Terahertz responses of the high-temperature metallic phase and photoinduced metallic state in the ferroelectric charge-ordered organic salt α- (ET ) 2 I3

The thermal and photoinduced phase transitions between the ferroelectric charge-ordered (CO) insulator state and the metallic state in the layered organic salt α- (ET) 2 I3 (ET: [bis(ethylenedithio)]-tetrathiafulvalene) were investigated using terahertz time-domain spectroscopy. Characteristic increases were observed in the optical conductivity and the dielectric constant upon increasing the temperature to just below the insulator-to-metal transition temperature (TCO), which reflects the partial or precursory collapse of the ferroelectric state and charge ordering. The excitation of this partially melted CO state near TCO led to the formation of a photoinduced macroscopic metallic state, whose conductivity appeared to be greater than that of a high-temperature metallic state. At T? TCO, a microscopic metallic state was formed within the rigid CO state.