Xue Qikun

Kondo Effect in Self-Assembled Manganese Phthalocyanine Monolayer on Pb Islands

The Kondo effect in two-dimensional manganese phthalocyanine (MnPc) self-assembled monolayer films on Pb(111) islands is studied by low-temperature scanning tunneling microscopy. Variation of the Kondo temperature from 50 K to 300 K at different molecule adsorption sites is revealed. It is shown that the variation is mainly due to the change in the width of d orbital, rather than the shift of its energy. The two-dimensional dI/dV mapping reveals the periodic modulation of the Kondo resonance in the self-assembled MnPc monolayer.

Two-Step Oxidation of Pb(111) Surfaces *

We report on a two-step method for oxidation of Pb(111) surfaces, which consists of low temperature (90K) adsorption of O2 and subsequent annealing to room temperature. In situ scanning tunnelling microscopy observation reveals that oxidation of Pb(111) can occur effectively by this method, while direct room temperature adsorption results in no oxidation. Temperature-dependent adsorption behaviour suggests the existence of a precursor state for O2 adsorption on Pb(111) surfaces and can explain the oxidation-resistance of clean Pb(111) surface at room temperature.

Modifying Quantum Well States of Pb Thin Films via Interface Engineering

We demonstrate the importance of interface modification on improving electron confinement by preparing Pb quantum islands on Si(111) substrates with two different surface reconstructions, i.e., Si(111)-7 × 7 and Si(111)-Root3 × Root3-Pb (hereafter, 7 × 7 and R3). Characterization with scanning tunneling microscopy/spectroscopy shows that growing Pb films directly on a 7 × 7 surface will generate many interface defects, which makes the lifetime of quantum well states (QWSs) strongly dependent on surface locations. On the other hand, QWSs in Pb films on an R3 surface are well defined with small variations in linewidth on different surface locations and are much sharper than those on the 7 × 7 surface. We show that the enhancement in quantum confinement is primarily due to the reduced electron-defect scattering at the interface.

Growth of Co nanoclusters on Si3N4 surface formed on Si(111)

We have grown high density Co clusters with a narrow-sized distribution on the Si3N4(0001)-(8 x 8) surface. In the submonolayer regime, Co clusters tend to keep a certain size (similar to1.45 nm in diameter) irrespective of coverage. With increasing coverage above 0.92 ML, two new clusters with certain but larger sizes are formed. This novel growth behaviour can be explained by the quantum size effect [Phys. Rev. Lett. 90 (2003) 185506]. It is found that the Co cluster size distribution can be improved by post annealing. Even at high temperature (700degrees C), no reaction of Co with Si3N4 is observed, indicating that Si3N4 (0001)-(8 x 8) is a promising substrate for growth of magnetic nanostructures.

Modulation of Step Heights of Thin Pb Films by the Quantum Size Effect Observed by Non-Contact Atomic Force Microscopy

Using a home-made Q-plus sensor, simultaneous scanning tunneling microscopy (STM) and atomic force microscopy (AFM) measurements were performed on the wedge-shaped Pb islands grown on Si(111)-7 × 7. Atomic resolved AFM images were observed. The contrast of AFM topography shows no dependence on the sample bias (tip is grounded), while the simultaneously obtained tunneling current image exhibits strong bias dependence due to quantum well states (QWS). Furthermore, In the AFM mode, neighboring Pb films with one monolayer (ML) thickness difference within the same Pb island show the same apparent height, which means that the apparent step heights of Pb films oscillate with a bilayer periodicity, being consistent with previous observations by helium atom scattering, x-ray diffraction, and STM. The possible reasons underlying the oscillation of apparent step heights in AFM topography are discussed.

Self-Assembly of TBrPP-Co Molecules on an Ag/Si(111) Surface Studied by Scanning Tunneling Microscopy *

Self-assembly of TBrPP-Co molecules on a Si(111)-√3 × √3 Ag substrate is studied by low-temperature scanning tunneling microscopy. With the same adsorbed amount (0.07 ML), the molecules deposited by low-temperature evaporation show three kinds of ordered structures whereas those deposited by high-temperature evaporation have size-dependent ordered structures. The distinct differences in the self-assembly structures and in the electron density of states inside the molecule near the Fermi energy demonstrate that the Br atoms of the molecule desorb at the higher evaporation temperature.

