A. Varykhalov

Giant Rashba splitting in graphene due to hybridization with gold

Graphene in spintronics is predominantly considered for spin current leads of high performance due to weak intrinsic spin-orbit coupling of the graphene π electrons. Externally induced large spin-orbit coupling opens the possibility of using graphene in active elements of spintronic devices such as the Das-Datta spin field-effect transistor. Here we show that Au intercalation at the graphene-Ni interface creates a giant spin-orbit splitting (~100 meV) of the graphene Dirac cone up to the Fermi energy. Photoelectron spectroscopy reveals the hybridization with Au 5d states as the source for this giant splitting. An ab initio model of the system shows a Rashba-split spectrum around the Dirac point of graphene. A sharp graphene-Au interface at the equilibrium distance accounts for only ~10 meV spin-orbit splitting and enhancement is due to the Au atoms in the hollow position that get closer to graphene and do not break the sublattice symmetry.Graphene in spintronics [1] has so far primarily meant spin current leads of high performance because the intrinsic spin-orbit coupling of its π electrons is very weak [2–4]. If a large spin-orbit coupling could be created by a proximity effect, the material could also form active elements of a spintronic device such as the Das-Datta spin field-effect transistor [5], however, metal interfaces often compromise the band dispersion of massless Dirac fermions [6]. Our measurements show that Au intercalation at the graphene-Ni interface creates a giant spin-orbit splitting (∼ 100 meV) in the graphene Dirac cone up to the Fermi energy. Photoelectron spectroscopy reveals hybridization with Au5d states as the source for the giant spin-orbit splitting. An ab initio model of the system shows a Rashba-split dispersion with the analytically predicted gapless band topology around the Dirac point of graphene and indicates that a sharp graphene-Au interface at equilibrium distance will account for only ∼ 10 meV spin-orbit splitting. The ab initio calculations suggest an enhancement due to Au atoms that get closer to the graphene and do not violate the sublattice symmetry.

Dirac cone protected by non-symmorphic symmetry and three-dimensional Dirac line node in ZrSiS

Materials harbouring exotic quasiparticles, such as massless Dirac and Weyl fermions, have garnered much attention from physics and material science communities due to their exceptional physical properties such as ultra-high mobility and extremely large magnetoresistances. Here, we show that the highly stable, non-toxic and earth-abundant material, ZrSiS, has an electronic band structure that hosts several Dirac cones that form a Fermi surface with a diamond-shaped line of Dirac nodes. We also show that the square Si lattice in ZrSiS is an excellent template for realizing new types of two-dimensional Dirac cones recently predicted by Young and Kane. Finally, we find that the energy range of the linearly dispersed bands is as high as 2 eV above and below the Fermi level; much larger than of other known Dirac materials. This makes ZrSiS a very promising candidate to study Dirac electrons, as well as the properties of lines of Dirac nodes.

(pi-pi) electronic order in iron arsenide superconductors

The distribution of valence electrons in metals usually follows the symmetry of the underlying ionic lattice. Modulations of this distribution often occur when those electrons are not stable with respect to a new electronic order, such as spin or charge density waves. Electron density waves have been observed in many families of superconductors, and are often considered to be essential for superconductivity to exist. Recent measurements seem to show that the properties of the iron pnictides are in good agreement with band structure calculations that do not include additional ordering, implying no relation between density waves and superconductivity in these materials. Here we report that the electronic structure of Ba1-xKxFe2As2 is in sharp disagreement with those band structure calculations, and instead reveals a reconstruction characterized by a (π, π) wavevector. This electronic order coexists with superconductivity and persists up to room temperature (300 K).

Tolerance of topological surface states towards magnetic moments: Fe on Bi2Se3.

We study the effect of Fe impurities deposited on the surface of the topological insulator Bi(2)Se(3) by means of core-level and angle-resolved photoelectron spectroscopy. The topological surface state reveals surface electron doping when the Fe is deposited at room temperature and hole doping with increased linearity when deposited at low temperature (~8 K). We show that in both cases the surface state remains intact and gapless, in contradiction to current belief. Our results suggest that the surface state can very well exist at functional interfaces with ferromagnets in future devices.

Ir(111) surface state with giant Rashba splitting persists under graphene in air.

Spin currents which allow for a dissipationless transport of information can be generated by electric fields in semiconductor heterostructures in the presence of a Rashbatype spin-orbit coupling. The largest Rashba effects occur for electronic surface states of metals but these cannot exist but under ultrahigh vacuum conditions. Here, we reveal a giant Rashba effect (αR ≈ 1.5 · 10 −10 eVm) on a surface state of Ir(111). We demonstrate that its spin splitting and spin polarization remain unaffected when Ir is covered with graphene. The graphene protection is, in turn, sufficient for the spinsplit surface state to survive in ambient atmosphere. We discuss this result along with evidences for a topological protection of the surface state.

