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We theoretically study the role of charge fluctuation in a molecular family θ-(BEDT-TTF)₂X, which situates just on the verge of the charge order phase. In the compounds which is just near the phase boundary, exotic behaviors such as highly non-linear conduction [1] and charge-cluster glass state [2] are observed. In this system, existence of geometrical frustration in the charge arrangement has been pointed out [3], while long-period charge fluctuation is often observed by x-ray scattering technique. Here we numerically investigate interacting fermion models on the anisotropic triangular lattice especially focusing on the role of long-range Coulomb repulsion beyond nearest-neighbors [4]. We find that it stabilizes the long-period charge order/fluctuating state over the previously-known commensurate charge ordered states. Implications to the experiments will be discussed.

[1] F. Sawano et al, Nature 437 (2005) 522.
[2] F. Kagawa et al, Nature Physics 9 (2013) 419.
[3] J. Merino, H. Seo, M. Ogata, Phys. Rev. B 71 (2005) 125111; H. Seo, K. Tsutsui, M. Ogata, and J. Merino, J. Phys. Soc. Jpn. 75 (2006) 114707.
[4] M. Naka and H. Seo, in preparation.