Research on superconductivity of epitaxial FeSe films

The newly discovered iron-based superconductors in 2008 have become the second class of high temperature superconductor besides the cuprates. Among all these iron-based compounds, β-FeSe has the simplest chemical structure and can be an archetype system for unraveling the mechanism of superconductivity. By using state-of-the-art molecular beam epitaxy (MBE) technique, high-quality superconducting FeSe single crystalline films with finely controllable topography and composition have been successfully prepared, with their superconducting properties extensively investigated. The low-temperature scanning tunneling spectra reveal that the superconducting gap in the quasiparticle density of states is visible down to two unit cells of FeSe films, and evidence for a gap function with nodal lines. Electron pairing with two-fold symmetry has also been demonstrated by direct imaging of quasiparticle excitations in the vicinity of magnetic vortex cores, Fe adatoms and Se vacancies. The two-fold pairing symmetry is supported by our observation of striped electronic nanostructures in the slightly Se-doped samples, primarily due to the orbital ordering. Twin boundaries run at approximately 45° to the Fe-Fe bond directions, and noticeably suppress the superconducting gap and pin magnetic vortices. This is likely caused by the increased Se height in the vicinity of twin boundaries, providing the first local evidence for the importance of this height to the mechanism of superconductivity. Furthermore, we reveal signatures of a bosonic mode in the local quasiparticle density of states of superconducting FeSe films, whose energy reduces with decreasing gap magnitude Ω . Recently, the growth recipe has been further extended to grow FeSe film on SrTiO 3 (001) substrate, leading to the high- T c superconductivity in the very FeSe/SrTiO 3 interface. A superconducting gap as large as 20 meV and the magnetic field induced vortex state revealed by in situ scanning tunneling microscopy suggest that the superconductivity of single-unit-cell FeSe films could occur around 77 K. Our subsequent transport and measurements of the one-unit-cell thick FeSe films on insulating SrTiO 3 substrates with non-superconducting FeTe protection layers provide definitive evidence for high temperature superconductivity with an onset T c above 40 K which are much higher than T c ~9.4 K for bulk FeSe, respectively. The finding provides a completely new avenue and method to reveal the pairing mechanism of high- T c superconductors, and to further enhance T c , stirring up worldwide increasing attention in the field of high- T c superconductors and material sciences.

Molecular Beam Epitaxy Growth and Scanning Tunneling Microscopy Study of Pyrite CuSe 2 Films on SrTiO 3

We perform molecular beam epitaxy growth and scanning tunneling microscopy study of copper diselenide (CuSe2) films on SrTiO3(001). Using a Se-rich condition, the single-phase pyrite CuSe2 grows in the Stranski—Krastanov (layer-plus-island) mode with a preferential orientation of (111). Our careful inspection of both the as-grown and post-annealed CuSe2 films at various temperatures invariably shows a Cu-terminated surface, which, depending on the annealing temperature, reconstructs into two distinct structures and . The Cu termination is supported by the depressed density of states near the Fermi level, measured by in-situ low temperature scanning tunneling spectroscopy. Our study helps understand the preparation and surface chemistry of transition metal pyrite dichalcogenides thin films.

Topological insulator and the quantum anomalous Hall effect

The quantum Hall effect (QHE) is a quantum phenomenon that occurs on a macroscopic length scale as the result of the non-trivial topological property of a two-dimensional electron system in a strong magnetic field. It has long been expected that QHE can be realized without an external magnetic field such that the effect can be applied in electric devices that consume little energy. The zero-magnetic-field QHE can appear in thin films of magnetic topological insulators as the quantized version of the anomalous Hall effect; i.e., the quantum anomalous Hall effect (QAHE). Here, we review how the ideas of the topological insulator and QAHE were developed and how the QAHE was finally experimentally realized in thin films of a magnetically doped topological insulator. We also discuss the prospect of application of the QAHE in low-energy-consuming devices.

STM observation of pit formation and evolution during the epitaxial growth of Si on Si（001） surface

Pit formation and surface morphological evolution in Si(001) homoepitaxy are investigated by using scanning tunneling microscopy. Anti-phase boundary is found to give rise to initial generation of pits bound by bunched DB steps. The terraces break up and are reduced to a critical nucleus size with pit formation. Due to anisotropic kinetics, a downhill bias diffusion current, which is larger along the dimer rows through the centre area of the terrace than through the area close to the edge, leads to the prevalence of pits bound by {101} facets. Subsequent annealing results in a shape transition from {101}-faceted pits to multi-faceted pits.