Investigations On The Two-dimensional Topological Properties Of WTe₂ And Bi Films

Topological insulator is a new class of quantum material.The two-dimensional topological insulator（2D-TI）has an energy gap in the bulk and gapless state on the edge.The edge state is derived from the nontrivial topology of the bulk and protected by time-inversion-symmetry.It has spin-momentum locking property and is prohibited from backscattering by non-magnetic impurities.These novel properties lead to the important prospects of 2D-TIs for applications in spintronic devices and topological quantum computing.To date,lots of materials have been theoretically predicted as 2D-TIs and some of them are considered to be ideal 2D-TIs,which have large bulk gap,simple electronic structure and controllable topological phase.However,most of them are yet confirmed experimentally.In this thesis,we have studied the topological edge states of 1T′-WTe₂ and Bi（111）films that are theoretically predicted to be 2D-TIs.At the same time,we have also discovered the novel electronic states of Bi（110）/NbSe₂ heterojunction.The main results are summarized as follows:（1）A one-dimensional electronic state residing at the step edge of WTe₂ is observed,which exhibits remarkable robustness against edge imperfections.First principles calculations rigorously verify the edge state has a topological origin,and its topological nature is unaffected by the presence of the substrate.Our study supports the existence of topological edge states in 1T′-WTe₂,which may envision in-depth study of its topological physics and device applications.In addition,embedded nanoribbons are found at the 1T’-WTe₂ surface,which exhibit one-dimensional electronic properties.The domain boundaries on both sides of the nanoribbons exhibit an in-plane electric polarization effect.（2）Atomically flat thin films of Bi（111）have been grown on superconductor 2H-NbSe₂substrate by MBE.Two types of nonmagnetic edge structures,i.e.,a conventional zigzag edge and a 2×1 reconstructed edge,coexist alternately at the boundaries of single bilayer islands,the topological edge states of which exhibit remarkably different energy and spatial distributions.Prominent edge states are persistently visualized at the edges of both single and double bilayer Bi islands,regardless of the underlying thickness of Bi（111）thin films.We provide an explanation for the topological origin of the observed edge states that is verified with first-principles calculations.Our results clarify the long-standing controversy regarding the topology of Bi（111）thin films and reveal the tunability of topological edge states via edge modifications.Due to the superconducting proximity effect,the topological edge state of the Bi（111）film is a topological superconductor,which is expected to host Majorana zero modes if the edge state is terminated with magnetic insulators.（3）Atomically flat thin films of Bi（110）have been grown on 2H-NbSe₂ substrate by MBE.The local density of state（LDOS）of the Bi（110）/NbSe₂ heterostructure not only laterally exhibits nanoscale spatial modulation that correlates with the Moire?pattern but also drastically changes as a function of Bi film thickness in the vertical direction.Such layer-dependent modulations lead to an unusual superconducting proximity effect.Our results of both vertical and lateral modulations are corroborated by density functional theory（DFT）calculations,which explain their physical origins from the stacking order of Bi（110）on NbSe₂ and the strong quasi-covalent interlayer interaction.The strong substrate-modulated electronic properties of layered heterostructure presented here emphasize a profound impact on designing tunable properties,with atomic precision,of materials beyond van de Waals（vdW）epitaxy.