Electronic ferroelectricity in molecular organic crystals.

Electronic ferroelectricity in molecular organic crystals is reviewed from a theoretical perspective. In particular, we focus on the charge-driven-type electronic ferroelectricity where electronic charge order without inversion symmetry induces a spontaneous electric polarization in quarter-filling systems. Two necessary conditions to realize this type of ferroelectricity are the dimer-type lattice structure and alternate electronic charge alignments. Some prototypical organic compounds are introduced. In particular, κ-type BEDT-TTF organic salts, which are termed the dimer-Mott insulating systems, are focused on. Recent developments in the theoretical researches for dielectric and magnetodielectric properties, a collective dipole excitation and a possibility of superconductivity induced by polar charge fluctuation are reviewed. Some perspectives are presented.