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

在由DCNQI分子氘代引入的有效压力的静水压力下，（DMe-DCNQI）_2Cu体系发生了金属-绝缘体（金属-绝缘体-金属）的凹入转变，并伴随着三重晶格畸变。在这个系统中，不仅电性能，而且磁性能在整个过渡期间急剧变化。我们一直在研究样品的比热和潜热，其中“化学压力”可以通过使用选择性氘代（DMe-DCNQI）_2Cu在温区（0.5K<T<100 K）使用热，利用这些实验结果，我们成功地精确地测定了金属态和绝缘体态的熵随温度的变化关系。高温和低温M-I转变。在绝缘体相中，电子被局域化，并且对比热和熵没有γ T贡献。另一方面，Cu^<2+>的自旋也表现出自由度.该自旋系统在6.8K发生反铁磁相变，由比热积分得到自旋系统的熵。它在2- 20 K的温度范围内生长，在20 K以上，自旋系统变得完全随机，由于这种自由而产生的熵取恒定值。绝缘相自旋系统的熵和相变潜热给出了金属相电子系统熵的信息。我们可以计算出DMe-Cu体系在两相中的自由能，重建DMe-Cu体系神秘而复杂的相图。我们阐明了金属相中的这些γ T贡献与绝缘体相中的Cu^<2+>自旋自由度之间的竞争驱动了重入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.