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

Abstract

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 ${\ensuremath{\delta}}^{4/3}$ with $\ensuremath{\delta}$ 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 ${\ensuremath{\delta}}^{2},$ 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.