Tricritical Behavior in Charge-Order System

TL;DR

This study investigates the tricritical behavior in charge-order systems using mean-field, Monte Carlo, and Hartree-Fock methods, revealing divergence of fluctuations, critical exponents, and conductivity singularities near the tricritical point.

Contribution

It provides a detailed analysis of the tricritical point in charge-order systems, including critical exponents and conductivity behavior, using both classical and quantum models.

Findings

Divergence of charge susceptibility and next-nearest-neighbor correlation susceptibility at the tricritical point.

Derived critical exponents for susceptibilities and conductivity singularity.

Demonstrated the change in critical exponent p_t between canonical and grand-canonical ensembles.

Abstract

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 chi_c and the susceptibility of the next-nearest-neighbor correlation chi_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 chi_D0. The singularity of conductivity at the tricritical point is clarified. We show that the singularity of the conductivity sigma is governed by that of the carrier density and is given as |sigma-sigma_c|=|g-g_c|^{p_t}Alog{|g-g_{c}|}+B), where g is the effective interaction of the Hubbard model, sigma_c g_c represents the critical conductivity(interaction) and A and B are constants, respectively. Here, in the canonical ensemble, we obtain p_t=2beta_t=1/2 at the tricritical point. We also show that p_t changes into p_{t}'=2beta=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.