E. V. Chulkov

University of Duisburg-Essen, Essen, North Rhine-Westphalia, Germany

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Publications (403)1281.45 Total impact

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    ABSTRACT: We present a comprehensive ab initio study of structural, electronic, lattice dynamical and electron-phonon coupling properties of the Bi(111) surface within density functional perturbation theory. Relativistic corrections due to spin-orbit coupling are consistently taken into account. As calculations are carried out in a periodic slab geometry, special attention is given to the convergence with respect to the slab thickness. Although the electronic structure of Bi(111) thin films varies significantly with thickness, we found that the lattice dynamics of Bi(111) is quite robust and appears converged already for slabs as thin as 6 bilayers. Changes of interatomic couplings are confined mostly to the first two bilayers, resulting in super-bulk modes with frequencies higher than the optic bulk spectrum, and in an enhanced density of states at lower frequencies for atoms in the first bilayer. Electronic states of the surface band related to the outer part of the hole Fermi surfaces exhibit a moderate electron-phonon coupling of about 0.45, which is larger than the coupling constant of bulk Bi. States at the inner part of the hole surface as well as those forming the electron pocket close to the zone center show much increased couplings due to transitions into bulk projected states near Gamma_bar. For these cases, the state dependent Eliashberg functions exhibit pronounced peaks at low energy and strongly deviate in shape from a Debye-like spectrum, indicating that an extraction of the coupling strength from measured electronic self-energies based on this simple model is likely to fail.
    09/2014;
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    ABSTRACT: Angular resolved photoemission spectroscopy in combination with ab initio calculations show that trace amounts of carbon doping of the Bi_{2}Se_{3} surface allows the controlled shift of the Dirac point within the bulk band gap. In contrast to expectation, no Rashba-split two-dimensional electron gas states appear. This unique electronic modification is related to surface structural modification characterized by an expansion of the top Se-Bi spacing of ≈11% as evidenced by surface x-ray diffraction. Our results provide new ways to tune the surface band structure of topological insulators.
    Physical Review Letters 09/2014; 113:116802. · 7.73 Impact Factor
  • Phys. Rev. Lett. 09/2014; 113(11):116802.
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    ABSTRACT: The electron–phonon coupling parameters in the vicinity of the point, , for electronic quantum well states in epitaxial lead films on a Si(1 1 1) substrate are measured using 5, 7 and 12 ML films and femtosecond laser photoemission spectroscopy. The values in the range of 0.6–0.9 were obtained by temperature-dependent line width analysis of occupied quantum well states and found to be considerably smaller than the momentum averaged electron–phonon coupling at the Fermi level of bulk lead, (λ = 1.1–1.7). The results are compared to density functional theory calculations of the lead films with and without interfacial stress. It is shown that the discrepancy can not be explained by means of confinement effects or simple structural modifications of the Pb films and, thus, is attributed to the influence of the substrate on the Pb electronic and vibrational structures.
    Journal of Physics Condensed Matter 08/2014; 26(35):352001. · 2.22 Impact Factor
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    ABSTRACT: By means of relativistic density functional theory (DFT) calculations we study electron band structure of the topological insulator (TI) Bi$_2$Se$_3$ thin films deposited on the ferromagnetic insulator (FMI) EuS substrate. In the Bi$_2$Se$_3$/EuS heterostructure, the gap opened in the spectrum of the topological state has a hybridization character and is shown to be controlled by the Bi$_2$Se$_3$ film thickness, while magnetic contribution to the gap is negligibly small. We also analyzed the effect of Eu doping on the magnetization of the Bi$_2$Se$_3$ film and demonstrated that the Eu impurity induces magnetic moments on neighboring Se and Bi atoms an order of magnitude larger than the substrate-induced moments. Recent magnetic and magneto-transport measurements in EuS/Bi$_2$Se$_3$ heterostructure are discussed.
    07/2014;
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    ABSTRACT: We apply both analytical and ab-initio methods to explore heterostructures composed of a threedimensional topological insulator (3D TI) and an ultrathin normal insulator (NI) overlayer as a proof ground for the principles of the topological phase engineering. Using the continual model of a semi-infinite 3D TI we study the surface potential (SP) effect caused by an attached ultrathin layer of 3D NI on the formation of topological bound states at the interface. The results reveal that spatial profile and spectrum of these near-surface states strongly depend on both the sign and strength of the SP. Using ab-initio band structure calculations to take materials specificity into account, we investigate the NI/TI heterostructures formed by a single tetradymite-type quintuple or septuple layer block and the 3D TI substrate. The analytical continuum theory results relate the near-surface state evolution with the SP variation and are in good qualitative agreement with those obtained from density-functional theory (DFT) calculations. We predict also the appearance of the quasi-topological bound state on the 3D NI surface caused by a local band gap inversion induced by an overlayer.
    06/2014;
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    ABSTRACT: We have clarified that a topological insulator, Bi2Te2Se, shows two surface states with gigantic Rashba-type spin-splitting located at a binding energy deeper than the topological surface state. The magnitude of the Rashba parameter, as well as the momentum splitting, is found to be large enough to realize a number of nanometer-sized spintronic devices. This novel finding paves the way to studies of gigantic Rashba systems that are suitable for future spintronic applications.
