Thermal Study of Reentrant Metal-Insulator Transition of DCNQI-Cu System

在DCNQI分子氘化引入的有效压力的静水压力下，（DMe-DCNQI）_2Cu体系表现出金属-绝缘子的重入式转变（金属-绝缘子-金属），并伴有三重晶格畸变。在这个系统中，不仅电学性质，而且磁学性质在整个转变过程中都发生了巨大的变化。本文研究了选择性氘化（DMe-DCNQI）_2Cu在0.5K<T<100K温度范围内的比热和潜热，利用热、脉冲和差热分析方法对样品的“化学压力”进行了精确控制。在高温和低温M-I跃迁附近的实验结果，我们成功地精确地确定了金属态和绝缘体态熵的温度依赖性。在绝缘体阶段，电子是局域化的，没有γ - mat对比热和熵的贡献。另一方面，出现了Cu^<2+>自旋的自由。该自旋体系在6.8K时发生了反铁磁相变。由自旋系统产生的熵，由比热积分得到。生长在2-20K的温度范围内。在20K以上，自旋系统变得完全随机，由于这种自由，熵取恒定值。绝缘体相自旋系统的熵和跃迁时的潜热给出了金属相电子系统熵的信息。我们可以计算出两相电子系统的自由能，并重建了DMe-Cu系统神秘而复杂的相图。我们澄清了金属相中的gammaT贡献和绝缘体相中的Cu^<2+>自旋自由之间的竞争驱动了可重入M-I跃迁。

Under hydrostatic pressure of effective pressure introduced by deuteration of DCNQI molecules, (DMe-DCNQI)_2Cu system exhibits the reentrant metal-insulator transition (metal-insulator-metal) accompanied by three fold lattice distortion. In this system, not only electrical properties but also magnetic properties drastically change across the transition. We have been studying the specific heat and latent heat on samples, in which "chemical pressure" can be precisely controlled by using selectively deuterated (DMe-DCNQI)_2Cu in the temperature region (0.5K<T<100K) using the thermal, pulse and DTA methods.We have succeeded in precisely determining temperature dependence of entropy of the metallic and insulator states by the these experimental results in the vicinity of the higher temperature and lower temperature M-I transitions. In the insulator phase, electrons are localized and there is no gammaT contribution to the specific heat and entropy. On the other hand, there appears the freedom of Cu^<2+> spin. This spin system undergoes the antiferromagnetic phase transition at 6.8K.The entropy due to the spin system obtained by integrating the specific heat. It grows in the temperature range of 2-20K.Above 20K the spin system becomes completely random and entropy due to this freedom take a constant value. The entropy of the spin system in the insulator phase and the latent heat at the transition gives the information of the entropy of electron system in the metallic phase. We can calculate the free-energy of the electronic system in both phases and reconstruct the mysterious and complicated phase diagram of DMe-Cu system. We clarified that the competition between these gammaT contribution in metallic phase and freedom of Cu^<2+> spins in insulator phase drives the reentrant M-I transition.