Yi-Lin Wang

Landau quantization of topological surface states in Bi2Se3.

1 Department of Physics, Tsinghua University, Beijing 100084, China 2 Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China 3 Microsoft Research, Station Q, University of California, Santa Barbara, CA 93106, USA 4 Department of Physics, Stanford University, Stanford CA 94305, USA 5 Physikalisches Institut, Universität Würzburg, D-97074 Würzburg, Germany * These authors contributed equally to this work. ¶ To whom correspondence should addressed. Email: xc@mail.tsinghua.edu.cn, xcma@aphy.iphy.ac.cn

Direct observation of nodes and twofold symmetry in FeSe superconductor.

Scanning tunneling spectroscopy suggests an orbital ordering mechanism for electron pairing in an iron-based superconductor. We investigated the electron-pairing mechanism in an iron-based superconductor, iron selenide (FeSe), using scanning tunneling microscopy and spectroscopy. Tunneling conductance spectra of stoichiometric FeSe crystalline films in their superconducting state revealed evidence for a gap function with nodal lines. Electron pairing with twofold symmetry was demonstrated by direct imaging of quasiparticle excitations in the vicinity of magnetic vortex cores, Fe adatoms, and Se vacancies. The twofold pairing symmetry was further supported by the observation of striped electronic nanostructures in the slightly Se-doped samples. The anisotropy can be explained in terms of the orbital-dependent reconstruction of electronic structure in FeSe.

Topological insulator Bi2Se3 thin films grown on double-layer graphene by molecular beam epitaxy

Atomically flat thin films of topological insulator Bi2Se3 have been grown on double-layer graphene formed on 6H–SiC(0001) substrate by molecular beam epitaxy. By a combined study of reflection high energy electron diffraction and scanning tunneling microscopy, we identified the Se-rich condition and temperature criterion for layer-by-layer growth of epitaxial Bi2Se3 films. The as-grown films without doping exhibit a low defect density of 1.0±0.2×1011/cm2, and become a bulk insulator at a thickness of ten quintuple layers, as revealed by in situ angle resolved photoemission spectroscopy measurement.Atomically flat thin films of topological insulator Bi{sub 2}Se{sub 3} have been grown on double-layer graphene formed on 6H-SiC(0001) substrate by molecular beam epitaxy. By a combined study of reflection high energy electron diffraction and scanning tunneling microscopy, we identified the Se-rich condition and temperature criterion for layer-by-layer growth of epitaxial Bi{sub 2}Se{sub 3} films. The as-grown films without doping exhibit a low defect density of 1.0{+-}0.2x10{sup 11}/cm{sup 2}, and become a bulk insulator at a thickness of ten quintuple layers, as revealed by in situ angle resolved photoemission spectroscopy measurement.

Landau quantization and the thickness limit of topological insulator thin films of Sb2Te3.

We report the experimental observation of Landau quantization of molecular beam epitaxy grown Sb{2}Te{3} thin films by a low-temperature scanning tunneling microscope. Different from all the reported systems, the Landau quantization in a Sb{2}Te{3} topological insulator is not sensitive to the intrinsic substitutional defects in the films. As a result, a nearly perfect linear energy dispersion of surface states as a 2D massless Dirac fermion system is achieved. We demonstrate that four quintuple layers are the thickness limit for a Sb{2}Te{3} thin film being a 3D topological insulator. The mechanism of the Landau-level broadening is discussed in terms of enhanced quasiparticle lifetime.

Fermi-level tuning of epitaxial Sb2Te3 thin films on graphene by regulating intrinsic defects and substrate transfer doping.

High-quality Sb2Te3 films are obtained by molecular beam epitaxy on a graphene substrate and investigated by in situ scanning tunneling microscopy and spectroscopy. Intrinsic defects responsible for the natural p-type conductivity of Sb2Te3 are identified to be the Sb vacancies and Sb(Te) antisites in agreement with first-principles calculations. By minimizing defect densities, coupled with a transfer doping by the graphene substrate, the Fermi level of Sb2Te3 thin films can be tuned over the entire range of the bulk band gap. This establishes the necessary condition to explore topological insulator behaviors near the Dirac point.

Properties of copper (fluoro-)phthalocyanine layers deposited on epitaxial graphene

We investigate the atomic structure and electronic properties of monolayers of copper phthalocyanines (CuPc) deposited on epitaxial graphene substrate. We focus in particular on hexadecafluorophthalocyanine (F(16)CuPc), using both theoretical and experimental (scanning tunneling microscopy - STM) studies. For the individual CuPc and F(16)CuPc molecules, we calculated the electronic and optical properties using density functional theory (DFT) and time-dependent DFT and found a red-shift in the absorption peaks of F(16)CuPc relative to those of CuPc. In F(16)CuPc, the electronic wavefunctions are more polarized toward the electronegative fluorine atoms and away from the Cu atom at the center of the molecule. When adsorbed on graphene, the molecules lie flat and form closely packed patterns: F(16)CuPc forms a hexagonal pattern with two well-ordered alternating α and β stripes while CuPc arranges into a square lattice. The competition between molecule-substrate and intermolecular van der Waals interactions plays a crucial role in establishing the molecular patterns leading to tunable electron transfer from graphene to the molecules. This transfer is controlled by the layer thickness of, or the applied voltage on, epitaxial graphene resulting in selective F(16)CuPc adsorption, as observed in STM experiments. In addition, phthalocyanine adsorption modifies the electronic structure of the underlying graphene substrate introducing intensity smoothing in the range of 2-3 eV below the Dirac point (E(D)) and a small peak in the density of states at ∼0.4 eV above E(D).

Suppression of Superconductivity by Twin Boundaries In FeSe

Low-temperature scanning tunneling microscopy and spectroscopy are employed to investigate twin boundaries in stoichiometric FeSe films grown by molecular beam epitaxy. Twin boundaries can be unambiguously identified by imaging the 90° change in the orientation of local electronic dimers from Fe site impurities on either side. Twin boundaries run at approximately 45° to the Fe-Fe bond directions, and noticeably suppress the superconducting gap, in contrast with the recent experimental and theoretical findings in other iron pnictides. Furthermore, vortices appear to accumulate on twin boundaries, consistent with the degraded superconductivity there. The variation in superconductivity 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.

Tailoring Phthalocyanine Metalation Reaction by Quantum Size Effect

We here report our experimental study of the quantum size effect modulated metalation reaction of phthalocyanine (H(2)Pc) by low temperature scanning tunneling microscopy. When iron atoms were deposited onto Pb(111) thin films (2-5 nm thick) precovered by a self-assembled H(2)Pc monolayer, a surface metalation reaction to iron phthalocyanine (FePc) was observed. The amount of the FePc products was found to change prominently whenever the film thickness varies by one atomic layer and exhibits thickness-dependent oscillatory behavior. We show that the oscillation can be well-understood by the quantum size effect in the Pb thin films. The present study gives direct proof for tailoring a surface chemical reaction by quantum confinement.

Interaction-induced quantum anomalous Hall phase in (111) bilayer ofLaCoO3

In the present paper, the Gutzwiller density functional theory (LDA+G) has been applied to study a bilayer system of LaCoO

Evidence for Berezinskii-Kosterlitz-Thouless transition in atomically flat two-dimensional Pb superconducting films

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