    New Journal of Physics 06/2014; 16(6):065016. · 4.06 Impact Factor
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    ABSTRACT: Spintronics, or spin electronics, is aimed at efficient control and manipulation of spin degrees of freedom in electron systems. To comply with demands of nowaday spintronics, the studies of electron systems hosting giant spin-orbit-split electron states have become one of the most important directions providing us with a basis for desirable spintronics devices. In construction of such devices, it is also tempting to involve graphene, which has attracted great attention because of its unique and remarkable electronic properties and was recognized as a viable replacement for silicon in electronics. In this case, a challenging goal is to make graphene Dirac states spin-polarized. Here, we report on absolutely new promising pathway to create spin-polarized Dirac states based on coupling of graphene and polar-substrate surface states with giant Rashba-type spin-splitting. We demonstrate how the spin-helical Dirac states are formed in graphene deposited on the surface of BiTeCl. This coupling induces spin separation of the originally spin-degenerate graphene states and results in fully helical in-plane spin polarization of the Dirac electrons.
    06/2014;
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    ABSTRACT: Using a first-principles Green's function approach we study magnetic properties of the magnetic binary tetradymite chalcogenides Bi2Se3, Bi2Te3, and Sb2Te3. The magnetic coupling between transition-metal impurities is long range, extends beyond a quintuple layer, and decreases with increasing number of d electrons per 3d atom. We find two main mechanisms for the magnetic interaction in these materials: the indirect exchange interaction mediated by free carriers and the indirect interaction between magnetic moments via chalcogen atoms. The calculated Curie temperatures of these systems are in good agreement with available experimental data. Our results provide deep insight into exchange interactions in magnetic binary tetradymite chalcogenides and open a way to design new materials for promising applications.
    Physical Review B 04/2014; 89:165202. · 3.66 Impact Factor
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    ABSTRACT: High-resolution spin- and angle-resolved photoemission spectroscopy measurements were performed on the three-dimensional topological insulator Bi2Te2.4Se0.6, which is characterized by enhanced thermoelectric properties. The Fermi level position is found to be located in the bulk energy gap independent of temperature and it is stable over a long time. Spin textures in the Dirac-cone state at energies above and below the Dirac point as well as in the Rashba-type valence band surface state are observed in agreement with theoretical prediction. The calculations of the surface electronic structure demonstrate that the fractional stoichiometry induced disorder within the Te/Se sublattice does not influence the Dirac-cone state dispersion. In spite of relatively high resistivity, temperature dependence of conductivity shows a weak metallic behavior that could explain the effective thermoelectric properties of the Bi2Te2.4Se0.6 compound with the in-plane Seebeck coefficient reaching −330 μV/K at room temperature.
    Physical review. B, Condensed matter 03/2014; 89:125416. · 3.77 Impact Factor
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    ABSTRACT: A general expression is introduced for the tracer diffusivity in complex periodic energy landscapes with more than one distinct hop rate in two- and three-dimensional diluted systems (low coverage, single-tracer limit). For diffusion in two dimensions, a number of formulas are presented for complex combinations of hop rates in systems with triangular, rectangular and square symmetry. The formulas provide values in excellent agreement with Kinetic Monte Carlo simulations, concluding that the diffusion coefficient can be directly determined from the proposed expressions without performing such simulations. Based on the diffusion barriers obtained from first principles calculations and a physically-meaningful estimate of the attempt frequencies, the proposed formulas are used to analyze the diffusion of Cu, Ag and Rb adatoms on the surface and within the van der Waals (vdW) gap of a model topological insulator, Bi$_{2}$Se$_{3}$. Considering the possibility for adsorbate intercalation from the terraces to the vdW gaps at morphological steps, we infer that, at low coverage and room temperature: (i) a majority of the Rb atoms bounce back at the steps and remain on the terraces, (ii) Cu atoms mostly intercalate into the vdW gap, the remaining fraction staying at the steps, and (iii) Ag atoms essentially accumulate at the steps and gradually intercalate into the vdW gap. These conclusions are in good qualitative agreement with previous experiments.
    02/2014;
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    ABSTRACT: Using high-resolution angle-resolved photoemission spectroscopy, we elucidate the Rashba splitting of ΔkF=0.003 Å−1 near the Fermi level (EF) in the Shockley surface state of Cu(110) at the Y⎯⎯⎯ point of the surface Brillouin zone. The observed energy-band dispersion exhibits a kink structure at ∼−20 meV, which is a clear indication of band renormalization caused by an electron-phonon interaction. The electron-phonon coupling parameter is found to be λep=0.17±0.02 based on the experimentally obtained real part of the self-energy. First-principle calculations yield λep=0.160 and ΔkF=0.004 Å−1 at EF, which are fully consistent with the experimental results. In addition, the contributions of the electron-electron and electron-phonon interactions to the linewidth of the surface state at the Y⎯⎯⎯ point are experimentally determined to be Γee∼9 meV and Γep∼7 meV, respectively. We demonstrate that the Rashba splitting must be resolved by photoemission line-shape analysis for an accurate determination of the electron self-energy and coupling parameters.
    Physical Review B 02/2014; 89(8):085404. · 3.66 Impact Factor
