PRESSURE CONTROL OF ORGANIC CONDUCTOR-DISCOVERY OF NEW TYPE OF NARROW GAP SEMICONDUCTORS

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Many organic conductors exhibit temperature-independent conductivity. Usually, they are considered to be dirty metals of which the electrical conduction is governed by impurity scattering. One of the main results of this investigation is the finding of pure organic conductors which shows temperature independent conductivity. In those conductors, both carrier density and mobility vary strongly with temperature. Nevertheless, we see temperature independent conduction because the effect of the change in the density and mobility just cancel out. Up to now, we know no other materials with such curious character both among organic and inorganic conductors. According to the density change at low temperatures, we found that they are semiconductors with extremely narrow energy gap.Organic conductors such as α-(BEDT-TTF)_2I_3, α-(BEDT-TSeF)_2I_3, O-(BEDT-TTF)_2I_3 and α-(BEDT-STF)_2I_3 belongs to this category of materials in case they are placed under high pressures.Another characteristic behavior of those materials is observed at low temperature under magnetic field. The property of carriers varies with increasing field. There are three steps of change. At low field limit, carriers have the mass of about 0.02m. where m is the free electron mass. In the intermediate field, the mass rises to a value close to m. In the high fields such as 10T, carrier mass looks to be much heavier.