6 M ay 2 00 1 Far - infrared study of the insulator - metal transition in θ - ( BEDT - TTF )

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