Material Properties Of Multi-Weyl Semimetals

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 ?-3(z-4/Q-1 and ?-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.