Ferroelectric charge order enhanced by magnetic frustration in dimer Mott insulators

Abstract

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 $\ensuremath{\kappa}\text{\ensuremath{-}}(\mathrm{BEDT}\text{\ensuremath{-}}\mathrm{TTF}){}_{2}X$ 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 $\ensuremath{\kappa}\text{\ensuremath{-}}{(\mathrm{BEDT}\ensuremath{-}\mathrm{TTF})}_{2}{\mathrm{Cu}}_{2}{(\mathrm{CN})}_{3}$.