Studies On The Surface Reactivity Of Bi₂Te₃ And Electronic Structure Of 1 T-TiSe₂

With the discovery of the quantum Hall effect,the concept of topology order has been introduced into the field of the condensed matter physics which enriches the physical theory and guides people to discover more novel quantum states.In the family of the novel quantum states,the topological insulator(TI)is one of the most extensively studied materials characterized by a bulk band gap and a robust metallic surface state protected by time-reversal symmetry.The gapless surface state is forged when the bonding forces of the crystal are modified in a special manner through strong spin-orbital coupling.These surface states have a special helical spin-momentum texture which prohibits the backscattering of the electrons in the absence of magnetic impurities.Many exotic phenomena have also been revealed at the interfaces with a TI,such as the Majorana fermions at a TI/superconductor interface,the quantum anomalous Hall effect in a magnetic TI.Therefore,TIs have great potential in both fundamental science and future practical applications like spintronics and quantum computing.However,it remains many problems to be solved before we put them into use.The obstacles we meet are whether the material is stable in air or residual gases and how to get a better heteroepitaxial structure by tuning the surface structure of topological insulator.On the other hand,the condensed matter physics has also been enriched since the discovery of graphene at 2004.After that there emerged many two-dimensional layered materials with peculiar properties.Among them,1T-TiSe2,one of the transition mental dichalcogenides,is on the spot light since it not only has charge density wave(CDW)but also can get superconductivity by doping and applied pressure.The mechanism behind the CDW transition and superconductivity and the relationship between them are the key issues that need to be solved.In this thesis,the surface reactivity of Bi2Te3 towards water has been systematically investigated by molecular beam epitaxial(MBE)and scanning tunneling microscopy(STM).In addition,the topological insulator surface with point defects was obtained by adjusting the film growth parameters and the effect of strain on the topological surface states has also been studied.At last,the surface electronic properties of 1T-TiSe2 has been systematically investigated.The main results are as follows:1.Through MBE and self-flux method,high-quality Bi2Te3 film and bulk-cleaved samples have been prepared.We employ STM to directly probe the surface reaction of Bi2Te3 towards H2O.We find that only the top quintuple layer(QL)is reactive to H2O,resulting in a hydrated Bi bilayer which passivate the surface and prevent subsequent reaction.Based on the dI/dV measurements,we find that water molecules act as an electron dopant.As a result,the STS on the unreacted area during the water dosage process has not been affected.Further,by dosing water on the Bi2Te3 surface at low temperature,a reaction mechanism is proposed with H2Tre and hydrated Bi as the products.For comparison,we study the reaction between oxygen gas(O2)and the Bi2Te3 surface and detect that the surface morphology is almost unchanged.All our results indicate that the water indeed react with water.2.We study the morphology of Bi2Te3 surface at different film deposition rate,film thickness and substrate through MBE and STM system.On the Si(111)-7×7 substrate,we can easily get a strained surface when the deposition rate is 0.5 ML/min and the film thickness is about 10 quintuple layer(QL).From the STM images and geometric phase analysis(GPA),we can clearly see a complicated strain field around the point-like defects:tensile and compressive regions are alternatively distributed.Moreover,the scanning tunneling spectrum perform a gap-like structure,which shows that the local density of state near the strained area has also been affected.We can also clearly see a local electronic structure variation after we deposit some Pb atoms on the surface.3.We systematically studied the charge density wave(CDW)inhomogeneity and the CDW gap inhomogeneity where exists two areas:one for the normal CDW area and the other for "suppressed" CDW area.We also found a pseudogap structure on the 1T-TiSe2 surface which is confirmed by its temperature and magnet evolution.Through potassium(K)atom deposition,the pseudogap structure evolves into a gap structure with coherence peaks which will be smeared out by applied magnet field.We can clearly see that the CDW inhomogeneity is related to the Ti intercalation induced local band changes while the pseudogap structure is related to the electron doping by either Ti intercalation or K atom deposition by analyzing the defects distribution.