11,559 research outputs found
Four-atom period in the conductance of monatomic Al wires
We present first principles calculations based on density functional theory
for the conductance of monatomic Al wires between Al(111) electrodes. In
contrast to the even-odd oscillations observed in other metallic wires, the
conductance of the Al wires is found to oscillate with a period of 4 atoms as
the length of the wire is varied. Although local charge neutrality can account
for the observed period it leads to an incorrect phase. We explain the
conductance behavior using a resonant transport model based on the electronic
structure of the infinite wire.Comment: 4 pages, 5 figure
Interference and k-point sampling in the supercell approach to phase-coherent transport
We present a systematic study of interference and k-point sampling effects in
the supercell approach to phase-coherent electron transport. We use a
representative tight-binding model to show that interference between the
repeated images is a small effect compared to the error introduced by using
only the Gamma-point for a supercell containing (3,3) sites in the transverse
plane. An insufficient k-point sampling can introduce strong but unphysical
features in the transmission function which can be traced to the presence of
van Hove singularities in the lead. We present a first-principles calculation
of the transmission through a Pt contact which shows that the k-point sampling
is also important for realistic systems.Comment: 4 pages, 5 figures. Accepted for Phys. Rev. B (Brief Report
Rate theory for correlated processes: Double-jumps in adatom diffusion
We study the rate of activated motion over multiple barriers, in particular
the correlated double-jump of an adatom diffusing on a missing-row
reconstructed Platinum (110) surface. We develop a Transition Path Theory,
showing that the activation energy is given by the minimum-energy trajectory
which succeeds in the double-jump. We explicitly calculate this trajectory
within an effective-medium molecular dynamics simulation. A cusp in the
acceptance region leads to a sqrt{T} prefactor for the activated rate of
double-jumps. Theory and numerical results agree
Forces and conductances in a single-molecule bipyridine junction
Inspired by recent measurements of forces and conductances of bipyridine
nano-junctions, we have performed density functional theory calculations of
structure and electron transport in a bipyridine molecule attached between gold
electrodes for seven different contact geometries. The calculations show that
both the bonding force and the conductance are sensitive to the surface
structure, and that both properties are in good agreement with experiment for
contact geometries characterized by intermediate coordination of the metal
atoms corresponding to a stepped surface. The conductance is mediated by the
lowest unoccupied molecular orbital, which can be illustrated by a quantitative
comparison with a one-level model. Implications for the interpretation of the
experimentally determined force and conductance distributions are discussed
Simulation of Cu-Mg metallic glass: Thermodynamics and Structure
We have obtained effective medium theory (EMT) interatomic potential
parameters suitable for studying Cu-Mg metallic glasses. We present
thermodynamic and structural results from simulations of such glasses over a
range of compositions. We have produced low-temperature configurations by
cooling from the melt at as slow a rate as practical, using constant
temperature and pressure molecular dynamics. During the cooling process we have
carried out thermodynamic analyses based on the temperature dependence of the
enthalpy and its derivative, the specific heat, from which the glass transition
temperature may be determined. We have also carried out structural analyses
using the radial distribution function (RDF) and common neighbor analysis
(CNA). Our analysis suggests that the splitting of the second peak, commonly
associated with metallic glasses, in fact has little to do with the glass
transition itself, but is simply a consequence of the narrowing of peaks
associated with structural features present in the liquid state. In fact the
splitting temperature for the Cu-Cu RDF is well above . The CNA also
highlights a strong similarity between the structure of the intermetallic
alloys and the amorphous alloys of similar composition. We have also
investigated the diffusivity in the supercooled regime. Its temperature
dependence indicates fragile-liquid behavior, typical of binary metallic
glasses. On the other hand, the relatively low specific heat jump of around
indicates apparent strong-liquid behavior, but this can
be explained by the width of the transition due to the high cooling rates.Comment: 12 pages (revtex, two-column), 12 figures, submitted to Phys. Rev.
