Optical conductivity measurement of a dimer Mott-insulator to charge-order phase transition in a two-dimensional quarter-filled organic salt compound

We report a novel insulator-insulator transition arising from the internal charge degrees of freedom in the two-dimensional quarter-filled organic salt β-(meso-DMBEDT-TTF)₂PF₆. The optical conductivity spectra above T_c=70 K display a prominent feature of the dimer Mott insulator, characterized by a substantial growth of a dimer peak near 0.6 eV with decreasing temperature. The dimer peak growth is rapidly quenched as soon as a peak of the charge order appears below T_c, indicating a competition between the two insulating phases. Our infrared imaging spectroscopy has further revealed a spatially competitive electronic phase far below T _c, suggesting a nature of quantum phase transition driven by material-parameter variations.