Measurement And Manipulation Of The 2D Strong Correlated Electronic States

The single electron approximation has succeeded in explaining the electron motion in tradi-tional solid state materials.With the exploration of new materials in recent years,the electron mo-tion in some materials cannot be properly explained by the single electron approximation.Strong coulomb repulsion between electrons should not be ignored in this case.Mott insulators and some unconventional superconductors are strong correlated electronic systems.Their electronic struc-tures cannot be explained by the traditional band theory.Due to the unique properties of the strong correlated systems,they have become the focus of the research in condensed matter physics.In this thesis,I mainly focus on the manipulation and measurement of the strong correlated electronic states of 1T-TaS2,a Mott insulator at low temperature,and FeSe,a superconductor,by low temperature scanning tunneling microscopy.The main contents are as follows:?1?Strain induced smooth evolution from Mott insulator state to metallic state in 1T-TaS2.We found an area with complex morphology in the transition metal dichalcogenides 1T-TaS2.which is different from the normal flat cleavage plane.The complex morphology may indicate a rich strain in this area.We select an area with mosaic charge density wave domains for analysis.The strain induced mosaic state is rather stable when compared with the former reported voltage pulse induced metastable mosaic state.There is no obvious change for the strain induced mosaic state after a long time measurement and a thermal cycle from 4.5 K to 60 K.It contains not only the Mott insulator domain which is similar to the parent material,but also the metallic domains with different degrees of metallization.We also find a corrugation which is not far from the mosaic domain.Across the corrugation,we observe a smooth evolution from the Mott insulator state to a metallic state,and then back to the Mott insulator state.Through the analysis of the electronic density of states,we find that the valence band gradually moves to the lower Hubbard band,with the evolution from the Mott insulator state to the metallic state.The mergence of the valence band and the lower Hubbard band is consistent with the two-orbital Hubbard model.We also apply strain to the samples through different thermal expansion coefficients between the samples and substrates.Both mosaic states and corrugations are found reproducibly in the strained samples.?2?Study of intrinsic defect states in bulk FeSe superconductor.We apply scanning tun-neling microscopy to study four types of intrinsic defect states in bulk FeSe,including the type-?dumbbell,type-? dumbbell,top-layer Se-site defect,and inner-layer Se-site defect.For the type-?dumbbell,the topography under positive bias voltage shows two bright lobes.The topography un-der negative bias voltage mainly shows a dark feature perpendicular to the lobe direction.For the type-? dumbbell,the lobe structure under positive bias voltage is brighter than that of the type-?dumbbell.The topography under negative bias voltage shows a suppressed but still bright lobe structure.Through the comparison of electronic density of states measured in a large energy range between the defect sites and the clean area,the four types of intrinsic defects are determined to be the Fe vacancy,Se_Fe antisite defect,top-layer Se vacancy and possible inner-layer Se-site va-cancy.We also measured a three layer exfoliated FeSe flake by the annular dark-field scanning transmission electron microscopy.The four types of intrinsic defects are qualitatively determined by the bright contrast at the defect sites.The determination of defect types lays a foundation for the complex theoretical calculations and the explanation of the pairing symmetry in the future.?3?Study of electronic nematicity in bulk FeSe superconductor.We study the intrinsic electronic nematicity of FeSe in a clean area at both 77 K and 4.5 K.In addition to the traditional[100]nematicity which is along the a/b axes of the iron lattice,we also observe a[110]nematicity which is along the diagonal Fe-Fe direction.Through a precise location of each atom site,we extract the map of electronic nematicity at different temperatures.The map of electronic nematicity at 77 K shows the same sign over the field of view,while the map of electronic nematicity at 4.5 K shows different signs with nanometer sized domains.Interestingly,the[110]nematic order shows a[-50.50]meV gap at 4.5 K,which shares a similar behavior with the Neel spin fluctuations in inelastic neutron scattering experiment.We also analyze the[100]nematicity at different temperatures.The[100]nematicity is rather weak at 77 K,while it grows up at 4.5 K.The trade-off behavior between the[100]nematicity and[110]nematicity indicates a competition relationship between them.Our results may shed significant new lights on the origin of electronic nematicity in FeSe.