Theoretical Aspects of Charge Ordering in Molecular Conductors

Abstract

Esta es la versión de autor del artículo publicado en: This is an author produced version of a paper published in: El acceso a la versión del editor puede requerir la suscripción del recurso Access to the published version may require subscription (Received February 4, 2008) Theoretical studies on charge ordering phenomena in quarter-filled molecular (organic) conductors are reviewed. Extended Hubbard models including not only the on-site but also the inter-site Coulomb repulsion are constructed in a straightforward way from the crystal structures, which serve for individual study on each material as well as for their systematic understandings. In general the inter-site Coulomb interaction stabilizes Wigner crystal-type charge ordered states, where the charge localizes in an arranged manner avoiding each other, and can drive the system insulating. The variety in the lattice structures, represented by anisotropic networks in not only the electron hopping but also in the inter-site Coulomb repulsion, brings about diverse problems in low-dimensional strongly correlated systems. Competitions and/or co-existences between the charge ordered state and other states are discussed, such as metal, superconductor, and the dimer-type Mott insulating state which is another typical insulating state in molecular conductors. Interplay with magnetism, e.g., antiferromagnetic state and spin gapped state for example due to the spin-Peierls transition, is considered as well. Distinct situations are pointed out: influences of the coupling to the lattice degree of freedom and effects of geometrical frustration which exists in many molecular crystals. Some related topics, such as charge order in transition metal oxides and its role in new molecular conductors, are briefly remarked.

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

“…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. Thus, in the present article, we mainly use the term 'charge correlation' when we need to include various possibilities as the origin of these phenomena.…”

“…This is due to a peak in −V (q) near q = (1/3, 1/3) u f as shown in figure 10. Namely, although χ 0 (q) is maximized around (2/3, 1/4) f = (11/24, −5/24) u f , the effect of −V (q) is strong in the denominator of the RPA charge susceptibility equation (2), thereby minimizing the denominator and maximizing χ c (q) at (1/3, 1/3) u f . In addition to the c-axis threefold peak, a subdominant peak also appears around the wave vector close to (1/2, 0) u f , i.e.…”

“…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.…”

“…[3] 2 is one of the most interesting families of organic compounds that exhibit short-or long-range charge correlations. The crystal structure of θ-(BEDT-TTF) 2 X , where BEDT-TTF = bis(ethylenedithio)tetrathiafulvalene, consists of a stack of alternating layers of BEDT-TTF and the anion X . In the BEDT-TTF layers, the molecules form an anisotropic triangular lattice, and the anion X controls the angle between the BEDT-TTF molecules, which in turn determines the band structure.…”

“…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]. A number of analytical and numerical studies have been performed using the extended Hubbard model (coupled with the lattice in some cases) described in the following section.…”