magnetic field induced metal-nonmetal transition in organic conductors

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Two experiments were done in 1994. In the first one we examined if the magnetic field induced metal-nonmetal transition occurs in alpha- (BEDT-TSeF) _2I_3 of which the crystal structure and the electronic energy structure is very similar to those of alpha- (BEDT-TTF) _2I_3. The experimental results indicate that the field induced metal-nonmetal transition takes place in this materials as well as in alpha- (BEDT-TTF) _2I_3. However, the magnetic field strength about one order of magnitude higher was necessary to induce the transition. This may be due to low carrier mobility in this material.The next problem concems with the transport properties in the metallic region. We measured the conductivity and the Hall effect simultaneously in the temperature region above the metal-nonmetal transition. We found that the carrier density of this material decreases with the decreasing temperature, while the carrier mobility increases. The effect of the change in the carrier density and the mobility just canceled out to give the temperature independent conductivity. This is just the same we observed in alpha- (BEDT-TSeF) _2I_3.In the second experiment, we reexamined the low temperature Hall effect in alpha-(BEDT-TTF) _2I_3. A very large Hall angle observed at low temperatures suggests the fairly high carrier mobility in this materialThroughout the present project, we come to the conclusion that the magnetic field induced metal-nonmetal transition takes place in alpha- (BEDT-TTF) _2I_3 and alpha- (BEDT-TSeF) _2I_3 placed under high pressures and at low temperatures.