionic phase transition are studied in the one-dimensional extended Hubbard model with alternating potentials at half filling. The finite-temperature density-matrix renormalization-group method is used to treat the thermodynamic limit. In the ionic phase, the free-energy gain is proportional to delta(4/3) with delta being the degree of dimerization even near the phase boundary at low temperatures. In the neutral phase, the free-energy gain is quadratic for small transfer integral far from the phase boundary, but it is nonlinearly enhanced near the boundary. Large transfer integrals make the gain faster than delta(2), so that they facilitate dimerization. The dimerization in the neutral phase increases the ionicity and lowers the spin excitation energies. These lattice effects are in contrast to the effects of a staggered magnetic field. The relevance of these results to recently observed dimerization in the neutral phase of ClMePD-DMeDCNQI is discussed.