Evidence for electronically-driven ferroelectricity in the family of strongly correlated dimerized BEDT-TTF molecular conductors

TL;DR

This study provides evidence of electronically-driven ferroelectricity in a dimerized BEDT-TTF molecular conductor, highlighting the role of intra-dimer charge degrees of freedom in correlated systems and their potential to create novel multiferroic states.

Contribution

It demonstrates charge-order-driven ferroelectricity in a moderately dimerized BEDT-TTF system, bridging the understanding between different dimerization regimes.

Findings

Evidence of electronic ferroelectricity driven by charge order within dimers.

The material exhibits a moderate dimerization strength, bridging different electronic regimes.

Intra-dimer charge degrees of freedom are crucial for emergent correlated states.

Abstract

By applying measurements of the dielectric constants and relative length changes to the dimerized molecular conductor $\kappa$-(BEDT-TTF)$_2$Hg(SCN)$_2$Cl, we provide evidence for order-disorder type electronic ferroelectricity which is driven by charge order within the (BEDT-TTF)$_2$ dimers and stabilized by a coupling to the anions. According to our density functional theory calculations, this material is characterized by a moderate strength of dimerization. This system thus bridges the gap between strongly dimerized materials, often approximated as dimer-Mott systems at 1/2 filling, and non- or weakly dimerized systems at 1/4 filling exhibiting charge order. Our results indicate that intra-dimer charge degrees of freedom are of particular importance in correlated $\kappa$-(BEDT-TTF)$_2$X salts and can create novel states, such as electronically-driven multiferroicity or charge-order-induced quasi-1D spin liquids.