Optical evidence of competitive nature between charge-order and dimer-Mott insulators

A family of two-dimensional (2D) quarter-filled organic materials exhibits various intriguing electronic and magnetic states. These salts are essentially metallic due to the partially-filled band, however, several materials show the correlated insulating states such as charge-order and dimer-Mott insulators owing to strong correlation effects coupled with their unique internal degrees of freedom. In this talk, we show a competitive nature between charge-order and dimer-Mott insulating phases in the 2D quarter-filled organic salt $\beta$-(meso-DMBEDT-TTF)$_2$PF$_6$ through the optical conductivity measurements. This material has been known to exhibit charge ordering below $T_c$ = 70 K. We find optical evidence of a dimer-Mott insulating phase above $T_c$, indicating that the transition in this material is a transition from dimer-Mott to charge-order insulator. Below $T_c$, the optical peak feature of dimer-Mott insulator is significantly suppressed by the formation of charge order, implying a competition of these two insulators in this system. Furthermore our infrared imaging spectroscopy reveals a spatially inhomogeneous electronic state far below $T_c$, which is attributed to the competition between charge-order and dimer-Mott insulators.