Anomalous dielectric response in the dimer Mott insulator κ-(BEDT-TTF)2Cu2(CN)3

We have measured and analyzed the dielectric constant of the dimer Mott insulator kappa-(BEDT-TTF)(2)Cu-2(CN)(3), which is known as a playground for a spin-liquid state. Most unexpectedly, this particular organic salt has nontrivial charge degrees of freedom, being characterized by a relaxor-like dielectric relaxation below around 60 K. This is ascribed to the charge disproportionation within the dimer due to the intersite Coulomb repulsion. A possible microscopic model is suggested and discussed.