Insight Into The Topological Quantum States Of WTe₂ And Shiba States Of NbSe₂ With Scanning Tunneling Microscopy

After introducing the concept of topology into physics in recent 30 years,scientists have proposed a new kind of material,topology quantum material.This material with unique band structure will exhibit topological properties which are insensitive to sample details.Since the prediction of quantum spin Hall effect in monolayer graphene,different kinds of topological quantum materials have been predicted and confirmed successively,including topological insulators,topological superconductors and topological semimetals.These materials have abundant physical properties and novel electromagnetic transport phenomena,which are of great significance for developing low-energy spintronic devices and topological quantum computing.Scanning tunneling microscopy?STM?is a kind of surface analysis technique which probes local density of states of materials directly in real space.It can be applied to a variety of environment.It has high energy and space resolution.It can detect the occupied and unoccupied states at the same time,and can also access reciprocal space indirectly by quasiparticle interference?QPI?.STM has become a powerful technique in the field of surface science.In this paper,we investigated the topological properties of WTe₂ and Shiba states of NbSe₂ with STM.The main results are summarized as follows:1.To confirm the single-layer 1T'-WTe₂ to have quantum spin Hall effect,which is also known as two-dimensional topological insulator,we investigated the properties of step edge on cleaved WTe₂.In our experiments,we directly observed robust one-dimensional topological edge states.Its topological properties originate from the nontrivial topology of the bulk and are irrelevant to the details of the step edges.Its topological origin is further supported by first principles calculations.Combined with the theory,our experiment confirms the existence of topological states residing at step edges of single-layer 1T'-WTe₂.Our results are of great significance for further study of its properties and topological physics.2.To confirm the existence of topological Fermi arc states in the type-II Weyl semimetal candidate WTe₂,we study the quasiparticle interference of its electronic structures by spectroscopic-imaging scanning tunneling spectroscopy.Here,we observed the electron scattering on two types of WTe₂ surfaces unambiguously.Compared with first principles calculations,its scattering signal can be ascribed mainly to trivial surface states.We also address the QPI feature of nontrivial surface states from theoretical calculations.The experimental QPI patterns show some features that are likely related to the nontrivial Fermi arc states,whose existence is,however,not conclusive.Our study provides an indispensable clue for studying the Weyl semimetal phase in WTe₂.3.To obtain an easier manipulation of long-ranged Shiba states,we deposited Fe atoms on the surface of layered superconductor 2H-NbSe₂ at low temperature.We found the Shiba states on single Fe atom with threefold symmetry have only a short range of spatial extension.This result indicates that the magnetic impurities adsorbed on the surface may be affected by the interlayer interaction,and the environment is not a truly two-dimensional system,the spatial extension of Shiba state thus goes as the decay in three-dimensional system.In addition,we found Fe clusters with different sizes exhibit different Shiba states and the Shiba states are sensitive to adsorption configurations.Our results may provide help for the study of hybridization between Shiba states.