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RIKEN Discovery Research Institute Condensed Molecular Materials Laboratory Chief Scientist: Reizo Kato |
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| A conductor that produces current from light | |
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The potential uses for organic conductors in electric devices, solar cells and optical electronics have captivated scientists and engineers for years. To date a large number of organic conductors have been developed in laboratories throughout the world with the hope of developing flatter, more flexible electronic devices. These synthetic materials have been studied for the conductive properties induced when pressure or a magnetic field is applied. Now, researchers in the Condensed Molecular Materials Laboratory at RIKEN Institute in Japan have identified a new photo-induced conductive state. BACKGROUND  -(BEDT-TTF)3I2 , is an organic conductor and a member of the (BEDT-TTF)2I3 family. It has a crystal structure with alternating conductive and insulating layers of BEDT-TTF molecules and I3 anions. The transport phenomena in -(BEDT-TTF)3I2 are a bit different than other salts in this family, some of which are superconducting, in that electron behaviour exhibits a phase transition from a metal to a non-conductive insulator at 135K. This transition originates from the charge ordering due to the strong electron correlation effect. However, when hydrostatic pressure is applied, the metal-insulator transition can be suppressed to and the metallic properties sustained with constant conductivity for a wide range of temperatures (300K to 1.5K). When light is applied to this crystal at low temperatures, the material’s state changes between a charge ordered insulating state and a metallic state that produces a persistent photo-current. RESEARCH RESULTS Two gold wires were attached to the crystal. In a low temperature environment, a pulsed voltage of about 20V was applied to the crystal. Without light, the resistivity was very high (4MW  cm) at 4K. However, when a pulsed laser was used to excite the conductor, a persistent current was produced that continued to increase as long as the electric field was applied. Analysis showed that there were two conducting states created by the irradiation of light. The first lasts for less than 120ns and the carriers have high mobility. The second, “anomalous photocurrent”, conducting state became stronger after the first one, was affected by the timing of the voltage pulse, and was sustained as long as the electric field was applied. The resistivity was less than 0.4W cm  for both conducting states with a 7-fold drop compared with that in the charge ordered insulating state.FUTURE WORK The laboratory will continue to study various features of this photo-induced phenomenon. They hope to learn how this photo-induced transition is generated from the charge ordered insulating state.
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