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.

Exposing the (111) surface of the topological insulator Bi(2)Se(3) to carbon monoxide results in strong shifts of the features observed in angle-resolved photoemission. The behavior is very similar to an often reported "aging" effect of the surface, and it is concluded that this aging is most likely due to the adsorption of rest gas molecules. The spectral changes are also similar to those recently reported in connection with the adsorption of the magnetic adatom Fe. All spectral changes can be explained by a simultaneous confinement of the conduction band and valence band states. This is possible only because of the unusual bulk electronic structure of Bi(2)Se(3). The valence band quantization leads to spectral features which resemble those of a band gap opening at the Dirac point.

Using angle-resolved photoelectron spectroscopy and ab-initio GW calculations, we unambiguously show that the widely investigated three-dimensional topological insulator Bi2Se3 has a direct band gap at the Γ point. Experimentally, this is shown by a three-dimensional band mapping in large fractions of the Brillouin zone. Theoretically, we demonstrate that the valence band maximum is located at the Γ point only if many-body effects are included in the calculation. Otherwise, it is found in a high-symmetry mirror plane away from the zone center. PACS numbers: 71.15.m, 71.20.b, 71.70.Ej, Bismuth selenide has been widely studied for many years for its potential applications in optical recording systems [1], photoelectrochemical [2] and thermoelectric devices [3,4], and is nowadays commonly used in refrigeration and power generation. Recently, it has attracted increasing interest after its identification as a prototypical topological insulator (TI) [5,6]. Its surface electronic structure consists of a single Dirac cone around the surface Brillouin zone (SBZ) centreΓ, with the Dirac point (DP) placed closely above the bulk valence band states. In order to exploit the multitude of interesting phenomena associated with the topological surface states [7,8], it is necessary to access the topological transport regime, in which the chemical potential is near the DP and simultaneously in the absolute bulk band gap. Due to the close proximity of the DP and the bulk valence states at Γ, this is only possible if there are no other valence states in Bi 2 Se 3 with energies close to or higher than the DP. Therefore, it is crucial to establish if the bulk valence band maximum (VBM) in bismuth selenide is placed at Γ (and thus projected out toΓ) or at some other position within the Brillouin zone (BZ). As the bulk conduction band minimum (CBM) is undisputedly located at Γ [9,10], the question about the VBM location is identical to the question about the nature of the fundamental band gap in this TI, direct or indirect.The nature of the bulk band gap is thus of crucial importance for the possibility of exploiting the topological surface states in transport, but the position of the VBM in band structure calculations remains disputed. In a linearized muffin-tin orbital method (LMTO) calculation within the local density approximation (LDA), the VBM was found at the Γ point, implying that Bi 2 Se 3 is a direct-gap semiconductor [11]. Contrarily, by employing the full-potential linearized augmented-plane-wave method (FLAPW) within the generalized gradient approximation (GGA), the authors of Ref. 9 have found the VBM to be located on the Z − F line of the BZ, which is lying in the mirror plane. Similar results have been obtained in Ref. 12 with the plane-wave pseudopotential method (PWP) within the LDA. Various density functional theory (DFT) calculations of the surface band structure of Bi 2 Se 3 [5,7,13,14] also indicate that the VBM of bulk bismuth selenide is not located at the BZ center. The inclusion of many-body effects within the G...

