Anomalous metal-insulator transition in /spl alpha/-(BEDT-TTF)/sub 2/I/sub 3/ under high pressure

Summary form only given. /spl alpha/-(BEDT-TTF)/sub 2/I/sub 3/ undergoes a transition from a two dimensional metal to an insulator at 135K. With the increasing pressure, the transition moves towards low temperatures. Under the pressure above 15kbar, the transition is fully suppressed and the system remains metallic down to the lowest temperatures. In spite of a rather long history of the investigation, the character and the mechanism of this transition are not understood yet. In this work, we have investigated it from the point of view of the conductivity anisotropy. Conductivity measurements using the Montgomery method were done for samples under the pressure up to 20kbar and in the magnetic field up to 9T. In the metallic region, the conductivity anisotropy was found to be small indicating that the system is two dimensional metal. Below the transition temperature, the anisotropy becomes much stronger. The following observation is interesting. 1) Under the intermediate pressures, we have obtained anomalous conductivity data which indicate that the metal-nonmetal transition occurs at different temperatures for two crystal axes in the conductive plane. 2) In the low temperature metallic state under high pressures, the application of the magnetic field induces highly resistive electronic state. In this region, the conductivity anisotropy changes strongly with the strength of the magnetic field.