166 research outputs found
Rings sliding on a honeycomb network: Adsorption contours, interactions, and assembly of benzene on Cu(111)
Using a van der Waals density functional (vdW-DF) [Phys. Rev. Lett. 92,
246401 (2004)], we perform ab initio calculations for the adsorption energy of
benzene (Bz) on Cu(111) as a function of lateral position and height. We find
that the vdW-DF inclusion of nonlocal correlations (responsible for dispersive
interactions) changes the relative stability of eight binding-position options
and increases the binding energy by over an order of magnitude, achieving good
agreement with experiment. The admolecules can move almost freely along a
honeycomb web of "corridors" passing between fcc and hcp hollow sites via
bridge sites. Our diffusion barriers (for dilute and two condensed adsorbate
phases) are consistent with experimental observations. Further vdW-DF
calculations suggest that the more compact (hexagonal) Bz-overlayer phase, with
lattice constant a = 6.74 \AA, is due to direct Bz-Bz vdW attraction, which
extends to ~8 \AA. We attribute the second, sparser hexagonal Bz phase, with a
= 10.24 \AA, to indirect electronic interactions mediated by the metallic
surface state on Cu(111). To support this claim, we use a formal
Harris-functional approach to evaluate nonperturbationally the asymptotic form
of this indirect interaction. Thus, we can account well for benzene
self-organization on Cu(111).Comment: 13 pages, 7 figures, 3 tables, submitted for publication Accepted for
publication in Phys. Rev. B. This version contains improved notation (with
corresponding relabeling of figures), very small corrections to some
tabulated values, and corrections concerning lattice lengths and subsequent
discussion of commensurability of unit-cell dimension
Resonant thermal transport in semiconductor barrier structures
I report that thermal single-barrier (TSB) and thermal double-barrier (TDB)
structures (formed, for example, by inserting one or two regions of a few Ge
monolayers in Si) provide both a suppression of the phonon transport as well as
a resonant-thermal-transport effect. I show that high-frequency phonons can
experience a traditional double-barrier resonant tunneling in the TDB
structures while the formation of Fabry-Perot resonances (at lower frequencies)
causes quantum oscillations in the temperature variation of both the TSB and
TDB thermal conductances and .Comment: 4 pages. 4 figure.
Site determination and thermally assisted tunneling in homogenous nucleation
A combined low-temperature scanning tunneling microscopy and density
functional theory study on the binding and diffusion of copper monomers,
dimers, and trimers adsorbed on Cu(111) is presented. Whereas atoms in trimers
are found in fcc sites only, monomers as well as atoms in dimers can occupy the
stable fcc as well as the metastable hcp site. In fact the dimer fcc-hcp
configuration was found to be only 1.3 meV less favorable with respect to the
fcc-fcc configuration. This enables a confined intra-cell dimer motion, which
at temperatures below 5 K is dominated by thermally assisted tunneling.Comment: 4 pages, 4 figure
Band bending and quasi-2DEG in the metallized -SiC(001) surface
We study the mechanism leading to the metallization of the -SiC(001)
Si-rich surface induced by hydrogen adsorption. We analyze the effects of band
bending and demonstrate the existence of a quasi-2D electron gas, which
originates from the donation of electrons from adsorbed hydrogen to bulk
conduction states. We also provide a simple model that captures the main
features of the results of first-principles calculations, and uncovers the
basic physics of the process.Comment: accepted for publication in physica status solidi - Rapid Research
Letter
Ab initio structure modeling of complex thin-film oxides: thermodynamical stability of TiC/thin-film alumina
We present an efficient and general method to identify promising candidate
configurations for thin-film oxides and to determine structural characteristics
of (metastable) thin-film structures using ab initio calculations. At the heart
of this method is the complexity of the oxide bulk structure, from which a
large number of thin films with structural building blocks, that is motifs,
from metastable bulk oxide systems can be extracted. These span a coarse but
well-defined network of initial configurations for which density functional
theory (DFT) calculations predict and implement dramatic atomic relaxations in
the corresponding, resulting thin-film candidates. The network of thin-film
candidates (for various film thicknesses and stoichiometries) can be ordered
according to their variation in ab initio total energy or in ab initio
equilibrium Gibbs free energy. Analysis of the relaxed atomic structures for
the most favored structures gives insight into the nature of stable and
metastable thin-film oxides. We investigate ultrathin alumina nucleated on TiC
as a model system to illustrate this method.Comment: Submitted to PRB; 16 pages, 11 figure
Energy scaling law for nanostructured materials
The equilibrium binding energy is an important factor in the design of
