STM Study Of The Transition Metal Chalcogenides ZrTe₅ And WTe₂

In the past ten years,topological materials sparked extensive research interest among the international and domestic physicists,and a large number of significant theoretical and experimental results have emerged,which result in the establishment of topological Insulator system.Topological insulator is a new quantum material,with the strong spin orbit coupling,it is insulating in bulk,while metallic on the edge.The gapless edge states are described by the massless Dirac equation,completely determined by the topology of the bulk states,and protected by the time reversal symmetry,which giving rise to prohibition for backscattering of non-magnetic impurities and the spin and momentum locking.These novel characters led to the potential applications of topological insulators in spintronics,quantum computing,even the discovery of new matter states in the future.Although HgTe/CdTe quantum well has demonstrated the existence of the quantum spin Hall state,it's small bulk gap and complex preparation process increase the difficulties for further research and practical applications.In that case,it shows great importance to search for the ideal two-dimensional topological insulator.In addition,the influence of external magnetic field on the quantum spin Hall state is another interesting issue and attracted extensive attention of theoretical physicists,thus the direct experimental study becomes significant urgent.Meanwhile,topological semimetals have been widely attended and studied both in the theory and experiment after establishment of the conception.Among them,a typical topological semimetal WTe2 attracted much attention attributed to its exceptional properties,especially non-saturating magnetoresistance effect.The physical mechanism of the magnetoresistance effect is also a very interesting research topic.In this thesis,based on the above problems and the theoretical researching on the topology of the ZrTe5,we studied the electronic structure of ZrTe5 by low scanning tunneling microscopy(STM).And then,by means of studying the effect of magnetic field on the surface electron scattering of WTe2 via surface standing wave,we tried to explain the mechanism of giant magnetoresistance.The main results are as follows:1.We have observed the surface morphology of ZrTe5 using STM,and determined two types of steps on the surface,one parallel to the a-axis has long and quasi-one dimension features,the other basically parallel to the c-axis has non-uniform geometry.The dI/dV spectrum reveal bulk gap on the surface of ZrTe5,which suggests ZrTe5 is a weak three-dimensional topological insulator.Besides,we found that there are topological edge states located at the surface step edge in the bulk gap,which further confirms that the ZrTe5 surface is a two-dimensional topological insulator.The distinct quasi-particle interference patterns indicate that ZrTe5 has strong two-dimensional characteristic.The dI/dV mapping images show one-dimension characteristics of the topological edge states along the all different steps edges.These results are direct experimental evidence for topological property of ZrTe5.2.Using the technique of low temperature and high magnetic field,we study the evolution of topological edge states when the external magnetic field breaks the time reversal symmetry.We find that each characteristic peaks of topological edge states is split into two branches,with the corresponding energies shifting up and down,respectively.The intensity for higher-energy branches is lower than that for lower-energy one under relatively high magnetic fields,which is in agreement with the results of theoretical calculation.We arrived at the conclusions that the evolution of topological edge states are completely corresponds to the topology of bulk band,and the splitting of topological edge states are induced by Landau quantization and Zeeman splitting of bulk band.The results are of great significance for understanding the properties of quantum spin Hall states.3.The surface morphology and electronic structure of topological semimetal WTe2 were studied by STM,the research results showed that the crystal structure of single crystal WTe2 was consistent with the theoretical model,and the dI/dV spectrum demonstrated that WTe2 has semimetallic property.Furthermore,the effect of magnetic field on the surface electron scattering of WTe2 was studied by suing the surface standing wave method.It shows that more scattering bands were discovered under the magnetic field,it is presumed that the external magnetic field changes the spin structure.The results are of great significance to understand the giant magnetoresistance effect of WTe2.Meanwhile,Landau quantization was not observed due to specific band of WTe2 due to its special band shtructure.