Abstract
Two-dimensional quarter-filled organic solids are a promising class of materials to realize the strongly correlated insulating states called dimer Mott insulator and charge order. In their conducting layer, the molecules form anisotropic triangular lattices, harboring geometrical frustration effect, which could give rise to many interesting states of matter in the two insulators and in the metals adjacent to them. This review is concerned with the theoretical studies on such issue over the past ten years, and provides the systematic understanding on exotic metals, dielectrics, and spin liquids, which are the consequences of the competing correlation and fluctuation under frustration.
Highlights
The study on “frustrated” systems dates back to 1950, when Wannier found an exact solution of the triangular lattice Ising antiferromagnet [1]
= bis(ethylenedithio)tetrathiafulvalene is abbreviated as ET, and X stands for the monovalent anions with closed shell), whose stacking layers provide a quasi-two-dimensional electronic system spanned on a lattice of ET molecules with three-quarter filling of electrons
They have brought into the world the two states of matter, charge order and dimer Mott insulator, representing the field [7]
Summary
The study on “frustrated” systems dates back to 1950, when Wannier found an exact solution of the triangular lattice Ising antiferromagnet [1]. The consequences of frustration have been studied in the realm of the localized spin systems, and far less attention was paid to those of the electronic systems until very recently. The latter relatively unstudied issue is what the present review focuses on. In the normal metallic state well described in reciprocal-space, the frustration often based on the real space geometry of interactions is not of direct importance For this reason, the insulating states driven by the strong correlation and the metallic states in their very vicinity are our main focus, wherein the frustration effect is amenable to laboratory studies. We pick up some of the recent experimental findings on molecule-based materials related to frustration in Section 5, and finalize the review at Section 6
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