Relaxor ferroelectricity induced by electron correlations in a molecular dimer Mott insulator

We have investigated the dielectric response in an antiferromagnetic dimer-Mott insulator β'-(BEDT-TTF)₂ICl₂ with square dimer lattice, compared to a spin liquid candidate κ-(BEDT-TTF) ₂Cu₂(CN)₃. Temperature dependence of the dielectric constant shows a peak structure obeying Curie-Weiss law with a strong frequency dependence. We found an anisotropic glassy ferroelectricity by pyrocurrent measurements, which suggests the charge disproportionation resulting in an electric dipole in a dimer. Each ferroelectric and antiferromagnetic transition temperature is closely related to the antiferromagnetic interaction energy and a freezing temperature of dipole dynamics in a dimer, respectively. These correspondences suggest the possible charge-spin coupled degrees of freedom in the system.