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    ABSTRACT: Time-resolved two-photon photoemission was used to study the electronic structure and dynamics at the surface of SnSb2Te4, a p-type topological insulator. The Dirac point is found 0.32±0.03 eV above the Fermi level. Electrons from the conduction band minimum are scattered on a time scale of 43±4 fs to the Dirac cone. From there they decay to the partly depleted valence band with a time constant of 78±5 fs. The significant interaction of the Dirac states with bulk bands is attributed to their bulk penetration depth of ̃3 nm as found from density functional theory calculations.
    01/2014; 89(8).
  • S. S. Tsirkin, E. V. Chulkov
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    ABSTRACT: The energies and lifetimes of the image potential resonances at the Ȳ point on the Cu(110), Ag(110), and Au(110) surfaces are studied, and the energies of the image potential states on the Pd(110) surface are analyzed. It is shown that every quantum number n corresponds to a pair of image potential states (resonances) n + and n - at the Ȳ point. The average energy of a pair of eigenstates (resonances) at n ≥ 2 is well described by the formula Ē n = ( E n+ + E n-) = E 0 - (0.85 eV)/( n + δ)2, where δ is the quantum defect, E 0 = (1/2 m e )(ħπ/ a)2, and a is the lattice parameter. The splitting energy of a pair of eigenstates (resonances) obeys the law Δ E n = E n+ — E n- ∝ n -3. The lifetimes τ n± of image potential resonances are proportional to n 3.
    Journal of Experimental and Theoretical Physics 01/2014; 118(2). · 0.92 Impact Factor
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    ABSTRACT: The quasi two-dimensional electron gas on a metal film can transmit to the surface even minute mechanical disturbances occurring in the depth, thus allowing the gentlest of all surface probes, helium atoms, to perceive the vibrations of the deepest atoms via the induced surface-charge density oscillations. A density functional perturbation theory (DFPT) and a helium atom scattering study of the phonon dispersion curves in lead films of up to 7 mono-layers on a copper substrate show that: (a) the electron-phonon interaction is responsible for the coupling of He atoms to in-depth phonon modes; and (b) the inelastic HAS intensity from a given phonon mode is proportional to its electron-phonon coupling. The direct determination of mode-selected electron-phonon coupling strengths has great relevance for understanding superconductivity in thin films and two-dimensional systems.
    Physical Chemistry Chemical Physics 01/2014; · 4.20 Impact Factor
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    ABSTRACT: Carrying a large, pure spin magnetic moment of 7 μB per atom in the half-filled 4f shell, divalent europium is an outstanding element for assembling novel magnetic devices in which a two-dimensional electron gas may be polarized due to exchange interaction with an underlying magnetically-active Eu layer. Here we show that the Si-Rh-Si surface trilayer of the antiferromagnet EuRh2Si2 bears a surface state, which exhibits an unexpected and large spin splitting controllable by temperature. The splitting sets in below ~32.5 K, well above the ordering temperature of the Eu 4f moments (~24.5 K) in the bulk, indicating a larger ordering temperature in the topmost Eu layers. The driving force for the itinerant ferromagnetism at the surface is the aforementioned exchange interaction. Such a splitting may also be induced into states of functional surface layers deposited onto the surface of EuRh2Si2 or similarly ordered magnetic materials with metallic or semiconducting properties.
    Nature Communications 01/2014; 5:3171. · 10.74 Impact Factor
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    ABSTRACT: We consider a system consisting of a topological insulator with an array of magnetic adatoms interacting with the surface states of electrons. We find that the indirect coupling of the magnetic impurities results in a ferromagnetic ordering of the magnetic moments and is also responsible for the unusual linear dispersion of the surface magnons. We also show that the interaction of magnons with surface electrons essentially renormalizes the electron energy spectrum. The renormalized spectrum is nonlinear and can be characterized by a negative effective mass of electrons and holes for any k �not equal 0. The electron velocity near the Dirac point depends on the electron-magnon coupling..
    Physical Review B 01/2014; 89:075103. · 3.66 Impact Factor
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    ABSTRACT: First-principles calculations for the temporal characteristics of hole-phonon relaxation in the valence band of titanium dioxide and zinc oxide have been performed. A first-principles method for the calculations of the quasistationary distribution function of holes has been developed. The results show that the quasistationary distribution of the holes in TiO 2 extends to an energy level approximately 1 eV below the top of the valence band. This conclusion in turn helps to elucidate the origin of the spectral dependence of the photocatalytic activity of TiO 2 . Analysis of the analogous data for ZnO shows that in this material spectral dependence of photocatalytic activity in the oxidative reactions is unlikely.
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    ABSTRACT: The topological insulators Bi2Se3 and Bi2Te2Se have been shown to possess unoccupied topological surface states at the center of the surface Brillouin zone at energies around 1.3 eV above the Fermi level. Using time-resolved two-photon photoemission we study the electron dynamics of the unoccupied topological surface states, image-potential states and conduction bands on these surfaces.
    Journal of Electron Spectroscopy and Related Phenomena 01/2014; · 1.71 Impact Factor
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    ABSTRACT: Collective vibrational modes of crystal lattices, called phonons, determine fundamental material properties, such as their thermal and electrical conductivities. Bulk phonon spectra are influenced by point defects. More recently, the importance of phonons on nanostructures has come into the focus of attention. Here we show a spatially resolved phonon spectra of point defects that reveal distinctly different signatures for a cavity alone and an impurity atom fully integrated into the surface as opposed to one placed into a cavity. The spectra are indicative for delocalized phonons and localized vibrations, respectively, as confirmed by theory.
    Nature Communications 01/2014; 5:5089. · 10.74 Impact Factor

