Transport property of an organic conductor α-(BEDT-TTF)2I3 under high pressure -: Discovery of a novel type of conductor

We have found quite a new type of transport phenomenon in an organic crystal alpha-(BEDT-TTF)(2)I-3 under high pressure. Essentially, it is a semimetal or a narrow gap semiconductor. But, the transport property is peculiar. The conductivity of this the carrier (hole) density and mobility change by a about 6 orders of magnitude. They change in a manner so that the effects just cancel out giving rise to the temperature independent conductivity. At low temperatures, the system is in a state with high mobility (3 x 10(5) cm(2)/V.sec) and low carrier density (5 x 10(15) cm(-3)). This state has been found to be very sensitive to magnetic field. We propose a mechanism for the extremely strong temperature dependence of the carrier density. It is based on the band structure and takes the thermal effect into consideration.