Shining Bao

Coexistence of the topological state and a two-dimensional electron gas on the surface of Bi 2 Se 3

The surface of a topological insulator plays host to an odd number of linearly-dispersing Dirac fermions, protected against back-scattering by time-reversal symmetry. Such characteristics make these materials attractive not only for studying a range of fundamental phenomena in both condensed matter and particle physics, but also for applications ranging from spintronics to quantum computation. Here, we show that the single Dirac cone comprising the topological state of the prototypical topological insulator Bi(2)Se(3) can co-exist with a two-dimensional electron gas (2DEG), a cornerstone of conventional electronics. Creation of the 2DEG is tied to a surface band-bending effect, which should be general for narrow-gap topological insulators. This leads to the unique situation where a topological and a non-topological, easily tunable and potentially superconducting, metallic state are confined to the same region of space.

Large Tunable Rashba Spin Splitting of a Two-Dimensional Electron Gas in Bi2Se3

We report a Rashba spin splitting of a two-dimensional electron gas in the topological insulator Bi(2)Se(3) from angle-resolved photoemission spectroscopy. We further demonstrate its electrostatic control, and show that spin splittings can be achieved which are at least an order-of-magnitude larger than in other semiconductors. Together these results show promise for the miniaturization of spintronic devices to the nanoscale and their operation at room temperature.

Stability of theBi2Se3(111) topological state: Electron-phonon and electron-defect scattering

The electron dynamics of the topological surface state on Bi2Se3(111) is investigated by temperature-dependent angle-resolved photoemission. The electron-phonon coupling strength is determined in a spectral region for which only intraband scattering involving the topological surface band is possible. The electron-phonon coupling constant is found to be lambda=0.25(5), more than an order of magnitude higher than the corresponding value for intraband scattering in the noble metal surface states. The stability of the topological state with respect to surface irregularities was also tested by introducing a small concentration of surface defects via ion bombardment. It is found that, in contrast to the bulk states, the topological state can no longer be observed in the photoemission spectra and this cannot merely be attributed to surface defect-induced momentum broadening.

Electronic structures of CuPc on a Ag(110) surface

Copper phthalocyanines (CuPc) on a Ag(110) surface have been studied by ultraviolet photoemission spectroscopy (UPS). On depositing CuPc organic films, the features from the substrate 3d valence fade and four new features corresponding to the adsorbed molecules emerge at 1.68, 4.45, 6.36 and 9.20 eV below the Fermi level. These features shift in binding energy with increasing thickness of the organic films. In the case of a monolayer, angle-resolved UPS measurements suggest that the molecular plane is nearly parallel to the substrate. Further theoretical calculation indicates that the adsorption of CuPc on a hollow site is the most favourable configuration, and the separation between the adsorbate and the substrate is about 2.7 A.

Study on the interaction between tetracene and Cu(110) surface

The electronic structure of tetracene on Cu (110) surface has been studied by using ultraviolet photoemission spectroscopy (UPS). The emission features from the organic molecule are located from 1 to 10 eV below the Fermi level, and they shift in binding energy with increasing the coverage of the organic material. For the surface with multilayer of tetracene, six well-resolved features were found at 1.90, 3.40, 4.70, 5.95, 6.95, and 9.15 eV below the Fermi level, respectively. On the surface with a lower coverage of tetracene, angle-resolved UPS measurements suggest that the molecular plane is parallel to the substrate. Density functional theory calculation confirms the flat-lying adsorption mode and shows that the tetracene molecule prefers to be adsorbed on the long bridge site with its long axis in the [110] azimuth.

Monolayer structure of tetracene on Cu (100) surface: parallel geometry.

The geometrical arrangement of tetracene on Cu (100) surface at monolayer coverage is studied by using scanning tunneling microscopy measurement and density functional theory (DFT) calculations. Tetracene molecule is found to be oriented with its molecular plane parallel to the substrate surface, and no perpendicular geometry is observed at this coverage. The molecule is aligned either in the [011] or [011] direction due to the fourfold symmetry of the Cu (100) surface. DFT calculations show that the molecule with the flat-lying mode has larger adsorption energy than that with the upright standing mode, indicating that the former is the more stable structure. With the flat-lying geometry, the carbon atoms prefer to be placed between surface Cu atoms. The molecular center prefers to be located at the bridge site between two nearest surface Cu atoms.

Interfaces between 8-hydroxyquinoline aluminum and cesium as affected by their deposition sequences

Abstract The electronic structures of tris(8-hydroxyquinoline) aluminum (Alq3) deposited on clean or Cs pre-covered Ag substrates have been studied by ultraviolet photoelectron spectroscopy. Interface of Cs deposited on Alq3 has also been prepared for comparison. For a low coverage of Cs on Ag, deposition of Alq3 on top of the Cs cannot induce any new electronic features. The low work function of the Cs reduces the barrier height of electron injection at the Alq3/Cs/Ag contact to 0.3 eV, as compared to 1.6 eV for the Alq3/Ag contact. For high Cs coverage, the Cs may diffuse as neutral atoms and undergo oxidation into the Alq3 layer and form a new gap state at 0.9 eV above the Alq3 highest occupied state, which is the same as that of Cs deposited on the Alq3.

Investigation on the orderly growth of thick zinc phthalocyanine films on Ag"100… surface

The growth of zinc phthalocyanine (ZnPc) on Ag(100) surface from monolayer to multilayer was investigated by low-energy electron diffraction, x-ray diffraction, and high-resolution electron energy loss spectroscopy (HREELS). At monolayer coverage, ZnPc molecules form an ordered film with molecular planes parallel to the substrate. The same structure is maintained as the film thickness increases. HREELS analysis shows that intermolecular π-π interaction dominates during the film growth from monolayer to multilayer. The π-d interaction between the adsorbates and the substrate is only applicable in the first adlayer. Stronger intermolecular-layer interaction is observed at higher coverages.

Interface between poly (9,9-dioctylfluorene) and alkali metals: cesium, potassium, sodium, and lithium

In this article we study the interface between poly (9,9-dioctylfluorene) (PFO) and different alkali metals (Cs, K, Na, and Li) by photoelectron spectroscopy. The low work-function alkali metals led to low or no electron injection barrier at the PFO interface. From the ultraviolet photoelectron spectroscopy, alteration of electronic structures upon Cs, K, Na, or Li doping into PFO represented a charge transfer process among them. Two new gap states known as bipolaron states were found above the highest-occupied molecular orbital of PFO. Variations in the intensity and feature of these gap states with increasing coverage of the alkali metals were correlated with changes of Cu200a1s shakeup peaks acquired from x-ray photoelectron spectroscopy. From the deduced energy level diagram, it is suggested that the new gap states may reduce the radiative recombination of holes and electrons in the polymer light-emitting devices. Films exposed either to residual gases at a pressure of 2.0×10−9 mbar for 3 h or to small am...

Surface states on a topologically nontrivial semimetal: The case of Sb(110)

The electronic structure of Sb(110) is studied by angle-resolved photoemission spectroscopy and first-principles calculations, revealing several electronic surface states in the projected bulk band gaps around the Fermi energy. The dispersion of the states can be interpreted in terms of a strong spin-orbit splitting. The bulk band structure of Sb has the characteristics of a strong topological insulator with a Z2 invariant ν0 = 1. This puts constraints on the existence of metallic surface states and the expected topology of the surface Fermi contour. However, bulk Sb is a semimetal, not an insulator, and these constraints are therefore partly relaxed. This relation of bulk topology and expected surface-state dispersion for semimetals is discussed.