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

The thermal and photoinduced phase transitions between the ferroelectric charge-ordered (CO) insulator state and the metallic state in the layered organic salt alpha-(ET)(2)I-3 (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 (T-CO), which reflects the partial or precursory collapse of the ferroelectric state and charge ordering. The excitation of this partially melted CO state near T-CO 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 << T-CO, a microscopic metallic state was formed within the rigid CO state.