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 λ = 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. PACS numbers: 73.20.At,71.70.Ej, Topological insulators are one of of the most intriguing subjects of current condensed matter physics [1-3]. Despite of their insulating bulk, these materials support metallic edge and surface states with an unconventional spin texture [4,5], electron dynamics [6, 7] and stability. Exploiting these properties is the key to several applications, e. g. in spintronics and quantum computing. Moreover, several novel physical phenomena are predicted in connection with the topological states [8-10].The stable existence of a gap-closing surface state [11][12][13] is a property derived from the bulk band structure of a topological insulator, rather than a mere coincidence. The topological state is also stable in a dynamical sense. A hallmark is the absence of back-scattering near nonmagnetic defects. Edge states in the quantum spin Hall effect, a two-dimensional topological insulator, are completely protected from (elastic) scattering [11] whereas the scattering phase space is strongly reduced for surface states on a three dimensional topological insulator [7,14], preventing localization by weak disorder.The stability of the topological state is essential for many phenomena and applications, however, only a few experimental studies have addressed this issue. Experimental measurements using angle resolved photoemission spectroscopy (ARPES) and scanning tunnelling microscopy (STM) have shown that the topological surface states are robust against a small number of adsorbates [15,16] and detectable at room temperature [15], but other results question their stability with respect to surface scattering processes [17]. Here we determine the electron-phonon (el − ph) coupling strength on the topological insulator Bi 2 Se 3 (111) [18,19]. In the absence of defects, el−ph scattering can be expected to be the dominant scattering mechanism at finite temperature and it is therefore of exceptional importance for any application.We concentrate on the spectral region in which only the topological state exists and thus only intraband scattering is possible and we show that while the el − ph coupling constant λ is of moderate size, it is surprisi...

Rubidium adsorption on the surface of the topological insulator Bi(2)Se(3) is found to induce a strong downward band bending, leading to the appearance of a quantum-confined two-dimensional electron gas state (2DEG) in the conduction band. The 2DEG shows a strong Rashba-type spin-orbit splitting, and it has previously been pointed out that this has relevance to nanoscale spintronics devices. The adsorption of Rb atoms, on the other hand, renders the surface very reactive, and exposure to oxygen leads to a rapid degrading of the 2DEG. We show that intercalating the Rb atoms, presumably into the van der Waals gaps in the quintuple layer structure of Bi(2)Se(3), drastically reduces the surface reactivity while not affecting the promising electronic structure. The intercalation process is observed above room temperature and accelerated with increasing initial Rb coverage, an effect that is ascribed to the Coulomb interaction between the charged Rb ions. Coulomb repulsion is also thought to be responsible for a uniform distribution of Rb on the surface.

Angle-resolved photoelectron spectroscopy is used for a detailed study of the electronic structure of the topological insulator Bi 2 Se 3 . Nominally stoichiometric and calcium-doped samples were investigated. The pristine surface shows the topological surface state in the bulk band gap. As time passes, the Dirac point moves to higher binding energies, indicating an increasingly strong downward bending of the bands near the surface. This time-dependent band bending is related to a contamination of the surface and can be accelerated by intentionally exposing the surface to carbon monoxide and other species. For a sufficiently strong band bending, additional states appear at the Fermi level. These are interpreted as quantised conduction band states. For large band bendings, these states are found to undergo a strong Rashba splitting. The formation of quantum well states is also observed for the valence band states. Different interpretations of similar data are also discussed.

The electron-phonon (e-p) interaction in Pentacene (Pn) films grown on Bi (001) was investigated using photoemission spectroscopy. The spectra reveal thermal broadening from which we determine an e-p mass enhancement factor of λ = 0.36 ± 0.05 and an effective Einstein energy of ω E = 11 ± 4 meV. From ω E it is inferred that dominant contributions to the e-p effects observed in ARPES comes from intermolecular vibrations. Based on the experimental data for λ we extract an effective Peierls coupling value of g eff = 0.55. The e-p coupling narrows the HOMO bandwidth by 15 ± 8% between 75 K and 300 K. 79.60.Fr, 71.20.Rv, 72.80.Le, 73.50.Gr Published in Phys. Rev. Lett. 104, 047601 (2010) For decades theorists have been working to explain the surprisingly high carrier mobilities and temperature dependence of organic semiconductors (OSCs). Pioneering work toward understanding the carrier transport mechanism in OSCs was done by

Separation of ordered and disordered phases is found along the growth axis of a Bi2Te2Se crystal rod. A high bulk resistivity is found in phase II, which is an ideal candidate for investigation of the transport properties of the surface states. The bulk insulating feature of phase II is caused by the opposing effects of different types of defects.