Charge Ordering in Organic ET Compounds

TL;DR

This paper theoretically investigates charge ordering in organic ET compounds, revealing stripe-type charge patterns stabilized by Coulomb interactions and comparing results with experimental data to understand different insulating phases.

Contribution

It provides a unified theoretical framework for understanding charge ordering in various ET compound structures, incorporating quantum fluctuations and comparing with experimental observations.

Findings

Stripe-type charge ordered states are stabilized by Coulomb interactions.

The spatial pattern of charge stripes depends on model anisotropy.

Charge patterns in insulating phases are deduced from theoretical models.

Abstract

The charge ordering phenomena in quasi two-dimensional 1/4-filled organic compounds (ET)_2X (ET=BEDT-TTF) are investigated theoretically for the $\theta$ and $\alpha$-type structures, based on the Hartree approximation for the extended Hubbard models with both on-site and intersite Coulomb interactions. It is found that charge ordered states of stripe-type are stabilized for the relevant values of Coulomb energies, while the spatial pattern of the stripes sensitively depends on the anisotropy of the models. By comparing the results of calculations with the experimental facts, where the effects of quantum fluctuation is incorporated by mapping the stripe-type charge ordered states to the S=1/2 Heisenberg Hamiltonians, the actual charge patterns in the insulating phases of $\theta$-(ET)_2MM'(SCN)_4 and $\alpha$-(ET)_2I_3 are deduced. Furthermore, to obtain a unified view among the $\theta$, $\alpha$ and $\kappa$-(ET)_2X families, the stability of the charge ordered state in competition with the dimeric antiferromagnetic state viewed as the Mott insulating state, which is typically realized in $\kappa$-type compounds, and with the paramagnetic metallic state, is also pursued by extracting essential parameters.