Optical Conductivity Spectra of Charge-Crystal and Charge-Glass States in a Series of θ-Type BEDT-TTF Compounds

In the 3/4-filled band system θ-(BEDT-TTF)2X with a two-dimensional triangular lattice, charge ordering (CO) often occurs due to strong inter-site Coulomb repulsion. However, geometrical frustration of lattice can prohibit long-range CO, resulting in charge-glass state which configurations are randomly distributed. Here, we investigate states orthorhombic and monoclinic θ-type BEDT-TTF salts by measuring electrical resistivity optical conductivity spectra. We find substantial difference between systems. The an isotropic exhibits larger low-energy excitations than that one anisotropic becomes more metallic as isotropy increases. These results be understood different formation mechanisms two systems: system, originates from geometric equilateral leading 3-fold COs, whereas isosceles is described superposition insulating 2-fold stripe COs.