Theoretical aspects of charge correlations in θ-(BEDT-TTF)₂X

Abstract

A review is given on the theoretical studies of charge correlations in θ-(BEDT-TTF) 2 X . Various studies show that within a purely electronic model on the θ-type lattice with on-site U and 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 ∼ V c , and V p > V c , respectively. In the realistic parameter regime of V p ∼ V c , there is competition between the c-axis three fold state and 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) 2 X may be more complicated than expected. Future problems regarding the competition between different types of charge correlations are discussed.

Infrared and Raman Studies of Charge Ordering in Organic Conductors, BEDT-TTF Salts with Quarter-Filled Bands

“…After the theoretical prediction of the CO ground state for α-(BEDT-TTF) 2 I 3 by Kino and Fukuyama [13], CO in two-dimensional organic compounds was suggested in α-(BEDT-TTF) 2 I 3 [14], and more clearly shown in θ-(BEDT-TTF) 2 RbZn(SCN) 4 [15]. Stimulated by the experimental findings of CO, theoretical studies were conducted on the role of intersite Coulomb interaction in CO [16][17][18], lattice distortion accompanied by CO [19][20][21], the relationship between CO fluctuation and superconductivity (SC) [22], and quantum criticality at the edge of CO [23]. Subsequently, several experimental and theoretical studies have been conducted [24][25][26][27].…”

Geometrical frustration effects on charge-driven quantum phase transitions

“…Several theoretical studies have aimed at reproducing the different CO patterns realized in this class of materials, either within the framework of the EHM itself or its generalizations, including longer ranged electronic interactions and various types of electron-lattice interactions. 20,21 In the present work we focus on the following open issues: (i) How does the strength of the local Coulomb correlations modify the nature of the metallic phase as well as its CO instabilities? (ii) What are the effects of geometrical frustration in the electron motion arising from the triangular molecular arrangement?…”

“…37 This issue is of particular importance to actual materials, as the relative values of the transfer integrals t c and t p can be tuned experimentally by applying pressure or by chemical substitution, which modifies the relative angles between neighboring molecules. 38,39 As a general observation, negative values of the ratio t c /t p produce the highest charge ordering temperatures, 20,38 while vanishing or positive values lead to glassy [X = CsCo(SCN) 4 , X = CsZn(SCN) 4 ] 40 or even superconducting (X = I 3 ) ground states. 20,38,41 From a more theoretical point of view, how the system evolves from a perfectly isotropic triangular lattice at t c = ±t p to a square lattice at t c = 0 remains an open issue.…”

“…32 More interesting from our perspective is the isotropically interacting case, V p = V c ≡ V . 20,28,29,[32][33][34][35][36] There, because of the frustration of intermolecular interactions induced by the triangular lattice geometry, an alternative charge-ordering pattern with threefold periodicity is favored with respect to the (degenerate) striped arrangements, illustrated in Fig. 1(b).…”

Theories on Frustrated Electrons in Two-Dimensional Organic Solids

“…Here, it is beyond our scope to discuss more than a recent fragment of the amounts of experiments given on these two families. We refer the interested reader to the recent articles [81,82].…”