Dielectric Properties of Organic Charge-Transfer Salts

The BEDT-TTF-based charge-transfer salts have attracted considerable attention due to their often intriguing dielectric properties. An example is $\kappa$-(BEDT-TTF)$_2$Cu[N(CN)$_2$]Cl. It was recently found to exhibit multiferroicity, for which a new electric-dipole driven mechanism was proposed [1]. The polar moment in this system was suggested to arise from the dimerization of the BEDT-TTF molecules, combined with charge order. Another interesting recent example is $\alpha$-(BEDT-TTF)$_2$I$_3$, which shows the signature of relaxor-ferroelectric behavior [2]. Here, we will present an overview of the dielectric properties of the above systems and provide new results on $\kappa$-(BEDT-TTF)$_2$Hg(SCN)$_2$Cl, which also seems to show relaxor-ferroelectric behavior in its charge-ordered state. In addition, we present measurements of $\delta$-(EDT-TTF-CONMe$_2$)$_2$Br. This compound lacks dimerization, but exhibits charge order already at room temperature. $[1]$ P. Lunkenheimer \textit{et al.}, Nat. Mater. \textbf{11}, 755 (2012). $[2]$ P. Lunkenheimer \textit{et al.}, Phys. Rev. B \textbf{91}, 245132 (2015).