Multiferroicity in the Mott Insulating Charge-Transfer Salt $\kappa-({\rm BEDT-TTF})_{2}{\rm Cu}[{\rm N}({\rm CN})_{2}]{\rm Cl}$

The recently proposed multiferroic state of the charge-transfer salt κ-(BEDT-TTF)
₂
Cu[N(CN)
₂
]Cl has been studied by dc-conductivity, magnetic susceptibility and measurements of the dielectric constant on various differently prepared single crystals. In the majority of crystals, we confirm the existence of an order-disorder-type ferroelectric state, which coincides with antiferromagnetic order. This phenomenology rules out scenarios which consider an inhomogeneous, short-range ordered ferroelectric state. Measurements of the dielectric constant and the magnetic susceptibility on the same crystals reveal that both transitions lie very close to each other or even collapse, indicating that both types of order are intimately coupled to each other. We address issues of the frequency dependence of the dielectric constant ε' and the dielectric loss ε" and discuss sample-to-sample variations.