Charge ordering and antiferromagnetic exchange in layered molecular crystals of theθtype

Abstract

We consider the electronic properties of layered molecular crystals of the type -D 2 A where A is an anion and D is a donor molecule such as bis-͑ethylenedithia-tetrathiafulvalene͒ ͑BEDT-TTF͒, which is arranged in the -type pattern within the layers. We argue that the simplest strongly correlated electron model that can describe the rich phase diagram of these materials is the extended Hubbard model on the square lattice at one-quarter filling. In the limit where the Coulomb repulsion on a single site is large, the nearest-neighbor Coulomb repulsion V plays a crucial role. When V is much larger than the intermolecular hopping integral t the ground state is an insulator with charge ordering. In this phase antiferromagnetism arises due to a novel fourth-order superexchange process around a plaquette on the square lattice. We argue that the charge ordered phase is destroyed below a critical nonzero value V, of the order of t. Slave-boson theory is used to explicitly demonstrate this for the SU(N) generalization of the model, in the large-N limit. We also discuss the relevance of the model to the all-organic family ␤Љ-(BEDT-TTF) 2 SF 5 Y SO 3 where Y ϭCH 2 CF 2 , CH 2 , CHF.

Study of the triangular latticetVmodel nearx=13

“…Our meanfield treatment follows closely McKenzie et al 6 The general formalism is the same except for an arithmetic difference in Eq. (14) and some minor differences in the analysis.…”

“…At the end of the calculation, we will put N =2, S = 1 2 , and our specific choices when defining H fb are such that this will reproduce the slave boson Hamiltonian for the spin-1 2 model. Proceeding as in McKenzie et al 6 and Kotliar and Liu, 5 we write the path integral in the radial gauge 17…”

“…Kotliar and Liu 5 studied the infinite-U Hubbard model on a two-dimensional square lattice in a systematic large-N treatment and found that the residual interaction from the nodouble-occupancy constraint favors d-wave superconductivity close to half filling. More recently, McKenzie et al 6 applied the analysis of Kotliar and Liu to the extended Hubbard model on the square lattice at a quarter filling and found a transition to the ͱ 2 ϫ ͱ 2 CDW, which is driven by the nearest-neighbor repulsion. Merino and McKenzie 7 studied superconducting instabilities of the Fermi liquid near this transition.…”

Spin exchange and superconductivity in at−J′−Vmodel for two-dimensional quarter-filled systems

“…However, well in the charge ordered insulating phase it is known that the spins order antiferromagnetically due to the presence of a next-nearest-neighbor spin exchange interaction. 2 This spin interaction results from a "ring" exchange process appearing at fourth order in t and acts along the diagonals of the square lattice reading: JЈ =4t 4 /9V 3 in the U → ϱ and V ӷ t limits. Exact diagonalization on 16-site clusters indicate that the ͑ /2, /2͒ antiferromagnetic spin arrangement follows closely the ͑ , ͒ checkerboard arrangement of the charge 10 as V / t is increased from the metal to the charge ordered phase.…”

“…1,2 They are examples of strongly correlated electron systems for which their electronic states are theoretically described by a two-dimensional ͑2D͒ extended Hubbard model at 3 / 4 filling of electrons ͑1 / 4 filling of holes͒ for the HOMO of the BEDT-TTF molecules. 3 The nearest-neighbor intermolecular Coulomb interaction V is a crucial ingredient as, at one-quarter filling, the on site Coulomb repulsion U by itself cannot describe charge ordering phenomena.…”