Purely electronic terahertz polarization in dimer Mott insulators

Abstract

We theoretically describe purely electronic polarization modes in terahertz frequency region in dimer Mott insulators $\ensuremath{\kappa}\text{\ensuremath{-}}{(\text{BEDT-TTF})}_{2}\text{X}$. The unusual low-frequency modes arise from the coupling between the oscillation of intradimer electric dipole moments and that of alternating interdimer bond orders. These collective modes play an important role in the dynamical dielectric properties of the dimer Mott insulators. Near the phase boundary of the dimer Mott transition, the ferroelectric ground state is realized by introducing electron-lattice coupling as a result of the softening of one of these collective modes.