Theoretical Study On Modulation Of Topological States In Bismuthene And ZrSnTe

Since proposed in 1980 s,a large amount of studies have been conducted surrounding the topic of quantum spin Hall effect.This exotic spin-resolved transportation behavior of electrons is regarded as a possible way to realize pure spin current,which is an essential concept in spintronics.Two dimensional topological insulators,as the corresponding materials of quantum spin Hall effect,has been studied over a decade.Being quite different from traditional classification of band insulator and metal,two dimensional topological insulator has insulating bulk states and conducting edge states.What's more,the edge states are chiral,with opposite spin orientation moving in the opposite direction.Considering they are the result of the topology of the band structure of the bulk,these edge states are quite robust,protected by time reversal symmetry.This makes topological insulator to be a potential candidate of a new generation of low-energy-dissipation spintronic devices.Comparing with three dimensional topological insulators,two dimensional insulators have simpler structures,and are easier to be tuned and fabricated into devices.To get a better understanding of two dimensional insulator,so as to design and optimize materials,it is crucial to study the stability of the topological states in two dimensional insulator and how to manipulate them.By using the density functional theory,we employed first-principles calculation to study how will the topological state evolve under the effects of strain,different strength of interlayer coupling and chemical doping,which can be summarized as the following two points:Firstly,we studied a quantum spin Hall insulator system: Bismuthene,which has similar hexagonal structure as the well-known graphene.Manipulating the interlayer coupling by changing the interlayer distance between two bilayers,we show that a topological phase transition from a topological nontrivial insulator state to a Dirac semimetal state and then to a normal band insulator state.This arouses a seemingly contradiction with the fact that monolayer bismuthene is actually topologically nontrivial,but it is understandable since we consider the two bilayers as a whole system.Detailed analysis and explanation was carried out through first-principles calculation and a tight-binding model.With these tools we can pin down the critical point of the topological phase transition and its origin.In addition,we designed a model device which may help observe this unique topological transition.Secondly,we tried to manipulate the topological state in a 2D topological insulator with chemical doping.We tried to tune the topological band of WHM-type structure by saturating the dangling bonds with F,Cl.Take ZrSnTe as an example,this material is a nodal-line semimetal in its 3D bulk phase and can be exfoliated to 2D,which is predicted to be a 2D topological insulator.However the particle-hole asymmetry in this system is so strong that the Dirac cones is away far from the Fermi level.Saturating the dangling bonds in the system with F or Cl,we found that the saturated structure can weaken the particle-hole asymmetry and move the Dirac points back to the Fermi level.Besides,it can also eliminate the irrelevant bands near the Fermi level and still have nontrivial topology.We think this may make ZrSnTe-Cl a better platform for further study on Nodal-line semimetals.The studies in this thesis will help to deepen the understanding of the topological states in topological insulators and enrich the avenues to manipulate the topological properties.So it might be meaningful to the future study and application of this exotic type of materials.