Tricritical behavior in charge-order system

Tricritical point in charge-order systems and its criticality are studied for a microscopic model by using the mean-field approximation and exchange Monte Carlo method in the classical limit as well as by using the Hartree-Fock approximation for the quantum model. We study the extended Hubbard model and show that the tricritical point emerges as an endpoint of the first-order transition line between the disordered phase and the charge-ordered phase at finite temperatures. Strong divergences of several fluctuations at zero wavenumber are found and analyzed around the tricritical point. Especially, the charge susceptibility χ_c and the susceptibility of the next-nearest-neighbor correlation χ_R are shown to diverge and their critical exponents are derived to be the same as the criticality of the susceptibility of the double occupancy χ_D(0). The singularity of conductivity at the tricritical point is clarified. We show that the singularity of the conductivity σ is governed by that of the carrier density and is given as |σ - σ_c| ∼ |g - g_c|^pt (A log |g - g_c| + B), where g is the effective interaction of the Hubbard model, σ_c (g_c) represents the critical conductivity(interaction) and A and B are constants, respectively. Here, in the canonical ensemble, we obtain p_t = 2β_t = 1/2 at the tricritical point. We also show that pt changes into p′_t = 2β = 1 at the tricritical point in the grand-canonical ensemble when the tricritical point in the canonical ensemble is involved within the phase separation region. The results are compared with available experimental results of organic conductor (DI-DCNQI)₂Ag.