### 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 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)_{2}Ag.

Original language | English |
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Article number | 064705 |

Journal | Journal of the Physical Society of Japan |

Volume | 75 |

Issue number | 6 |

DOIs | |

Publication status | Published - 2006 Jun 1 |

Externally published | Yes |

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### Keywords

- Charge order
- Charge susceptibility
- Doublon susceptibility
- Extended Hubbard model
- Organic conductor
- Phase separation
- Tricritical point

### ASJC Scopus subject areas

- Physics and Astronomy(all)

### Cite this

*Journal of the Physical Society of Japan*,

*75*(6), [064705]. https://doi.org/10.1143/JPSJ.75.064705