Bremen Center for Computational Materials Science

Schönau am Königssee, Germany

Bremen Center for Computational Materials Science

Schönau am Königssee, Germany
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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.

Eelbo T.,University of Hamburg | Waaniowska M.,University of Hamburg | Thakur P.,European Synchrotron Radiation Facility | Gyamfi M.,University of Hamburg | And 8 more authors.
Physical Review Letters | Year: 2013

We investigate the electronic and magnetic properties of single Fe, Co, and Ni atoms and clusters on monolayer graphene (MLG) on SiC(0001) by means of scanning tunneling microscopy (STM), x-ray absorption spectroscopy, x-ray magnetic circular dichroism (XMCD), and ab initio calculations. STM reveals different adsorption sites for Ni and Co adatoms. XMCD proves Fe and Co adatoms to be paramagnetic and to exhibit an out-of-plane easy axis in agreement with theory. In contrast, we experimentally find a nonmagnetic ground state for Ni monomers while an increasing cluster size leads to sizeable magnetic moments. These observations are well reproduced by our calculations and reveal the importance of hybridization effects and intra-atomic charge transfer for the properties of adatoms and clusters on MLG. © 2013 American Physical Society.

Gargiulo F.,Ecole Polytechnique Federale de Lausanne | Autes G.,Ecole Polytechnique Federale de Lausanne | Virk N.,Ecole Polytechnique Federale de Lausanne | Barthel S.,University of Bremen | And 7 more authors.
Physical Review Letters | Year: 2014

Hydrogen adatoms and other species covalently bound to graphene act as resonant scattering centers affecting the electronic transport properties and inducing Anderson localization. We show that attractive interactions between adatoms on graphene and their diffusion mobility strongly modify the spatial distribution, thus fully eliminating isolated adatoms and increasing the population of larger size adatom aggregates. Such spatial correlation is found to strongly influence the electronic transport properties of disordered graphene. Our scaling analysis shows that such aggregation of adatoms increases conductance by up to several orders of magnitude and results in significant extension of the Anderson localization length in the strong localization regime. We introduce a simple definition of the effective adatom concentration x∗, which describes the transport properties of both random and correlated distributions of hydrogen adatoms on graphene across a broad range of concentrations. © 2014 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.

PubMed | University of Bremen, MAX IV Laboratory, Bremen Center for Computational Materials Science and Lund University
Type: | Journal: Physical chemistry chemical physics : PCCP | Year: 2016

The growth, morphology, structure, and stoichiometry of ultrathin praseodymium oxide layers on Ru(0001) were studied using low-energy electron microscopy and diffraction, photoemission electron microscopy, atomic force microscopy, and X-ray photoelectron spectroscopy. At a growth temperature of 760 C, the oxide is shown to form hexagonally close-packed (A-type) Pr

Wang R.,Beijing Computational Science Research Center Haidian | Zhang Y.,Northwestern University | Bi F.,Beijing Computational Science Research Center Haidian | Bi F.,Anhui University of Science and Technology | And 4 more authors.
Nanoscale | Year: 2016

Understanding of the electroluminescence (EL) mechanism in optoelectronic devices is imperative for further optimization of their efficiency and effectiveness. Here, a quantum mechanical approach is formulated for modeling the EL processes in nanoscale light emitting diodes (LED). Based on non-equilibrium Green's function quantum transport equations, interactions with the electromagnetic vacuum environment are included to describe electrically driven light emission in the devices. The presented framework is illustrated by numerical simulations of a silicon nanowire LED device. EL spectra of the nanowire device under different bias voltages are obtained and, more importantly, the radiation pattern and polarization of optical emission can be determined using the current approach. This work is an important step forward towards atomistic quantum mechanical modeling of the electrically induced optical response in nanoscale systems. © 2016 The Royal Society of Chemistry.

Oppenlander C.,University of Regensburg | Korff B.,Bremen Center for Computational Materials Science | Frauenheim T.,Bremen Center for Computational Materials Science | Niehaus T.A.,University of Regensburg
Physica Status Solidi (B) Basic Research | Year: 2013

We present dynamical transport calculations based on a tight-binding approximation to adiabatic time-dependent density functional theory (TD-DFTB). The reduced device density matrix is propagated through the Liouville-von Neumann equation. For the model system, 1,4-benzenediol coupled to aluminum leads, we are able to confirm the equality of the steady state current resulting from a time-dependent calculation to a static calculation in the conventional Landauer framework. We also investigate the response of the junction subjected to alternating bias voltages with frequencies up to the optical regime. Here we can clearly identify capacitive behaviour of the molecular device and a significant resonant enhancement of the conductance. The results are interpreted using an analytical single level model comparing the device transmission and admittance. In order to aid future calculations under alternating bias, we shortly review the use of Fourier transform techniques to obtain the full frequency response of the device from a single current trace. © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

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.

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.

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