Negligible Surface Reactivity of Topological Insulators Bi2Se3 and Bi2Te3 towards Oxygen and Water

The long-term stability of functional properties of topological insulator materials is crucial for the operation of future topological insulator based devices. Water and oxygen have been reported to be the main sources of surface deterioration by chemical reactions. In the present work, we investigate the behavior of the topological surface states on Bi2X3 (X = Se, Te) by valence-band and core level photoemission in a wide range of water and oxygen pressures both in situ (from 10(-8) to 0.1 mbar) and ex situ (at 1 bar). We find that no chemical reactions occur in pure oxygen and in pure water. Water itself does not chemically react with both Bi2Se3 and Bi2Te3 surfaces and only leads to slight p-doping. In dry air, the oxidation of the Bi2Te3 surface occurs on the time scale of months, in the case of Bi2Se3 surface of cleaved crystal, not even on the time scale of years. The presence of water, however, promotes the oxidation in air, and we suggest the underlying reactions supported by density functional calculations. All in all, the surface reactivity is found to be negligible, which allows expanding the acceptable ranges of conditions for preparation, handling and operation of future Bi2X3-based devices.

Momentum dependence of the superconducting gap in Ba 1 − x K x Fe 2 As 2

D. V. Evtushinsky,1 D. S. Inosov,1,2 V. B. Zabolotnyy,1 A. Koitzsch,1 M. Knupfer,1 B. Buchner,1 M. S. Viazovska,3 G. L. Sun,2 V. Hinkov,2 A. V. Boris,2,4 C. T. Lin,2 B. Keimer,2 A. Varykhalov,5 A. A. Kordyuk,1,6 and S. V. Borisenko1 1Institute for Solid State Research, IFW Dresden, P.O. Box 270116, D-01171 Dresden, Germany 2Max-Planck-Institute for Solid State Research, Heisenbergstrasse 1, D-70569 Stuttgart, Germany 3Max-Planck-Institute for Mathematics, Vivatsgasse 7, 53111 Bonn, Germany 4Department of Physics, Loughborough University, Loughborough, LE11 3TU. United Kingdom 5BESSY GmbH, Albert-Einstein-Strasse 15, 12489 Berlin, Germany 6Institute of Metal Physics, National Academy of Sciences of Ukraine, 03142 Kyiv, Ukraine Received 24 September 2008; revised manuscript received 30 December 2008; published 17 February 2009

Observation of quantum-tunnelling-modulated spin texture in ultrathin topological insulator Bi2Se3 films

Understanding the spin-texture behaviour of boundary modes in ultrathin topological insulator films is critically essential for the design and fabrication of functional nanodevices. Here, by using spin-resolved photoemission spectroscopy with p-polarized light in topological insulator Bi2Se3 thin films, we report tunnelling-dependent evolution of spin configuration in topological insulator thin films across the metal-to-insulator transition. We report a systematic binding energy- and wavevector-dependent spin polarization for the topological surface electrons in the ultrathin gapped-Dirac-cone limit. The polarization decreases significantly with enhanced tunnelling realized systematically in thin insulating films, whereas magnitude of the polarization saturates to the bulk limit faster at larger wavevectors in thicker metallic films. We present a theoretical model that captures this delicate relationship between quantum tunnelling and Fermi surface spin polarization. Our high-resolution spin-based spectroscopic results suggest that the polarization current can be tuned to zero in thin insulating films forming the basis for a future spin-switch nanodevice.

Strength of correlation effects in the electronic structure of iron.

The strength of electronic correlation effects in the spin-dependent electronic structure of ferromagnetic bcc Fe(110) has been investigated by means of spin and angle-resolved photoemission spectroscopy. The experimental results are compared to theoretical calculations within the three-body scattering approximation and within the dynamical mean-field theory, together with one-step model calculations of the photoemission process. This comparison indicates that the present state of the art many-body calculations, although improving the description of correlation effects in Fe, give too small mass renormalizations and scattering rates thus demanding more refined many-body theories including nonlocal fluctuations.

Anisotropic two-dimensional electron gas at SrTiO3(110)

Significance Although still in its infancy, electronics based on all-oxide materials is a rapidly developing field, and strontium titanate is its key player. For this area to thrive, an atomic-scale control and understanding of the materials’ surfaces and interfaces needs to be achieved. A SrTiO3 crystal with (110) orientation automatically forms an overlayer that is more insulating than the bulk and chemically less reactive, akin to the native SiO2 on conventional wafer. With appropriate doping a two-dimensional electron gas (2DEG) forms underneath the SrTiO3(110) surface. This (110) 2DEG is very different from (001): The effective mass here depends on the quantum number, and a completely flat band can be realized. Such a flat band bears good prospects for, among others, magnetism and thermoelectricity. Two-dimensional electron gases (2DEGs) at oxide heterostructures are attracting considerable attention, as these might one day substitute conventional semiconductors at least for some functionalities. Here we present a minimal setup for such a 2DEG––the SrTiO3(110)-(4 × 1) surface, natively terminated with one monolayer of tetrahedrally coordinated titania. Oxygen vacancies induced by synchrotron radiation migrate underneath this overlayer; this leads to a confining potential and electron doping such that a 2DEG develops. Our angle-resolved photoemission spectroscopy and theoretical results show that confinement along (110) is strikingly different from the (001) crystal orientation. In particular, the quantized subbands show a surprising “semiheavy” band, in contrast with the analog in the bulk, and a high electronic anisotropy. This anisotropy and even the effective mass of the (110) 2DEG is tunable by doping, offering a high flexibility to engineer the properties of this system.