High frequency electrical conductivity of Organic conductors

在1992年的实验中，我们发现α-（BEDT-TTF）_2I_3体系在135 K下通过金属态和绝缘态的相界时，微波电导率表现出反常的各向异性。在沿晶轴a方向的电场中，电导率通过金属-绝缘体转变单调减小。而在沿着b方向的磁场中，电导率在转变温度以上增加，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异常各向异性是否取决于我们测量电导率的频率。我们采用蒙哥马利方法，同时测量了样品沿着a方向和b方向的电导率，发现电导率的反常各向异性与频率无关。我们还发现在135 K的相变并不像我们预期的那么简单。我们发现电导率的各向异性不仅在转变温度（135 K）发生变化，而且在低于转变温度0.5K左右的温度下也发生变化。在1992年的研究中，我们发现在很高的压力下，金属-绝缘体转变被抑制，以至于系统在低温下仍然保持金属状态，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。我们称这种新相为“磁场感应绝缘态”，并对这种体系进行了霍尔效应测量。我们发现，在磁场诱导的绝缘体阶段，载流子密度变得很低。关于场致绝缘体的第二个发现是，在恒定温度下，载流子密度强烈地依赖于磁场的强度。

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.