15,091 research outputs found
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
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
Functional agrobiodievrsity - a novel approach to optimize pest control in fruit production
Functional agrobiodiversity was implemented in organic apple orchards in Denmark, by perennial flower strips. Two techniques were tested with the purpose of enhancing natural enemies and reducing damage of the rosy apple aphid
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
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
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
Unbiased sampling of globular lattice proteins in three dimensions
We present a Monte Carlo method that allows efficient and unbiased sampling
of Hamiltonian walks on a cubic lattice. Such walks are self-avoiding and visit
each lattice site exactly once. They are often used as simple models of
globular proteins, upon adding suitable local interactions. Our algorithm can
easily be equipped with such interactions, but we study here mainly the
flexible homopolymer case where each conformation is generated with uniform
probability. We argue that the algorithm is ergodic and has dynamical exponent
z=0. We then use it to study polymers of size up to 64^3 = 262144 monomers.
Results are presented for the effective interaction between end points, and the
interaction with the boundaries of the system
An Economic and Environmental Analysis of Slurry Separation
With increased pressure to redistribute animal manure in order to lower the environmental pressure from agriculture, it seems obvious to consider processing slurry into nutrient rich fractions which can easily be transported. In this paper, an overall analysis of four different separation technologies is presented. The four technologies are Decanter, Funki Manura 2000, Green Farm Energy and Staring. These technologies are all implemented on a full scale in Denmark. In this paper both the economic and environmental aspects are considered, looking at the entire chain from stable to the field. The total investments range from 50,000 to 4 million Euros and the total net costs are from 1 to 7 Euros per tonne for the four different technologies. One of the clear environmental benefits is a better utilisation of phosphorus, but using phytase in feeding is a cheaper first step when reducing phosphorus surplus. Improved nitrogen utilisation is only apparent with the Staring and Green Farm Energy concepts. The conclusion is that the Funki Manura 2000 system is too expensive and the Decanter system a fairly cheap way to reduce phosphorus levels, but other benefits are limited. Staring and Green Farm Energy show the greatest potential, but these systems have not been running long enough to validate the expected results included in this paper.Resource /Energy Economics and Policy,
Conduction Mechanism in a Molecular Hydrogen Contact
We present first principles calculations for the conductance of a hydrogen
molecule bridging a pair of Pt electrodes. The transmission function has a wide
plateau with T~1 which extends across the Fermi level and indicates the
existence of a single, robust conductance channel with nearly perfect
transmission. Through a detailed Wannier function analysis we show that the H2
bonding state is not involved in the transport and that the plateau forms due
to strong hybridization between the H2 anti-bonding state and states on the
adjacent Pt atoms. The Wannier functions furthermore allow us to derive a
resonant-level model for the system with all parameters determined from the
fully self-consistent Kohn-Sham Hamiltonian.Comment: 5 pages, 4 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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