High frequency electrical conductivity of Organic conductors

我们在1992年的实验中发现，当系统在135K温度下通过金属态和绝缘态的相界时，α-(BEDT-TTF)_2I_3的微波电导率表现出反常的各向异性。在晶轴a方向的电场中，电导率通过金属-绝缘体转变单调降低。而在沿 b 方向的场中，电导率在转变温度上方增加，然后在转变温度以下突然下降。我们今年进行的第一个实验是检查电导率的反常各向异性是否取决于我们测量电导率的频率。我们采用了蒙哥马利法，可以同时测量a方向和b方向的电导率。我们发现电导率的反常各向异性与频率无关。我们还发现135K时的相变并不像我们想象的那么简单。我们发现电导率的各向异性不仅在转变温度（135K）下发生变化，而且在低于转变温度约0.5K的温度下也发生变化。我们所做的第二个实验是压力下金属-绝缘体转变的调查。在我们1992年所做的研究中，我们发现在非常高的压力下，金属-绝缘体转变受到抑制，使得系统在低温下仍保持金属性。当我们对这样的系统施加磁场时，我们发现金属-绝缘体转变再次出现。我们将这种新相称为“磁场感应绝缘态”。我们已经对此类系统进行了霍尔效应测量。我们发现，在磁场感应绝缘体阶段，载流子密度变得非常低。关于场感绝缘体的第二个发现是恒定温度下的载流子密度强烈依赖于磁场的强度。

In the experiment, we did in 1992, we discovered that the microwave conductivity of alpha-(BEDT-TTF)_2I_3 shows an anomalous anisotropy when the system passes the phase boundary between the metallic state and the insulating state at 135K. In the electric field in the a-direction of the crystal axis, the conductivity decreases monotonically through the metal-insulator transition. While, in the field along the b-direction, the conductivity increases just above the transition and then decreases abruptly below the transition temperature.The first experiment, we performed this year is to check if the anomalous anisotropy of the conductivity depends on the frequency in which we have measure the conductivity. We have adopted the Montgomery method by which we can measure the conductivity along the a-direction and the b-direction, simultaneously.We found that the anomalous anisotropy of the conductivity is independent of the frequency. We also found that the phase transition at 135K is not so simple as we have expected. We have found that the anisotrpopy of the conductivity changes not only at the transition temperature(135K) but also at the temperature about 0.5K below the transition temperature.The second experiment we have done is the survey of the metal-insulator transition under the pressures. In the investigation we did in 1992, we found that in a very high pressure, the metal-insulator transition is suppressed so that the system remains metallic down to low temperatures.When we apply a magnetic field on such a system, we found that the metal-insulator transition appears again. We call this new phase as the "magnetic field induced insulating state".We have done the Hall effect measurement for such systems. We found that in the magnetic field induced insulator phase, the carrier density becomes very low. The second discovery on the field induced insulator is that the carrier density at a constant temperature depends strongly on the strength of the magnetic field.