Photoemission spectroscopy and first-principles studies of two-dimensional Dirac and Weyl materials
Abstract
Two-dimensional (2D) materials have attracted tremendous research interest since the breakthrough of graphene. As the nature of the low dimension, 2D materials are thin, flexible, and easy for fabrication. More interestingly, it emerges a lot of novel and unique properties such as 2D magnetic and 2D superconductivity, which attracts a huge amount of research and opens a door for the next generation of electronics and spintronics. Dirac and Weyl semimetals are topological matter with gapless electronic excitations protected by topology and symmetry. Their nontrivial electron band topology leads to protected surface or edge states and novel responses to applied electric and magnetic fields. Those states are described by the models of relativistic chiral fermions as quasi-particles, which raise a platform to study particle physics in condensed matter physics and lead to more possibilities for spintronics and quantum calculation. The combination of low-dimension and novel band topology would emerge a lot of novel physics properties and applications, but due to the lack of a dimensional constraint, 2D Dirac and Weyl systems are rare. In this research, we use the symmetry analysis to find the possible candidate of 2D Dirac and Weyl materials, the molecular beam epitaxy (MBE), and the in-situ scanning tunning microscopy (STM) are used to experimentally synthesize and adjust the 2D thin films, the combination of angle-resolved photoemission spectroscopy (ARPES) and first-principles calculation is used to examine the electronic properties of 2D Dirac and Weyl matter. Specifically, there are three different 2D Dirac or Weyl systems discussed in this thesis. The first one focuses on black phosphorous structures, and symmetryenforced Dirac states, unpinned 2D Dirac states, and 2D Weyl fermions are discussed in this system. The second one studies the interaction between Dirac states based on the Moiré lattice. The third one is mainly about the 2D magnetic material and its heterostructures which are the candidates of the 2D magnetic Weyl matters.
Degree
Ph. D.