Publication Stats

5k Citations
1,281.45 Total Impact Points

Institutions

  • 2014
    • University of Duisburg-Essen
      Essen, North Rhine-Westphalia, Germany
  • 2010–2014
    • Hiroshima University
      • Graduate School of Science
      Hirosima, Hiroshima, Japan
    • Technische Universität Kaiserslautern
      • Fachbereich für Physik
      Kaiserslautern, Rhineland-Palatinate, Germany
    • Ikerbasque - Basque Foundation for Science
      Bilbo, Basque Country, Spain
  • 2001–2014
    • Donostia International Physics Center
      San Sebastián, Basque Country, Spain
  • 1979–2014
    • Tomsk State University
      Tomsk, Tomsk, Russia
  • 1997–2013
    • Universidad del País Vasco / Euskal Herriko Unibertsitatea
      • Departamento de Física de Materiales
      Leioa, Basque Country, Spain
  • 2012
    • Karlsruhe Institute of Technology
      • Institute for Theoretical Solid State Physics
      Carlsruhe, Baden-Württemberg, Germany
    • Martin Luther University of Halle-Wittenberg
      • Institute of Physics
      Halle-on-the-Saale, Saxony-Anhalt, Germany
  • 2002–2011
    • Philipps University of Marburg
      Marburg, Hesse, Germany
  • 1990–2011
    • Russian Academy of Sciences
      • Institute of Strength Physics and Materials Science of the Siberian Branch
      Moscow, Moscow, Russia
  • 2008–2010
    • Center of Materials Physics
      San Sebastián, Basque Country, Spain
    • University of Gothenburg
      • Institutionen för fysik
      Göteborg, Vaestra Goetaland, Sweden
  • 2009
    • Aarhus University
      • Department of Physics and Astronomy
      Aarhus, Central Jutland, Denmark
  • 2001–2009
    • Université Paris-Sud 11
      • Institut des Sciences Moléculaires d’Orsay (ISMO)
      Orsay, Île-de-France, France
  • 2004
    • Freie Universität Berlin
      • Institute of Experimental Physics
      Berlin, Land Berlin, Germany