| Over the past decade,the emergence of topological states and topological materials refresh the recognition of physics and materials.First-principles calculations method provides an important linkage between topological theory and experimental explorations by powerful prediction of realistic topological systems.In topological materials,the metallic surface and/or edge states protected by global symmetry possess many superior properties,including zero-mass,high mobility,defect tolerance and near dissipationless transport,etc.Therefore,people are increasingly interested in exploring functional applications of topological materials,such as the realization of topological superconductors that can be used for fault-tolerant topological quantum computation,application of the next generation of spintronics,improvement of catalytic performance of topological surface states(TSSs)serving as an effective electron bath and so on.In this thesis,using the first-principles calculations,we study topological materials and explore their functionality from different angles.The present thesis is organized as follows:Chapter 1 mainly contains three sections:topological materials,two-dimensional(2D)superconductivity,and the functional application of topological catalysis,respectively.we first make a brief introduction about the fundamental physics and concepts of the topological materials and the corresponding progress.Then we review the development of 2D superconductivity,and in particular introduce the research progress of Ising superconductivity emerged very recently in this field.Finally,we briefly discuss the functional application of topological materials in catalysis,and mainly focus on three types of topological catalysts according to the classification of topological materials.Chapter 2 mainly introduces the calculation methods and theoretical background involved in this thesis,including density functional theory,Wannier charge centers method,Eliashberg equation with McMillan formula,Gibbs free energy,and the packages used in this thesis.In Chapter 3,using first-principles approaches,we first show that a hydrostatic pressure over a critical value of 2.5 GPa can induce a p-d band inversion in a NbSe2 monolayer,introducing topological phase transition(Z2=1)to the system while still preserving its superconducting property.Next,we demonstrate substitution of Se by Te can function as an even more superior approach of chemical pressuring in inducing the nontrivial p-d band inversion in an alloyed monolayer of NbSe2-xTex when x≥0.8,originating from the dual effects of a larger atomic radius and stronger SOC of Te.We also evaluate the in-plane upper critical fields at different pressures or doping concentrations,and confirm the enhanced Ising superconductivity nature.Encouragingly,we have further carried out preliminary experimental studies to confirm that the NbSe2-xTex samples indeed exhibit enhanced Ising superconductivity.Collectively,the central findings presented here offer appealing new approaches towards definitive materialization of topological Ising superconductivity in 2D systems.In Chapter 4,using density functional theory,we have systematically investigated the structural,electronic,and topological properties of the Pb3Bi alloy grown on the Ge(111)substrate.We first identified the existence of three distinct phases,labeled as T1,H3,and T4,which are of high symmetry and energetically nearly degenerate.All three phases have been shown to possess giant Rashba enhanced SOC and corresponding spin splittings.Next,we showed that each of the saddle-and parabolic-like band dispersions can result in the emergence of type-II van Hove singularity,whose positions can be substantially tuned by the giant Rashba effects to the vicinity of the Fermi level.More importantly,we identified the presence of topological states hosted by both the H3 and T4 structures,with the openings of topologically nontrivial gaps of 25 and 50 meV,respectively.Given the energetic affinity of the three phases,our findings point to the feasibility of realizing quantum phase transitions between the quantum spin Hall effect and topological superconductivity within the same materials platform.The present study is expected to give new insights into searching for topological quantum states including 2D topological superconductivity based on Rashba systems.In Chapter 5,using first-principles approaches within density functional theory,we demonstrate that the three-dimensional topological insulator of Bi2Se3 covered with a single layer of ZnSe can function as an ideal platform for HER,as measured by the optimal hydrogen adsorption free energy,positioning the heterostructural system at the peak of the HER volcano curve.The underlying mechanism for the maximal activity is attributed to the subtle yet elegant synergistic effect:on one hand,the existence of dangling bonds on the ZnSe overlayer causes the enhanced local bonding;on the other hand,the TSSs serve as an electron bath to additionally enhance such bonding nature.In addition,We also identify precise tunability in the vertical location of the TSSs by the ZnSe overlayer thickness,a finding that can be exploited for other functionalities of such topological insulator heterostructures.Collectively,The present study provides an important linkage between the topological insulators as a new class of quantum matter and catalytic materials for clean energy.In Chapter 6,using first-principles approaches,we reveal the atomistic growth mechanisms and robust topological properties.Our calculations show that hydrogen passivation plays a decisive role as a surfactant in facilitating the multilayer growth and improving the overall quality of the stanene films.On the other hand,we investigate systematically the dependence of the topological properties of stanene on the film thickness,hydrogen passivation,and substrate,and obtain robust quantum spin Hall effects under diverse physical conditions.The robustness of the nontrivial topology is attributable to the strong spin-orbit coupling of the Bi substrate.All these results are also confirmed by our preliminary experimental studies.The present findings open an appealing avenue toward the realization of one-dimensional topological superconductivity in few-layer stanene.In Chapter 7,we summarize all the projects involved in this thesis and further comment on an outlook of the development of the topological materials. |
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