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We have found quite a new type of transport phenomenon in an organic crystal α-(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 ×10 5 cm 2 /V·sec) and low carrier density (5 ×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.
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