Passenger ride comfort technology for transport aircraft situations
A brief overview is given of NASA research in ride comfort and of the resultant technology. Three useful relations derived from the technology are presented together with five applications of these relations to illustrate their effectiveness in addressing various ride comfort situations of passenger transports
Spatially resolved quantum plasmon modes in metallic nano-films from first principles
Electron energy loss spectroscopy (EELS) can be used to probe plasmon
excitations in nanostructured materials with atomic-scale spatial resolution.
For structures smaller than a few nanometers quantum effects are expected to be
important, limiting the validity of widely used semi-classical response models.
Here we present a method to identify and compute spatially resolved plasmon
modes from first principles based on a spectral analysis of the dynamical
dielectric function. As an example we calculate the plasmon modes of 0.5-4 nm
thick Na films and find that they can be classified as (conventional) surface
modes, sub-surface modes, and a discrete set of bulk modes resembling standing
waves across the film. We find clear effects of both quantum confinement and
non-local response. The quantum plasmon modes provide an intuitive picture of
collective excitations of confined electron systems and offer a clear
interpretation of spatially resolved EELS spectra.Comment: 7 pages, 7 figure
Plasmons on the edge of MoS2 nanostructures
Using ab initio calculations we predict the existence of one-dimensional
(1D), atomically confined plasmons at the edges of a zigzag MoS2 nanoribbon.
The strongest plasmon originates from a metallic edge state localized on the
sulfur dimers decorating the Mo edge of the ribbon. A detailed analysis of the
dielectric function reveals that the observed deviations from the ideal 1D
plasmon behavior result from single-particle transitions between the metallic
edge state and the valence and conduction bands of the MoS2 sheet. The Mo and S
edges of the ribbon are clearly distinguishable in calculated spatially
resolved electron energy loss spectrum owing to the different plasmonic
properties of the two edges. The edge plasmons could potentially be utilized
for tuning the photocatalytic activity of MoS2 nanoparticles
Fully selfconsistent GW calculations for molecules
We calculate single-particle excitation energies for a series of 33 molecules
using fully selfconsistent GW, one-shot GW, Hartree-Fock (HF), and
hybrid density functional theory (DFT). All calculations are performed within
the projector augmented wave (PAW) method using a basis set of Wannier
functions augmented by numerical atomic orbitals. The GW self-energy is
calculated on the real frequency axis including its full frequency dependence
and off-diagonal matrix elements. The mean absolute error of the ionization
potential (IP) with respect to experiment is found to be 4.4, 2.6, 0.8, 0.4,
and 0.5 eV for DFT-PBE, DFT-PBE0, HF, GW[HF], and selfconsistent GW,
respectively. This shows that although electronic screening is weak in
molecular systems its inclusion at the GW level reduces the error in the IP by
up to 50% relative to unscreened HF. In general GW overscreens the HF energies
leading to underestimation of the IPs. The best IPs are obtained from one-shot
GW calculations based on HF since this reduces the overscreening.
Finally, we find that the inclusion of core-valence exchange is important and
can affect the excitation energies by as much as 1 eV.Comment: 10 pages, 5 figure
Towards electron transport measurements in chemically modified graphene: The effect of a solvent
Chemical functionalization of graphene modifies the local electron density of
the carbon atoms and hence electron transport. Measuring these changes allows
for a closer understanding of the chemical interaction and the influence of
functionalization on the graphene lattice. However, not only chemistry, in this
case diazonium chemistry, has an effect on the electron transport. Latter is
also influenced by defects and dopants resulting from different processing
steps. Here, we show that solvents used in the chemical reaction process change
the transport properties. In more detail, the investigated combination of
isopropanol and heating treatment reduces the doping concentration and
significantly increases the mobility of graphene. Furthermore, the isopropanol
treatment alone increases the concentration of dopants and introduces an
asymmetry between electron and hole transport which might be difficult to
distinguish from the effect of functionalization. The results shown in this
work demand a closer look on the influence of solvents used for chemical
modification in order to understand their influence
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