STM Study Of 2D Topological Insulators And Topological Superconductors

Topological materials have been intensively investigated both in theory and experiments in recent years and become one of the focus areas in condensed matter physics.Materials can be classified based on their topological properties after introducing topology into the band structures,such as topological insulators,topological semimetals and topological supercon-ductors,etc.Among them,topological insulators and topological superconductors are exten-sively studied due to their huge potential in backscattering-immune electron transportation and fault-tolerant quantum computation,respectively.Topological insulators(TI)have insulating states in the bulk,just like the normal insu-lators,while conducting boundary states connecting conduction and valence bands at their boundaries,such as the edges of 2D lattices and surface of 3D lattices.The existence of this kind of boundary states are determined by the topology of bulk bands,thus the boundary states are under the protection which is called”topological-protection”.Besides,owing to the restrict of time reversal symmetry,these boundary states have the character of spin-momentum lock-ing,making the electrons immune to backscattering during the transportation in the absence of magnetic defects.Recently,the 1T'phase of transition metal dichalcogenides are predicted to be a family of topological insulators,among which the 1T'-WTe₂is the most concerned candidates for it is the only one having 1T'nature ground state.The conducting edge channel of monolayer 1T'-WTe₂has been detected in experiments and it shows the theoretically pre-dicted quantized conductance of 2e²/h within a channnel length of 100 nm.1T'-WTe₂now has become one of the most promising 2D topological materials.While the short coherent-length limits its practical uasage,which is an urgent problem to address.This length limit is mainly caused by the overlap between edge states and bulk states in the absence of insulating gap.Thus it is crucial to open an insulating gap in 1T'-WTe₂to reduce the coupling between edge states and bulk states.Considering that strain tuning is effective for tuning 2D metarials,we in-duced starins into WTe₂films during the molecular beam epitaxial process by using deformed graphene substrate.Then the deformed WTe₂films were characterized by in-situ scanning tun-neling microscopy/spectroscopy.The resulsts show that the band structure of WTe₂films can be affected by deformations,and insulating gaps show up at somewhere of deformed regions,indicating a metal-insulator transition happens.After a systematic study using statistical and averaging methods,we find that the value of the insulating gap is possitively related with lat-tice b and negatively related with lattice a.In addition,the topological edge states are detected at the edges of insulating WTe₂films,demostrating that its nontrivial topological properties remain after applying external strains.These results are also supported by the orbital analysis combined with the first principle calculations.This results indicates that 1T'-WTe₂can be a good platform for tunable 2D topological insulator and it has the capability for next generation controllable topological electronic devices.Graphene-like materials-Xenes are aslo good candidates of 2D TI.This family of mate-rial have simple structure and tunable electronic properties.Among them,stanene stands out owing to its strong spin orbital coupling effect and intrinsic superconductivity.However,de-spite of numerous experimental studies,only a few evidences of topological edge states are reported.This is beacause that its topological property tremendously relies on the substrate,strain,chemical-functionalization and layer thickness.Thus searching for a sutible substrate to grow high-quality stanene films having robust topological properties is a big problem for this material system.Here we successfully grow 1-5 layer stanene films on Bi(111)substrate.The topological edge states are confirmed via in-situ scanning tunneling microscopy/spectroscopy in 1-5 layer stanene films.According to the first-principle-calculations,this robust topology is related to the coupling with substrate Bi(111).Besides,the localized edge states show a penetration depth as short as 1?2 nm at A-edge,availing the formation of paralleled dense multi-edge channels.This is partially confirmed by a multi-step area,where multiple paralleled topological edge states co-exist,even with a samll seperation distance of several nanometers.Topological superconductors are the superconducting version of topological insulators.The superconducting gap exist in the bulk and there are also the topological boundary states,called Majorana quasi-particles.The Majorana quasi-particles in system without time-reversal symmetry usually abbey non-Abelian statistics and are suitable for topological quantum com-putation.Among numerous candidates of topological superconductors,stanene is the simplest one which has a buckled honeycomb structure and consists only single element-Sn.Besides,stanene has superconductivity itself and require neither the participation of proximity effect like TI/superconductor heterostructure nor the precise ratio of different elements such as in iron based superconductor.Theoretically,stanene can privide intrinsic 2D p-wave pairing once the inter-orbital interaction surpassing intro-orbital interaction owing to the strong spin orbital coupling.Moreover,it can also sustain 1D topological superconductor when introducing the bulk band superconductivity into the 1D topological egde states by proximity effect.We suc-cessfully detected the signatures of superconducivity combined with the previously prooved topological edge states in few-layers stanene films grown on Bi(111)substrate,and studied the dependency between superconducting gap size and layer tickness and external magnetic field along z direction at 400 mk.The results show that the superconductivity emerges after stanene fully covering Bi(111)substrate and the superconducting gap size enlarges with in-creasing layer thickness and become 0.3 me V on 5th layer stanene.However,we did not find the signature of helical Majorana edge states,indicating that few-layers stanene is not an intrin-sic 2D p-wave superconductor.Thus the proximity scenario should be condidered to induce topological superconductivity based on few-layers stanene.Our results of superconductivity in few-layers stanene lay a foundation for further exploration of the topological superconductivity in this system.