Ferroelectric charge order enhanced by magnetic frustration in dimer Mott insulators

We investigate the effect of magnetic frustration on the dielectric properties of dimer Mott insulators. We adopt the two-dimensional quarter-filled extended Hubbard model for $\ensuremath{\kappa}\text{\ensuremath{-}}(\mathrm{BEDT}\text{\ensuremath{-}}\mathrm{TTF}){}₂X$ with an additional coupling with the dimer lattice motions, and numerically obtain the ground state. In the parameter range for strong magnetic frustration, an interdimer bond-length alternation mode that lifts the quasidegeneracy of classical spin configurations is strongly coupled to the electron system; this alternation mode occurs even with an extremely small electron-lattice coupling constant. Furthermore, the bond-length alternation induces in the dimers electric dipoles, which align along the same direction. A dimer Mott insulator phase with the ferroelectric charge order is obtained in the region. These results are consistent with the relaxorlike dielectric anomaly in $\ensuremath{\kappa}\text{\ensuremath{-}}{(\mathrm{BEDT}\ensuremath{-}\mathrm{TTF})}₂{\mathrm{Cu}}₂{(\mathrm{CN})}₃$.