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Surer B.,ETH Zurich | Troyer M.,ETH Zurich | Werner P.,ETH Zurich | Wehling T.O.,University of Bremen | And 6 more authors.
Physical Review B - Condensed Matter and Materials Physics | Year: 2012

We investigate the electronic structure of cobalt atoms on a copper surface and in a copper host by combining density-functional calculations with a numerically exact continuous-time quantum Monte Carlo treatment of the five-orbital impurity problem. In both cases we find low energy resonances in the density of states of all five Co d orbitals. The corresponding self-energies indicate the formation of a Fermi liquid state at low temperatures. Our calculations yield the characteristic energy scale-the Kondo temperature-for both systems in good agreement with experiments. We quantify the charge fluctuations in both geometries and suggest that Co in Cu must be described by an Anderson impurity model rather than by a model assuming frozen impurity valency at low energies. We show that fluctuations of the orbital degrees of freedom are crucial for explaining the Kondo temperatures obtained in our calculations and measured in experiments. © 2012 American Physical Society.

Varykhalov A.,Helmholtz Center Berlin | Marchenko D.,Helmholtz Center Berlin | Sanchez-Barriga J.,Helmholtz Center Berlin | Scholz M.R.,Helmholtz Center Berlin | And 6 more authors.
Physical Review X | Year: 2012

The appearance of massless Dirac fermions in graphene requires two equivalent carbon sublattices of trigonal shape. While the generation of an effective mass and a band gap at the Dirac point remains an unresolved problem for freestanding extended graphene, it is well established by breaking translational symmetry by confinement and by breaking sublattice symmetry by interaction with a substrate. One of the strongest sublattice-symmetry-breaking interactions with predicted and measured band gaps ranging from 400 meV to more than 3 eV has been attributed to the interfaces of graphene with Ni and Co, which are also promising spin-filter interfaces. Here, we apply angle-resolved photoemission to epitaxial graphene on Ni (111) and Co(0001) to show the presence of intact Dirac cones 2.8 eV below the Fermi level. Our results challenge the common belief that the breaking of sublattice symmetry by a substrate and the opening of the band gap at the Dirac energy are in a straightforward relation. A simple effective model of a biased bilayer structure composed of graphene and a sublattice-symmetry-broken layer, corroborated by densityfunctional-theory calculations, demonstrates the general validity of our conclusions.

Gyamfi M.,University of Hamburg | Eelbo T.,University of Hamburg | Waniowska M.,University of Hamburg | Wehling T.O.,University of Hamburg | And 7 more authors.
Physical Review B - Condensed Matter and Materials Physics | Year: 2012

We investigate the coupling of Ni monomers and trimers to graphene by means of atomically resolved scanning tunneling microscopy (STM). The precise adsorption site of the adatoms is determined experimentally. STM images reveal characteristic nodal structures above the Ni adatoms and trimers on graphene. First-principles calculations combined with symmetry considerations explain our experimental results by an orbitally controlled interaction of the adatoms and clusters with the Dirac electrons in graphene. © 2012 American Physical Society.

Niehaus T.A.,Bremen Center for Computational Materials Science | March N.H.,University of Antwerp | March N.H.,University of Oxford
Theoretical Chemistry Accounts | Year: 2010

The electron density n(r,t), which is the central tool of time-dependent density functional theory, is presently considered to be derivable from a one-body time-dependent potential V(r,t), via one-electron wave functions satisfying a time-dependent Schrödinger equation. This is here related via a generalized equation of motion to a Dirac density matrix now involving t. Linear response theory is then surveyed, with a special emphasis on the question of causality with respect to the density dependence of the potential. Extraction of V(r,t) for solvable models is also proposed. © Springer-Verlag 2009.

Yam C.,University of Hong Kong | Zheng X.,University of Hong Kong | Chen G.,University of Hong Kong | Wang Y.,Bremen Center for Computational Materials Science | And 2 more authors.
Physical Review B - Condensed Matter and Materials Physics | Year: 2011

To explore whether the density-functional theory-nonequilibrium Green's function formalism (DFT-NEGF) provides a rigorous framework for quantum transport, we carried out time-dependent density-functional-theory (TDDFT) calculations of the transient current through two realistic molecular devices, a carbon chain and a benzenediol molecule between two aluminum electrodes. The TDDFT simulations for the steady-state current exactly reproduce the results of fully self-consistent DFT-NEGF calculations even beyond linear response. In contrast, sizable differences are found with respect to an equilibrium, non-self-consistent treatment, which are related here to differences in the Kohn-Sham and fully interacting susceptibilities of the device region. Moreover, earlier analytical conjectures on the equivalence of static and time-dependent approaches in the low-bias regime are confirmed with high numerical precision. © 2011 American Physical Society.

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