Charge ordering in theta-(BEDT-TTF)_2 X materials

TL;DR

This paper theoretically studies charge ordering in theta-(BEDT-TTF)_2 X materials, revealing specific stripe patterns and bond dimerizations, and explains associated high-temperature and low-temperature transitions.

Contribution

It introduces a detailed theoretical model of charge and bond order patterns on an anisotropic triangular lattice specific to these materials.

Findings

Charge order forms horizontal stripe patterns with specific CO along different directions.

Bond dimerizations occur along all directions in the high spin state.

The theory accounts for observed high-temperature charge transition and low-temperature spin gap transition.

Abstract

We investigate theoretically charge ordered states on the anisotropic triangular lattice characteristic of the theta-(BEDT-TTF)_2 X materials. Using exact diagonalization studies, we establish that the charge order (CO) pattern corresponds to a ``horizontal'' stripe structure, with ...1100... CO along the two directions with larger electron hopping (p-directions), and ...1010... CO along the third direction (c-direction). The CO is accompanied by co-operative bond dimerizations along all three directions in the highest spin state. In the lowest spin state bonds along the p-directions are tetramerized. Our theory explains the occurence of a charge-induced high temperature transition as well as a spin gap transition at lower temperature.