# Charge orders in organic charge-transfer salts

## TL;DR

This paper investigates charge and magnetic phases in an extended two-orbital Hubbard model on a triangular lattice, revealing polar charge-ordered states that could explain ferroelectricity in organic salts, independent of magnetism.

## Contribution

It introduces a detailed microscopic model showing how ferroelectricity can arise from charge order without magnetic order in organic charge-transfer salts.

## Key findings

- Polar charge-ordered insulating phases are stabilized in the strongly correlated limit.
- Ferroelectricity can occur without antiferromagnetic order.
- Metallic states with unique charge patterns emerge in weak interaction regimes.

## Abstract

Motivated by recent experimental suggestions of charge-order-driven ferroelectricity in organic charge-transfer salts, such as $\kappa$-(BEDT-TTF)$_2$Cu[N(CN)$_2$]Cl, we investigate magnetic and charge-ordered phases that emerge in an extended two-orbital Hubbard model on the anisotropic triangular lattice at $3/4$ filling. This model takes into account the presence of two organic BEDT-TTF molecules, which form a dimer on each site of the lattice, and includes short-range intramolecular and intermolecular interactions and hoppings. By using variational wave functions and quantum Monte Carlo techniques, we find two polar states with charge disproportionation inside the dimer, hinting to ferroelectricity. These charge-ordered insulating phases are stabilized in the strongly correlated limit and their actual charge pattern is determined by the relative strength of intradimer to interdimer couplings. Our results suggest that ferroelectricity is not driven by magnetism, since these polar phases can be stabilized also without antiferromagnetic order and provide a possible microscopic explanation of the experimental observations. In addition, a conventional dimer-Mott state (with uniform density and antiferromagnetic order) and a nonpolar charge-ordered state (with charge-rich and charge-poor dimers forming a checkerboard pattern) can be stabilized in the strong-coupling regime. Finally, when electron-electron interactions are weak, metallic states appear, with either uniform charge distribution or a peculiar $12$-site periodicity that generates honeycomb-like charge order.