Richard C. Hatch

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.

Electron-Phonon Coupling in Crystalline Pentacene Films

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 lambda=0.36+/-0.05 and an effective Einstein energy of omega{E}=11+/-4 meV. From omega{E} it is inferred that dominant contributions to the e-p effects observed in angle-resolved photoemission spectroscopy come from intermolecular vibrations. Based on the experimental data for lambda we extract an effective Peierls coupling value of g{eff}=0.55. The e-p coupling narrows the highest occupied molecular orbital bandwidth by 15+/-8% between 75 and 300 K.

Electron–phonon coupling in quasi-free-standing graphene

Quasi-free-standing monolayer graphene can be produced by intercalating species like oxygen or hydrogen between epitaxial graphene and the substrate crystal. If the graphene was indeed decoupled from the substrate, one would expect the observation of a similar electronic dispersion and many-body effects, irrespective of the substrate and the material used to achieve the decoupling. Here we investigate the electron-phonon coupling in two different types of quasi-free-standing monolayer graphene: decoupled from SiC via hydrogen intercalation and decoupled from Ir via oxygen intercalation. The two systems show similar overall behaviours of the self-energy and a weak renormalization of the bands near the Fermi energy. The electron-phonon coupling is found to be so weak that it renders the precise determination of the coupling constant λ through renormalization difficult. The estimated value of λ is 0.05(3) for both systems.

Robust surface doping of Bi2Se3 by rubidium intercalation.

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.

Phase Separation and Bulk p‐n Transition in Single Crystals of Bi2Te2Se Topological Insulator

Bismuth chalcogenide (Bi 2 Ch 3 ) alloys are among the most extensively studied and commonly used thermoelectric materials. [ 1 ] They are also currently of great interest in condensedmatter physics as prototypical three-dimensional topological insulators (TIs), due to the existence of stable and topologically protected Dirac like states on the surface. [ 2 , 3 ] Shubnikov– de Haas and weak-fi eld Hall anomalies show a substantially enhanced surface current and mobility of the surface states over bulk Bi 2 Te 3 values. [ 4 ] However, it is challenging to investigate the charge-transport characteristics of the surface states directly, as the charge transport is dominated by the bulk properties. [ 3–5 ] Most studies of TIs in the Bi 2 Ch 3 family have focused on binary Bi 2 Se 3 and Bi 2 Te 3 systems. Both compounds are semiconductors, but they display high bulk conductivities due to intrinsic defects. [ 4–6 ] A conversion from nto p-type conduction in Bi 2 Te 3 thin fi lms is observed when the growth condition changes. [ 5b ] Recently, the ternary compound Bi 2 Te 2 Se has been suggested to be the best material for studies of the surface transport due to its large bulk resistivity. [ 3 , 6–8 ] However, controlling the bulk conductivity is diffi cult, even for Bi 2 Te 2 Se, due to unintentional doping by crystal defects. [ 6 , 8 ] Here we show that the diffi culty of making high-quality Bi 2 Te 2 Se single crystals originates from the internal features of the specifi c solid-state composition and phase separation in Bi 2 Te 2 Se. Bi 2 Ch 3 has the tetradymite-type rhombohedral structure, which can be described by the R-3m space group. The lattice can be regarded as a hexagonal layered structure in which the layers stack in the sequence of Ch 1 -Bi-Ch 2 -Bi-Ch 1 , where Ch is Te or Se in the present study. Each unit cell consists of three of these fi velayer groups, which are connected by bonds with a high degree of van der Waals character between the Ch 1 -Ch 1 layers. Early phase-diagram studies showed that Bi 2 Te x Se y ( x + y = 3) compounds form continuous solid solutions at temperatures above 500 ° C; however, the crystal structures tend to be ordered at

The electronic structure of clean and adsorbate-covered Bi2Se3: an angle-resolved photoemission study

Angle-resolved photoelectron spectroscopy is used for a detailed study of the electronic structure of the topological insulator Bi2Se3. 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.

High-temperature behavior of supported graphene: Electron-phonon coupling and substrate-induced doping

One of the salient features of graphene is the very high carrier mobility that implies tremendous potential for use in electronic devices. Unfortunately, transport measurements find the expected high mobility only in freely suspended graphen. When supported on a surface, graphene shows a strongly reduced mobility, and an especially severe reduction for temperatures above 200 K. A temperature-dependent mobility reduction could be explained by scattering of carriers with phonons, but this is expected to be weak for pristine, weakly-doped graphene. The mobility reduction has therefore been ascribed to the interaction with confined ripples or substrate phonons. Here we study the temperature-dependent electronic structure of supported graphene by angle-resolved photoemission spectroscopy, a technique that can reveal the origin of the phenomena observed in transport measurements. We show that the electron-phonon coupling for weakly-doped, supported graphene on a metal surface is indeed extremely weak, reaching the lowest value ever reported for any material. However, the temperature-dependent dynamic interaction with the substrate leads to a complex and dramatic change in the carrier type and density that is relevant for transport. Using ab initio molecular dynamics simulations, we show that these changes in the electronic structure are mainly caused by fluctuations in the graphene-substrate distance.

Comparison of acid- and non-acid-based surface preparations of Nb-doped SrTiO3 (001)

High-resolution angle-resolved photoemission spectroscopy (ARPES) and x-ray photoelectron spectroscopy (XPS) were used to study the relative effectiveness of acid- and non-acid-based surface preparations of Nb-doped SrTiO3 (STO) single crystals. ARPES measurements show that boiling STO in deionized water produces surfaces of similar quality to those etched with buffered HF (Kawasaki method), or HCl/HNO3 (Arkansas method). XPS measurements indicate this water-based surface preparation is more effective than acid-based methods at removing SrOx crystallites and leaving the surface TiO2-terminated.

Combined in-situ photoemission spectroscopy and density functional theory of the Sr Zintl template for oxide heteroepitaxy on Si(001)

Half-monolayer Sr on Si(001) is a Zintl template necessary for epitaxial growth of SrTiO3 on Si(001). The authors investigate the reconstruction in the atomic and electronic structure of Si(001) induced by sub-monolayer Sr deposition using in-situ x-ray/ultraviolet photoemission spectroscopy and density functional theory. Sub-monolayer Sr is deposited on Si(001) using molecular beam epitaxy and the structural evolution of the surface is monitored using reflection high-energy electron diffraction. Experimentally, the authors find that the ionization energy of Si(001) decreases as a function of Sr coverage from 4.82 eV for pure Si(001) to 3.97 eV for half-monolayer Sr on Si(001) due to charge transfer from Sr to Si. They calculate the ionization energy for sub-monolayer Sr on Si(001) by considering several atomistic models and find good agreement with experiment. The authors clearly establish the Zintl character of the template by measuring the surface core level shifts of Si(001) and half-monolayer Sr/Si(0...