Far-infrared study of the insulator-metal transition in θ-(BEDT-TTF)2RbZn(SCN)4 (BEDT-TTF ≡ bis (ethylene-dithio)tetrathiafulvalene)

We investigated the temperature-dependent infrared properties of the compound theta-(BEDT-TTF)(2)RbZn(SCN)(4). At temperatures above the metal-insulator transition, the optical conductivity remains finite in the low-energy region, but shows no conventional Drude feature typical of metals. Below the metal-insulator transition temperature, the low-energy spectral weight is significantly reduced. The opening of an optical gap at about 300 cm(-1) is observed clearly in polarization parallel to the donor stacking direction. Analysis of both the electronic and vibronic spectra suggests a redistribution of charges in BEDT-TTF molecules as temperature decreases across the metal-insulator transition, providing strong support for a charge-ordering state at low temperatures.