Effects of Electron-Lattice Coupling on Charge Order in \theta-(ET)2X

TL;DR

This study investigates how electron-lattice interactions influence charge order patterns in organic conductors, revealing that lattice distortions stabilize experimentally observed charge arrangements and induce phase transitions.

Contribution

It demonstrates that lattice coupling stabilizes horizontal charge order and explains phase transitions in heta-(ET)2X using an extended Hubbard model with Peierls-type interactions.

Findings

Lattice distortion stabilizes horizontal charge order.

Anisotropic Coulomb interactions favor horizontal order.

First-order transition from metallic to insulating state.

Abstract

Charge ordering accompanied by lattice distortion in quasi-two dimensional organic conductors \theta-(ET)2X (ET=BEDT-TTF) is studied by using an extended Hubbard model with Peierls-type electron-lattice couplings within the Hartree-Fock approximation. It is found that the horizontal-stripe charge-ordered state, which is experimentally observed in \theta-(ET)2RbZn(SCN)4, is stabilized by the self-consistently determined lattice distortion. Furthermore, in the presence of the anisotropy in nearest-neighbor Coulomb interactions Vij, the horizontal charge order becomes more stable than any other charge patterns such as diagonal, vertical and 3-fold-type states. At finite temperatures, we compare the free energies of various charge-ordered states and find a first-order transition from a metallic state with 3-fold charge order to the insulating state with the horizontal charge order. The role of lattice degrees of freedom in the realization of the horizontal charge order and the relevance to experiments on \theta-(ET)2X are discussed.