Theoretical Aspects of Charge Correlations in $θ$-(BEDT-TTF)$_2X$
A review is given on the theoretical studies of charge correlations in $θ$-(BEDT-TTF)$_2X$. Various studies show that within a purely electronic model on the $θ$-type lattice with the on-site $U$ and the nearest neighbor $V_p$ and $V_c$ interactions, the diagonal stripe, c-axis three-fold, and the vertical stripe charge correlations are favored in the regime $V_p< V_c$, $V_p\sim V_c$, and $V_p> V_c$, respectively. In the realistic parameter regime of $V_p\sim V_c$, there is a competition between c-axis three fold state and the diagonal stripe state. Since these are different from the experimentally observed a-axis three fold and the horizontal stripe charge correlations, additional effects have to be included in order to understand the experiments. The electron-lattice coupling, which tends to distort the lattice into the $θ_d$-type, is found to favor the horizontal stripe state, suggesting that the occurrence of this stripe ordering in the actual materials may not be of purely electronic origin. On the other hand, distant electron-electron interactions have to be considered in order to understand the a-axis three fold correlation, whose wave vector is close to the nesting vector of the Fermi surface. These studies seem to suggest that the minimal model to understand the charge correlation in $θ$-(BEDT-TTF)$_2X$ may be more complicated than expected. Future problems regarding the competition between different types of charge correlations are discussed.