On topological properties of materials: Topological insulators, Weyl and Dirac semimetals

Abstract

Topological materials are materials whose properties remain invariant under continuous transformations. Topological insulators are insulating in bulk and conducting at surface. In another class of topological materials, the material is a semimetal where their valence and conduction bands touch at, or near the Fermi level. Depending on whether the bands are non or doubly-degenerate, a topological material is called topological Weyl or Dirac semimetal, respectively. Standard topological materials possess three-dimensional crystal structures that are often formed as bulk crystals. However, the research has been focused lately on two-dimensional van der Waals topological materials due to their unique quantum electronic properties and their reduced dimensionality. This doctoral dissertation aims on presenting theoretical and experimental results that contribute to the research on two-dimensional van der Waals topological materials. Firstly, we fabricated epitaxial (SnBi2Te4)n(Bi2Te3)m natural van der Waals superlattices. The existence of topological surface states is confirmed by using first-principles calculations in combination with in-situ and synchrotron angle-resolved photoemission spectroscopy. Their presence is also correlated with appearance of the weak antilocalization effect observed with magnetotransport measurements. In addition, we report on the crucial orthorhombic non-centrosymmetric Weyl semimetal Td-phase direct room-temperature observation in epitaxial three monolayers MoTe2 films generated layer by layer on InAs substrates via molecular beam epitaxy. The lattice constants of our epitaxial orthorhombic phase are found to be significantly greater than the experimental values from bulk Td-phase of MoTe2 that were previously published. In this study, we claim that the stabilization of the Td-phase at room temperature in epitaxial thin films is significantly influenced by the expanded lattice parameters, as well as the energy position of the Weyl points. Moreover, the family of group IV transition metal ditellurides is studied with theoretical calculations, where HfTe2 and ZrTe2 are classified as type-I and type-II topological Dirac semimetals, respectively. In addition, a new type-III phase is proposed, which is achieved through a topological phase transition, by alloying the two materials in combination with in-plane strain. This strain may be used to alternate between the two types of Dirac semimetals. We successfully fabricated the desired alloy by using molecular beam epitaxy, and performed angle-resolved photoemission spectroscopy, where the topological Dirac semimetal behavior is displayed. Finally, theoretical results of the WTe2, and to a lesser extend MoTe2, in a van der Waals heterostructure with the CrTe2 two-dimensional ferromagnet are presented. WTe2 and MoTe2 are topological Weyl semimetals in large thicknesses and two-dimensional topological insulators down to the single layer, holding great opportunities on spintronic applications. Our findings suggest that an interfacial Dzyaloshinskii-Moriya interaction strong enough to create Néel-type skyrmion lattice is produced whose dynamics are tested under external magnetic fields and temperature. This study also shows that the generation and annihilation of magnetic skyrmions in the CrTe2/WTe2 van der Waals heterostructure can be achieved and that the motion of the magnetic skyrmions can be controlled by ultra-low spin-polarized currents.