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

在DCNQI分子氢化引入的有效压力静水压下，(DME-DCNQI)_2Cu体系发生重入金属-绝缘体(金属-绝缘体-金属)相变，并伴有三重晶格畸变。在这个系统中，不仅电学性质，而且磁学性质在转变过程中都发生了巨大的变化。我们用热法、脉冲法和差热分析等方法研究了样品的比热和潜热，其中的“化学压力”可以通过选择性地在0.5K&lt；T&lt；100K的温度范围内用(DME-DCNQI)_2Cu来精确地控制。我们成功地用这些实验结果在较高温度和较低温度的M-I转变附近精确地测定了金属和绝缘态的熵随温度的变化。在绝缘体相中，电子处于局域化状态，对比热和比熵没有贡献。另一方面，出现了自旋自由的现象。这个自旋系统在6.8K经历了反铁磁相变。这是由于通过积分比热得到的自旋系统的熵。它在2-20K的温度范围内生长。在20K之前，自旋系统变得完全无序，并且由于这种自由而产生的熵具有恒定值。绝缘体相中自旋系统的熵和相变时的潜热给出了金属相中电子系统的信息。我们可以在这两个相中计算电子系统的自由能，重建出神秘而复杂的二甲醚-铜系统相图。我们阐明了金属相中的这些GammaT贡献与绝缘体相中的Cu^&lt；2+&gt；自旋之间的竞争驱动了重入M-I相变。

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.