Effect of orbital degeneracy on the heavy electron systems

在重电子系统中，被称为近藤温度的特征能量尺度很小，因此可以人工控制反铁磁相和超导相之间的相变。理解这类量子临界行为是当前凝聚态物理研究的一个主题。本研究的主要目的是阐明轨道简并在量子相变中的作用。由于电子-电子相互作用的重要影响可以用微扰理论系统地处理，因此本研究采用了j-j耦合方案。主要研究结果如下：(1)建立了基于j-j耦合方案的Ce115体系的有效哈密顿量，明确了其磁性和超导性。利用电子能带结构计算研究了新发现的超导体PuCoGa_5的电子态。(2)报道了过渡金属化合物LiV_2O_4的重电子行为。为了理解重质量的起源，考虑轨道简并，在焦绿石晶格上建立了扩展的Hubbard型模型。澄清了模型的自旋、轨道和自旋-轨道联合涨落模式的性质。(3)针对UGe_2中观察到的超导现象的起源，建立了UGe_2的模型，探讨了两个均匀铁磁相之间一阶相变的可能性。事实证明，由于轨道简并的影响，这种跃迁是可能的。

In the heavy electron systems, a characteristic energy scale called a Kondo temperature is small and thus it is possible to controll artificially phase transition between an antiferromagnetic phase and a superconductive phase. Understanding of this type of quantum critical behaviors is a main theme of the current research of condensed matter physics. The main objective of this study was to clarify the role of orbital degeneracy concerning the quantum phase transition. For this study we used a j-j coupling scheme, since the important effect of electron-electron interaction may be treated by a perturbation theory in a systematic way. The main results obtained through the present study may be summarized as follows :(1) We have developed an effective Hamiltonian for the Ce115 system based on the j-j coupling scheme and clarified its magnetic as well as superconductive properties. Electronic states of PuCoGa_5, which is a newly found superconductor, are studied by electronic band structure calculations.(2) Heavy electron behaviors are reported for LiV_2O_4 which is a transition metal compound. To understand the origin of the heavy mass, an extended Hubbard type model was considered on the pyrochlore lattice taking account of the orbital degeneracy. Nature of spin, orbital, and spin-orbital combined fluctuation modes of the model are clarified.(3) Concerned with the origin of the superconductivity observed in UGe_2, possibility of the first order phase transition between two uniform ferromagnetic phases was investigated for a model constructed for UGe_2. It has turned out that this type of transition is possible due to the effect of orbital degeneracy.