Scanning Tunneling Microscopy Studies Of Quantum Materials In Correlated Electron System

The research of quantum condensed matter materials is mainly focusing on the combination of two central issues.The first one is the strongly correlated electron physics,where abundant physical phenomena have been generated,including local moment formation,correlated metallic states,quantum criticality,unconventional superconductivity,and so on.The second is the study of weakly correlated electron system,where nontrivial physics are triggered by strong spin orbit coupling（SOC）,such as topological insulators,topological semimetal,topological Mott insulator and so on.In present work,the potential Kitaev material Na₂IrO₃,2D superconductingα-Mo₂C crystal and topological nodal-line semimetal ZrSiSe/ZrSiTe/ZrGeSe are systematically studied by using scanning tunneling microscopy/spectroscopy（STM/S）,molecular beam epitaxy（MBE）and angular resolution photoelectron spectroscopy（ARPES）.These materials have different strengths of electron correlation and exhibit various peculiar physical properties.The main results are summarized as follows:1.Surface crystalline structure and Ir-O bond properties of strongly correlated oxide Na₂IrO₃An electronic gap of 420 meV is observed in Na₂IrO₃ at 77 K,which is far above the transition temperature of antiferromagnetic order（T_N≈15 K）,manifesting a novel J_eff=1/2Mott state rather than Slater-like insulator.In addition,both the surface morphologies and electronic properties of Na₂IrO₃ crystal are tuned by the annealing treatment.In particular,by functionalizing the STM tip with surface oxygen atom,ultrahigh resolution imaging has been achieved on the surface of Na₂IrO₃.Importantly,zigzag Ir-O-Ir bonds are observed at77 K,and this unusual asymmetry of bonds may dictate the zigzag magnetic order below15 K because of the strong spin-orbit interactions.2.Surface structure and superconductivity of 2Dα-Mo₂C crystalsConsiderable density of strain and defects are observed on the surface of ultrathinα-Mo₂C flakes.Moreover,due to the relative glide between different atomic layers,the surface Mo layers present different stacking modes.Through the analysis of the tunneling spectra,we found that ultrathinα-Mo₂C flake possesses a uniform and robust superconductivity,and the superconducting transition temperature T_c is significantly higher than the results of macroscopic transport measurements.A mechanism of superconducting enhancement is proposed based on the strain-induced strong coupling and the increased disordering originated from lattice defects.3.Band structures of topological nodal-line semimetal ZrSiSe/ZrSiTe/ZrGeSeTwo SOC induced gaps are observed in the tunneling spectra taken on both ZrSiSe and ZrSiTe surfaces,with values of 6 meV and 19 meV,respectively.Furthermore,the Fermi surface and energy band information of ZrSiSe/ZrSiTe/ZrGeSe materials were obtained by quasiparticle interference and ARPES experiments,as well as the entanglement between the bulk and the surface bands near the X point in Brillouin zone.Importantly,the time reversal symmetry was broken by introducing Fe atoms on the ZrSiTe surface.The degenerated Dirac crossings therefore develop into Weyl nodes near the X points,supported by the observation of Fermi arcs at the Dirac energy（+40 meV）.