Ordering and Dynamics of Multipolar Moments

本课题的主要研究结果如下：电荷有序可以看作是零阶多极有序。从这一观点出发，我们研究了Yb4As3的电荷有序，其中形成了Yb^&lt；3+&gt；离子的一维链。我们仔细推导了一个描述一维链的低能有效哈密顿量。因此，由于这种材料的独特结构，我们成功地证明了Dzyloshinksy-Moriya项的存在。此外，还对链中激发谱的性质进行了详细的研究，并与现有的中子散射实验进行了比较。本项目中最重要的问题是研究CeB6四极序相的多极动力学。我们以前研究过四极序的平衡性质。在这个项目下，我们扩展了我们的动力学研究，可以通过施加磁场和使用中子散射来实验探索。事实上，A·布韦在10年前的博士论文中就做过这样的实验。由于当时缺乏任何可靠的理论，他的实验结果没有得到认真的分析。根据我们的理论，场诱导的交错八极矩在CeB6中是重要的。我们将我们的理论与布韦的实验进行了比较，并取得了很好的一致性。作为第三个主题，我们研究了具有Γ8晶场基态的一般材料。它们包括TmTe和NPO2。在这样的系统中，四极以外的多极时刻至关重要。我们利用晶体结构的对称性研究了多极矩之间的相互作用。此外，我们还研究了TmTe中的场致反铁磁矩，并解释了实验中观察到的一些特征。

The main results in this project are as follows.The charge ordering may be viewed as the zeroth-order multipolar ordering. From this viewpoint we studied the charge ordering of Yb4As3, in which one-dimensional chains of Yb^<3+> ions are formed. We have carefully derived a low-energy effective Hamiltonian to describe the one-dimensional chain. As a result, we have succeeded in showing the presence of Dzyloshinksy-Moriya term due to the unique structure of this material. Moreover, the nature of the excitation spectrum in the chain has been studied in detail and compared with available neutron-scattering experiments.The most important problem in this project was to study multipolar dynamics in quadrupolar order phase of CeB6. The equilibrium properties of the quadrupolar order were studied by us before. Under this project we extended our study for dynamics, which can be explored experimentally by applying the magnetic field and by using neutron scattering. In fact A. Bouvet carried out such an experiment 10 years ago in his Ph. D. thesis. Because of lack of any reliable theory at that time his experimental results have not been seriously analyzed. According to our theory field-induced staggered octupolar moments are important in CeB6. We have compared our theory with Bouvet's experiment and have achieved a good agreement.As a third topic, we studied in general materials having Γ8 crystal-field ground state. They include TmTe and NpO2. In such systems multipolar moments beyond quadrupolar ones are crucial. We have studied interactions between multipolar moments using the symmetry of the crystal structure. In addition we have examined field-induced antiferromagnetic moments in TmTe and have explained some of the features observed in experiment.