11,718 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
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
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
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
Dynamic rotor mode in antiferromagnetic nanoparticles
We present experimental, numerical, and theoretical evidence for a new mode
of antiferromagnetic dynamics in nanoparticles. Elastic neutron scattering
experiments on 8 nm particles of hematite display a loss of diffraction
intensity with temperature, the intensity vanishing around 150 K. However, the
signal from inelastic neutron scattering remains above that temperature,
indicating a magnetic system in constant motion. In addition, the precession
frequency of the inelastic magnetic signal shows an increase above 100 K.
Numerical Langevin simulations of spin dynamics reproduce all measured neutron
data and reveal that thermally activated spin canting gives rise to a new type
of coherent magnetic precession mode. This "rotor" mode can be seen as a
high-temperature version of superparamagnetism and is driven by exchange
interactions between the two magnetic sublattices. The frequency of the rotor
mode behaves in fair agreement with a simple analytical model, based on a high
temperature approximation of the generally accepted Hamiltonian of the system.
The extracted model parameters, as the magnetic interaction and the axial
anisotropy, are in excellent agreement with results from Mossbauer
spectroscopy
Partly Occupied Wannier Functions
We introduce a scheme for constructing partly occupied, maximally localized
Wannier functions (WFs) for both molecular and periodic systems. Compared to
the traditional occupied WFs the partly occupied WFs posses improved symmetry
and localization properties achieved through a bonding-antibonding closing
procedure. We demonstrate the equivalence between bonding-antibonding closure
and the minimization of the average spread of the WFs in the case of a benzene
molecule and a linear chain of Pt atoms. The general applicability of the
method is demonstrated through the calculation of WFs for a metallic system
with an impurity: a Pt wire with a hydrogen molecular bridge.Comment: 5 pages, 4 figure
Graphene on metals: a Van der Waals density functional study
We use density functional theory (DFT) with a recently developed van der
Waals density functional (vdW-DF) to study the adsorption of graphene on Al,
Cu, Ag, Au, Pt, Pd, Co and Ni(111) surfaces. In constrast to the local density
approximation (LDA) which predicts relatively strong binding for Ni,Co and Pd,
the vdW-DF predicts weak binding for all metals and metal-graphene distances in
the range 3.40-3.72 \AA. At these distances the graphene bandstructure as
calculated with DFT and the many-body GW method is basically unaffected
by the substrate, in particular there is no opening of a band gap at the
-point.Comment: 4 pages, 3 figure
Inelastic Scattering in Metal-H2-Metal Junctions
We present first-principles calculations of the dI/dV characteristics of an
H2 molecule sandwiched between Au and Pt electrodes in the presence of
electron-phonon interactions. The conductance is found to decrease by a few
percentage at threshold voltages corresponding to the excitation energy of
longitudinal vibrations of the H2 molecule. In the case of Pt electrodes, the
transverse vibrations can mediate transport through otherwise non-transmitting
Pt -channels leading to an increase in the differential conductance even
though the hydrogen junction is characterized predominately by a single almost
fully open transport channel. In the case of Au, the transverse modes do not
affect the dI/dV because the Au d-states are too far below the Fermi level. A
simple explanation of the first-principles results is given using scattering
theory. Finally, we compare and discuss our results in relation to experimental
data.Comment: Accepted in Phys. Rev.
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