materials and devices. However, it presents great computational challenges for
materials built up from nanostructures. Here we investigate the binding-energy
scaling law from first-principles calculations. We show that the equilibrium
binding energy per atom between identical nanostructures can scale up or down
with nanostructure size. From the energy scaling law, we predict finite
large-size limits of binding energy per atom. We find that there are two
competing factors in the determination of the binding energy: Nonadditivities
of van der Waals coefficients and center-to-center distance between
nanostructures. To uncode the detail, the nonadditivity of the static multipole
polarizability is investigated. We find that the higher-order multipole
polarizability displays ultra-strong intrinsic nonadditivity, no matter if the
dipole polarizability is additive or not.Comment: 13 pages, 4 figures, 7 table
Graphene Nanogap for Gate Tunable Quantum Coherent Single Molecule Electronics
We present atomistic calculations of quantum coherent electron transport
through fulleropyrrolidine terminated molecules bridging a graphene nanogap. We
predict that three difficult problems in molecular electronics with single
molecules may be solved by utilizing graphene contacts: (1) a back gate
modulating the Fermi level in the graphene leads facilitate control of the
device conductance in a transistor effect with high on/off current ratio; (2)
the size mismatch between leads and molecule is avoided, in contrast to the
traditional metal contacts; (3) as a consequence, distinct features in charge
flow patterns throughout the device are directly detectable by scanning
techniques. We show that moderate graphene edge disorder is unimportant for the
transistor function.Comment: 8 pages, 6 figure
Sublattice ordering in a dilute ensemble of defects in graphene
Defects in graphene, such as vacancies or adsorbents attaching themselves to
carbons, may preferentially take positions on one of its two sublattices, thus
breaking the global lattice symmetry. This leads to opening a gap in the
electronic spectrum. We show that such a sublattice ordering may spontaneously
occur in a dilute ensemble defects, due to the long-range interaction between
them mediated by electrons. As a result sublattice-ordered domains may form,
with electronic properties characteristic of a two-dimensional topological
insulator.Comment: to appear in Europhysics Letter
Nonequilibrium thermodynamics of interacting tunneling transport: variational grand potential, density-functional formulation, and nature of steady-state forces
The standard formulation of tunneling transport rests on an open-boundary
modeling. There, conserving approximations to nonequilibrium Green function or
quantum-statistical mechanics provide consistent but computational costly
approaches; alternatively, use of density-dependent ballistic-transport
calculations [e.g., Phys. Rev. B 52, 5335 (1995)], here denoted `DBT', provide
computationally efficient (approximate) atomistic characterizations of the
electron behavior but has until now lacked a formal justification. This paper
presents an exact, variational nonequilibrium thermodynamic theory for fully
interacting tunneling and provides a rigorous foundation for frozen-nuclei DBT
calculations as a lowest order approximation to an exact nonequilibrium
thermodynamics density functional evaluation. The theory starts from the
complete electron nonequilibrium quantum statistical mechanics and I identify
the operator for the nonequilibrium Gibbs free energy. I demonstrate a minimal
property of a functional for the nonequilibrium thermodynamic grand potential
which thus uniquely identifies the solution as the exact nonequilibrium density
matrix. I also show that a uniqueness-of-density proof from a closely related
study [Phys. Rev. B 78, 165109 (2008)] makes it possible to provide a
single-particle formulation based on universal electron-density functionals. I
illustrate a formal evaluation of the thermodynamics grand potential value
which is closely related to the variation in scattering phase shifts and hence
to Friedel density oscillations. This paper also discusses the difference
between the here-presented exact thermodynamics forces and the often-used
electrostatic forces. Finally the paper documents an inherent adiabatic nature
of the thermodynamics forces and observes that these are suited for a
nonequilibrium implementation of the Born-Oppenheimer approximation.Comment: 37 pages, 3 Figure
Evaluation of New Density Functional with Account of van der Waals Forces by Use of Experimental H2 Physisorption Data on Cu(111)
Detailed experimental data for physisorption potential-energy curves of H2 on
low-indexed faces of Cu challenge theory. Recently, density-functional theory
has been developed to also account for nonlocal correlation effects, including
van der Waals forces. We show that one functional, denoted vdW-DF2, gives a
potential-energy curve promisingly close to the experiment-derived
physisorptionenergy curve. The comparison also gives indications for further
improvements of the functionals
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