Angle Resolved Photoemission Spectroscopy (ARPES) Studies On Topological Semimetals

Since the discovery of quantum spin Hall effect and topological phase in condensed-matter physics,the study of nontrivial topological phases has become a very important and hot topic in condensed-matter systems?CMSs?due to the novel properties and potential applications of the topological materials.Various novel topological phases have been theoretically predicted and experimentally confirmed in CMSs,such as topological insulators?TIs?,topological semimetals?TSMs?,topological superconductors?TSCs?and so on.TIs have been extensively studied during the past decade,and TSMs have been attracting significant interest in current condensed-matter physics research.TSMs are characterized by symmetry-protected band degeneracy in their electronic structure near the Fermi level?E_F?.The TSMs can be divided into two groups according to the dimensionality of band degeneracy in momentum space.The first group has zero-dimensional?0D?band degeneracy,the most famous examples are the Dirac and Weyl semimetals,in which two doubly-or singly-degenerate bands cross each other,forming the 4-fold Dirac points or 2-fold Weyl points.The fermionic excitations near Dirac and Weyl points are analogous to Dirac and Weyl fermions in high-energy physics,respectively.The second group has 4-or 2-fold band crossing along one-dimensional?1D?lines in momen-tum space.The materials with such nodal lines are called topological nodal-line semimetals?TNLSMs?.In contrast to the 0D nodal points,the 1D nodal lines form a much larger variety of possible configurations,such as nodal rings,nodal links,nodal chains,and nodal knots.As the most direct and powerful tool to probe the three-dimensional?3D?electronic structure,ARPES plays an important rule in the study of TSMs.In this dissertation,we use ARPES to systematically investigate the 3D electronic structures of several TSMs,and mainly made three achievements:1.We report the experimental realization of a Weyl semimetal?WSM?phase in TaAs by observing spin polarized Fermi arcs and the long-sought-after Weyl nodes.The projected locations at the nodes on the?001?surface match well to the Fermi arcs,providing undisputable experimental evidence for the existence of Weyl fermions in TaAs.2.We provide the first observation of crystal symmetry protected triply degenerate point in the electronic structure of symmorphic crystalline MoP.Quasiparticle excitations near a triply degenerate point are unconventional three-component fermions,beyond the conventional Dirac–Weyl–Majorana classification.We also observe pairs of Weyl points coexisting with the three-component fermions.This material thus represents a platform for studying the interplay between different types of fermions.Our experimental discovery opens up a way of exploring the new physics of unconventional fermions in CMSs.3.We clearly observe a nodal-chain structure formed by connecting nodal rings in the bulk electronic structure of TNLSM TiB₂.Furthermore,we provide experimental evidence of Dirac-cone surface states on the?001?cleaved surface,which are distinct from the usual drumhead SSs with a single flat band proposed in other two band TNLSMs.In solid state 'universe',due to the richness of the crystal symmetries of the 230 space groups,the diversity of crystal-symmetry protected band crossings is far more than we thought.And we strongly believe more and more types of TSMs with unconventional band crossings will be discovered in CMSs in the future.