Angle-resolved Photoemission Spectroscopy(ARPES) Studies On The Electronic Structure Of Several Novel Topological Systems

Since the discovery of the quantum Hall effect and topologically nontrivial states in condensed-matter physics,the topological electronic structure of materials have received extensive research attention both theoretically and experimentally due to the novel physical properties and potential applications of topological materials.Exploring new topological phases and novel topological electronic structures has become a very important research topic in condensed-matter systems.As the most direct and powerful tool to probe the energy band structure of materials,Angle-resolved Photoemission Spectroscopy（ARPES）plays an important role in the study of topological materials.ARPES can afford us various information including the energy dispersion,Fermion surface,spin and orbitals of electrons.The surface-sensitive properties of ARPES are very suitable for the study of the electronic structure of two-dimensional thin film systems.In this dissertation,we prepare various two-dimensional and three-dimensional topological systems and investigate their topological properties by ARPES.1.We achieve the 2D Su-Schrieffer-Heeger（SSH）model in condensed-matter materials.The Si-3×3 structure grown on Ag（001）is invstigated by ARPES combing first-principle calculations.We demonstrate that the Si lattice grown on Ag（001）hosts gapped Dirac cones at the Brillouin zone corners.Our tight-binding analysis reveals that the Dirac bands can be described by a 2D SSH model with anisotropic polarizations.The gap of the Dirac cone is driven by alternative hopping amplitudes in one direction and staggered potential energies in the other one and hosts topological nontrivial edge states.Our results establish an ideal platform to explore the rich physical properties of the 2D SSH model.2.We demonstrate that monolayer Al B₂ can be synthesized on Al（111）via molecular beam epitaxy.Our theoretical calculations revealed that the monolayer Al B₂hosts several Dirac cones along theΓ-M andΓ-K directions;these Dirac cones are protected by crystal symmetries and are thus resistant to external perturbations.The extraordinary electronic structure of the monolayer Al B₂ was confirmed via ARPES measurements.Our first-principle calculations also show the persistence of energy bands of freestanding Al B₂ in the Al B₂/Al（111）system,which indicates the relative weak interaction between the monolayer Al B₂ and the Al（111）substrate.Therefore the exfoliation and transfer of monolayer Al B₂ is possible.These results are likely to stimulate further research interest to explore the exotic properties arising from the interplay of Dirac fermions and superconductivity in two-dimensional materials.3.We research the superconductivity and Fermion surface nesting in Nb C,a transition metal carbide with various unusual properties.Transport,magnetic susceptibility,and specific heat measurements demonstrate that Nb C is a conventional superconductor with a superconducting transition temperature of 11.5 K.Our theoretical calculations show that Nb C is a type-II Dirac semimetal with strong Fermi-surface nesting,which is supported by our ARPES measurement results.We also observed the superconducting gaps of Nb C using ARPES and found the temperature dependence of the superconducting gap deviates significantly from the BCS theory.These intriguing superconducting and topological properties,combined with the high corrosion resistance,make Nb C an ideal platform for both fundamental research and device applications.4.We obtain crystalline Mg（0001）thin film in the ultrahigh vacuum system by molecular beam epitaxy.The Mg O（111）single crystal is used as the substrate.Oxygen superstructure of the Mg O（111）surface after high temperature annealing process is removed by Ar ion sputtering.A homogeneous epitaxy is demonstrated by reflection high-energy electron diffraction（RHEED）during the growth of Mg on Mg O（111）.The sharp stripes in RHEED pattern indicate the high quality of Mg（0001）surface.ARPES intensity maps and plots show clear surface states of the Mg（0001）surface at room temperature.The achievement of Mg（0001）with high surface quality also afford a platform for various research topics,e.g.,the topological electronic structure of alkaline metal and the adsorption of molecules or oxidation of the Mg（0001）surface.The growth of B on Mg（0001）can also achieve the monolayer Mg B₂ film,which is a superconducting film with high research and application potential.