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

Abstract

We have investigated the dielectric response in an antiferromagnetic dimer-Mott insulator $\ensuremath{\beta}$'-(BEDT-TTF)${}_{2}$ICl${}_{2}$ with square dimer lattice, compared to a spin liquid candidate $\ensuremath{\kappa}$-(BEDT-TTF)${}_{2}$Cu${}_{2}$(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.