Charge Ordering in Organic Conductors

Abstract

Present status of experiments on charge ordering (CO) phenomena observed in organic molecular conductors is reviewed. Experimental techniques to detect and analyze the CO, i.e., NMR, X-ray and Infrared/Raman spectroscopy, are described and the experimental results in typical materials exhibiting CO are summarized. (DI-DCNQI) 2 Ag, (TMTTF) 2 X, -(BEDT-TTF) 2 I 3 , -(BEDT-TTF) 2 RbZn(SCN) 4 and related materials are discussed. Charge disproportionation without long-range order observed in numbers of materials are also included.

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

“…Namely, in the x-ray scattering experiment, diffuse sheets (or planes) along the 2a + c and the 2a − c directions are observed at room temperature below, which is considered to be due to the effect of anions (the diffuse sheets are shown in the lower panels of figures 5 and 7) [9]. Actually, the wave vectors ( 2 3 , k, 1 3 ) (a-axis three fold) and (0, k, 1 2 ) (horizontal stripe) are on these diffuse sheets, and they 'grow' from these sheets as the temperature is lowered, suggesting the relevance of the anions. Kuroki considered the effect of these diffuse sheets phenomenologically by adding a term that enhances the charge susceptibility along the line 2a + c to the denominator of equation (2), and he found that the maximum value of the charge susceptibility saturates at low temperatures [42].…”

“…orderings and fluctuations have been one of the central issues in the physics of organic materials [1,2] 1 . θ-(BEDT-TTF) 2 X 1 The term 'charge ordering' is usually used in a sense that it is not related to the Fermi surface, while a 'charge density wave' occurs as a result of Fermi surface nesting.…”

“…orderings and fluctuations have been one of the central issues in the physics of organic materials [1,2] 1 . θ-(BEDT-TTF) 2 X 1 The term 'charge ordering' is usually used in a sense that it is not related to the Fermi surface, while a 'charge density wave' occurs as a result of Fermi surface nesting. As mentioned in the present article, the origin of the experimental observation regarding the charge correlation in θ -(BEDT-TTF) 2 X is still controversial, so that we cannot definitely state whether or not the Fermi surface is related to these phenomena at the present stage.…”

“…(0, k, 1 2 ) structure develops as the temperature is lowered, and the system becomes more insulating (figure 5). When an electric field is applied, the (0, k, 1 2 ) horizontal stripe ordering is degraded, resulting in a recovery of the metallic behavior and thus the nonlinear transport.…”

“…At around 200 K, the system undergoes a metal-insulator transition, accompanied by a structural phase transition to the so-called θ d phase [15], in which the unit cell is doubled in the c direction [6], [11][12][13][14], [16][17][18]. In the θ d phase, x-ray diffraction measurements have revealed that long-range horizontal stripe charge ordering with the modulation wave vector (0, 0, 1 2 ) takes place [4,5,11,12]. Theoretically, the origin of these charge correlations has been an issue of great interest [2].…”