Scanning Tunneling Microscopy Study Of Correlated Electrons And Topological States In Two-dimensional Layered Materials

In the past decade,a large number of newly-emerged two-dimensional（2D）materials have been successfully synthesized experimentally.The novel properties of two-dimensional materials,such as unconventional superconductivity,topological states,charge density wave（CDW）and intrinsic magnetic orders,provide the new platform for condensed matter physics and correlated electronic systems.Besides,the tunning of structure or chemical composition will also induce novel physical phenomena in two-dimensional materials.In this thesis,we investigated the surface structure,superconductivity and topological surface states of several two-dimensional materials by using scanning tunneling microscopy/spectroscopy.The following key results have been achieved:（1）Superconducting Mo₂C crystals with different Cr-doped concentration（0,1.8,2.7and 9.4 atom%）,have been successfully synthesized.Low temperature scanning tunneling microscopy/spectroscopy（STM/STS）,transport measurements and density functional theory（DFT）calculations are employed to investigate the interplay between charge ordered states and 2D superconductivity.Our result reveals that,upon Cr doping（2.7 atom%）,the superconductivity of Mo₂C is heavily suppressed,while incommensurate CDW phase appears in the temperatures above the superconducting phase,accompanied with a partial gap opening at Fermi surface.Theoretical calculations show that,the wave vector of CDW agrees well with the calculated Fermi surface nesting vectors.Subsequently,such CDW phase is prominently enhanced at a higher Cr content（9.4 atom%）and a smaller periodicity and a larger partial gap appears concurrently.Spatial-resolved and temperature-dependent tunneling spectroscopy reveals that such CDW phases exist only in a non-superconducting regime and could form long-range orders uniformly.The results contribute to further understanding of the relationship between charge ordering and superconductivity in 2D transition metal carbides.（2）We successfully fabricate a highly uniform ripple array on the continuous few-layered Bi₂Te₃ films on Sr Ti O₃ substrate.Utilizing the low temperature scanning tunneling microscope/spectroscopy（STM/S）,we found that well-defined uniform 1D periodic ripples are induced by interfacial strain.As the thickness of Bi₂Te₃ thin films increases,the release of the strain leads to the reduction of the averaged corrugation height and the periodicity of ripple array.The thickness-dependent tunneling spectra and quasiparticle interference investigation show that both the energy gap and the Fermi velocity of the Dirac surface states for 3 QL are greatly enhanced compared with 6 QL.Our experimental results demonstrate that a spatially uniform periodic ripple array produced by the epitaxial interface stress,can effectively enlarge the energy gap and increase the Fermi velocity of the Dirac surface states,which provide a new route for realizing a high-performance topological-based device.（3）Topological surface states of Mn Bi₂Te₄ single crystals are clearly revealed by spatially-resolved scanning tunneling spectroscopy and quasiparticle coherent scattering imaging（QPI）.Furthermore,an asymmetric QPI pattern observed at the energies near Dirac point of surface states.DFT calculations strongly suggest the magnetization of the topmost Mn layer can be orientated along the in-plane direction,which could explain previous observations of gapless TSSs.Furthermore,various bulk bandgaps observed in tunneling spectroscopy with the sizes ranging from 138 me V to 320 me V,imply the presence of abundant magnetic orders in the topmost Mn Bi₂Te₄ layer.Our results provide a comprehensive and in-depth understanding of correlations between topological electron structures and magnetic ordering of surface layer.