【作者】 孙勇；

【导师】 王安民；

【作者基本信息】 中国科学技术大学 ， 理论物理， 2017， 博士

【摘要】 多手征荷外尔半金属可以看作动量空间中相同手征性外尔点的聚合态,它受点群对称性保护。其低能色散关系在对称轴方向是线性的,而在剩下的两个方向上是高幂次的。因为拓扑特性的改变、态密度的增加、各向异性的非线性色散关系和修改的自旋动量锁定等特性,多手征荷外尔半金属态会呈现出不同于其单手征荷对应的,独特的光学和输运性质。研究这些性质不仅有其本身的理论意义,还可以帮助到实验中对多手征荷外尔半金属态的探测。本论文主要内容如下:第一章中,我们首先对拓扑材料的兴起与发展做了一个简要的梳理和回顾,然后系统分析了狄拉克半金属和外尔半金属的模型及对称性,并通过对外尔点拓扑性质的讨论切入到本论文的主要研究对象:多手征荷外尔半金属。在本章的最后,我们针对外尔半金属的材料预言和实验方法做了简要的概述。第二章中,我们研究了双手征荷外尔半金属的磁光导特性。在存在磁场的情况下,磁光导率最显著的特征是一系列谐振峰的出现,而谐振峰的相应频率与磁场强度成线性关系。另外,由于各向异性色散关系的影响,磁光导率是各向异性的,三个纵向分量中有两个存在线性背景,剩下一个存在恒定背景。同时我们还分析了化学势、温度、杂质散射和空穴粒子对称破缺等对磁光导率的影响。此外,我们的推导方法可以一般推广到多手征荷情况,据此可定性的给出多手征荷外尔半金属中磁光导率的一些重要性质。第三章中,我们系统的研究了任意手征荷Q外尔半金属中,两磁性杂质之间的 Ruderman-Kittel-Kasuya-Yosida (RKKY)相互作用。我们发现 RKKY 相互作用在Q ≥ 2的情形是各向异性的,并且它对费米能和杂质间距的依赖由手征荷直接控制。在外尔点连线和垂直平面两个方向上,相互作用的衰减分别为z-2/Q-1和ρ-3 (内稟情形分别为z-4/Q-1和ρ-Q-4)。随着手征荷的增加,RKKY相互作用变得愈发长程和各向异性,这使得有趣的磁序更容易形成,从而可能在自旋电子学中具有重要的应用。由于衰减指数严格依赖于手征荷的大小,该研究也可以作为实验中判断外尔半金属手征荷数的一个重要佐证。第四章中,我们对本博士论文做了一个简短的总结和展望。更多还原

【Abstract】 Multi-Weyl semimetal (m-WSM) can be viewed as a k-space merger of Weyl points with same chirality. It is protected by a point-group symmetry. The low-energy dispersion is linear along the symmetry axis, whereas nonlinear in the remaining two dimensions. Because of the change of topology, increase of density of states, nonlinear anisotropic dispersion relations, and the modified spin-momentum lock condition, m-WSMs exhibit unique optical and transport properties compared to 1-WSMs. Studying these properties no only has its inherent theoretical value, but also can help us to pin down m-WSM states in future experiments.The structure of the thesis is as follows:Chapter 1, We start with a brief review of the rise and development of common topological materials. We analyzed the model and symmetry of Dirac and Weyl semimet-als, and through the discussion of Weyl node topology we cut to our main research fo-cus: m-WSMs. At the end of this chapter, we gave a brief overview of the material predictions and experimental methods for WSM candidates.Chapter 2, We investigated the magneto-optical response of double Weyl semimet-als whose energy dispersion is intrinsically anisotropic. We find that in the presence of a magnetic field, the most salient feature of the optical conductivity is a series of resonant peaks with the corresponding frequencies scaling linearly with the strength of the mag-netic field. Besides, due to the anisotropic dispersion relations, the optical conductivity is also found to be anisotropic, with two of the three longitudinal components residing at a linear background and the rest one at a constant background. The effects of chemi-cal potential, temperature, impurity scattering, and particle-hole symmetry breaking to the optical conductivity are also studied. In addition, our derivation can be generalized to m-WSMs, which qualitatively gives some important properties of magneto-optical conductivity in general w-WSMs.Chapter 3, We systematically investigated the Ruderman-Kittel-Kasuya-Yosida(RKKY) interaction between two magnetic impurities in Weyl semimetals with arbi-trary monopole charge Q. We find that the RKKY interaction becomes intrinsically anisotropic for Q ≥ 2, and its dependence on Fermi energy and impurity separation is directly controlled by the monopole charge. Along the line connecting the two Weyl nodes and in the transverse plane, the power law decay of RKKY interactions versus impurity separation goes as z-2/Q-1 and ρ-3(z-4/Q-1 and ρ-Q-4 for the intrinsic case).With the increase of Q, the RKKY interaction becomes more long-ranged and more anisotropic, which makes interesting magnetic orders easier to form and thus may have important applications in spintronics. As the power law decay strictly depends on Q,this research can be used to help determine the chiral charge of given m-WSMs in real experiments.Chapter 4, We conclude our thesis and make a brief prospect.更多还原