magnetic field induced metal-nonmetal transition in organic conductors

1994年进行了两次实验。在第一个实验中，我们研究了α-（BEDT-TSeF）_2I_3是否发生磁场诱导的金属-非金属转变，它的晶体结构和电子能量结构与α-（BEDT-TTF）_2I_3非常相似。实验结果表明，该材料与α-（BEDT-TTF）_2I_3一样，都发生了场致金属-非金属相变。然而，需要高出一个数量级的磁场强度才能诱导转变。这可能是由于这种材料的载流子迁移率低。下一个问题与金属区的输运性质有关。我们在金属-非金属转变温度以上的温度区同时测量了电导率和霍尔效应。我们发现这种材料的载流子密度随着温度的降低而降低，而载流子迁移率则随着温度的降低而增加。载流子密度和迁移率变化的影响正好抵消，从而得到与温度无关的电导率。这与我们在α-（BEDT-TSeF）_2I_3中观察到的相同。在第二个实验中，我们重新研究了α-（BEDT-TTF）_2I_3的低温霍尔效应。在低温下观察到的很大的霍尔角表明这种材料具有相当高的载流子迁移率。在本项目中，我们得出的结论是，在高压和低温下，α-（BEDT-TTF）_2I_3和α-（BEDT-TSeF）_2I_3发生了磁场诱导的金属-非金属转变。

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.