New Development of the Quadrupole Interaction studied by Thulium Hexaboride (TmB_6)

本研究计划的目的是阐明近年来许多研究人员感兴趣的四极相互作用的本质。CeB_6是这一研究领域的热点材料之一，也是我们长期研究的对象。CeB_6只有一个4f电子，这是四足相互作用的起源。Tm的电子数为13-4f，则TmB_6与CeB_6对应，因为4f壳层充满14个电子。因此，TmB_6的四极性质是一个非常重要的研究课题。而TmB_6在常温常压下无相存在。我们找到了突破这个瓶颈的方法。在CeB_6中，La取代Ce并不改变其近藤型单重态离子的性质。相反，Ce_xLa_ B_6的取代研究中出现了许多新的信息<1-x>。我们用浮区法成功地生长出了高质量的Tm_xLa_<1-x>B_6、Tm_xYb_<1-x>B_6单晶。通过电阻率和磁化强度的测量，Tm_xYb_<1-x>B_6和Tm_xLa_ B_6分别在7.5K和4.5K发生相变<1-x>。很难考虑这些相变的磁序，因为Tm仅占25%。相变被认为是由于四极相互作用。因此，现在需要进行比热、中子散射等许多其他测量。

The purpose of this research project is to elucidate the essential nature of quadrupole interactions in which many researchers are interested in recently. One of the interested materials in this research area is CeB_6 that is investigated by us for long years. CeB_6 has only one 4f electron that is the origin of the quadrupode interactions. On the other hand, Tm has 13-4f electrons, then TmB_6 is counterpart to CeB_6 because 4f shell is full for 14 electrons. Therefore, the quadrupole nature of TmB_6 is very important research subject. However, TmB_6 is not found its phase in room temperature and in normal pressure. We found a method to break the bottleneck of this problem. In CeB_6, the Kondo-type singlet-ion nature is not changed by substitution of Ce by La. On the contrary, many new informations appeared in the investigations of Ce_xLa_<1-x>B_6 by the substitution. We succeeded to grow good quality of single crystals of Tm_xLa_<1-x>B_6, Tm_xYb_<1-x>B_6 by floating zone method. They shows phase transition at 7.5 K for Tm_xYb_<1-x>B_6 and at 4.5 K for Tm_xLa_<1-x>B_6 by the electrical resistivity and the magnetization measurements. It is hard to consider the magnetic order at these phase transitions because Tm is contained by only 25%. The phase transitions are thought to be attributed to quadrupole interactions. Therefore, many other measurements of specific heat, neutron scattering, etc. are needed now.