1,336 research outputs found
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
Simple models of the chemical field around swimming plankton
International audienceThe chemical field around swimming plankton depends on the swimming style and speed of the organism and the processes affecting uptake or exudation of chemicals by the organism. Here we present a simple model for the flow field around a neutrally buoyant self-propelled organism at low Reynolds number, and numerically calculate the chemical field around the organism. We show how the concentration field close to the organism and the mass transfer rates vary with swimming speed and style for Dirichlet (diffusion limited transport) boundary conditions. We calculate how the length of the chemical wake, defined as being the distance at which the chemical field drops to 10% of the surface concentration of the organism when stationary, varies with swimming speed and style for both Dirichlet and Neumann (production limited) boundary conditions. For Dirichlet boundary conditions, the length of the chemical wake increases with increasing swimming speed, and the self-propelled organism displays a significantly longer wake than the towed-body model. For the Neumann boundary conditions the converse is true; because swimming enhances the transport of the chemical away from the organism, the surface concentration of chemical is reduced and thus the wake length is reduced
Image-charge induced localization of molecular orbitals at metal-molecule interfaces: Self-consistent GW calculations
Quasiparticle (QP) wave functions, also known as Dyson orbitals, extend the
concept of single-particle states to interacting electron systems. Here we
employ many-body perturbation theory in the GW approximation to calculate the
QP wave functions for a semi-empirical model describing a -conjugated
molecular wire in contact with a metal surface. We find that image charge
effects pull the frontier molecular orbitals toward the metal surface while
orbitals with higher or lower energy are pushed away. This affects both the
size of the energetic image charge shifts and the coupling of the individual
orbitals to the metal substrate. Full diagonalization of the QP equation and,
to some extent, self-consistency in the GW self-energy, is important to
describe the effect which is not captured by standard density functional theory
or Hartree-Fock. These results should be important for the understanding and
theoretical modeling of electron transport across metal-molecule interfaces.Comment: 7 pages, 6 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
Electron transport through an interacting region: The case of a nonorthogonal basis set
The formula derived by Meir and Wingreen [Phys. Rev. Lett. {\bf 68}, 2512
(1992)] for the electron current through a confined, central region containing
interactions is generalized to the case of a nonorthogonal basis set. As in the
original work, the present derivation is based on the nonequilibrium Keldysh
formalism. By replacing the basis functions of the central region by the
corresponding elements of the dual basis, the lead- and central
region-subspaces become mutually orthogonal. The current formula is then
derived in the new basis, using a generalized version of second quantization
and Green's function theory to handle the nonorthogonality within each of the
regions. Finally, the appropriate nonorthogonal form of the perturbation series
for the Green's function is established for the case of electron-electron and
electron-phonon interactions in the central region.Comment: Added references. 8 pages, 1 figur
The age of 47Tuc from self-consistent isochrone fits to colour-magnitude diagrams and the eclipsing member V69
Our aim is to derive a self-consistent age, distance and composition for the
globular cluster Tucanae (Tuc; NGC104). First, we reevaluate the
reddening towards the cluster resulting in a nominal as
the best estimate. The of the components of the eclipsing binary
member V69 is found to be K from both photometric and spectroscopic
evidence. This yields a true distance modulus (random)(systematic) to Tuc when combined with existing measurements of
V69 radii and luminosity ratio. We then present a new completely
self-consistent isochrone fitting method to ground based and
cluster colour-magnitude diagrams and the eclipsing binary member V69. The
analysis suggests that the composition of V69, and by extension one of the
populations of Tuc, is given by [Fe/H], [O/Fe], and
on the solar abundance scale of Asplund, Grevesse & Sauval.
However, this depends on the accuracy of the model scale which is
50-75 K cooler than our best estimate but within measurement uncertainties. Our
best estimate of the age of Tuc is 11.8 Gyr, with firm () lower
and upper limits of 10.4 and 13.4 Gyr, respectively, in satisfactory agreement
with the age derived from the white dwarf cooling sequence if our determination
of the distance modulus is adopted.Comment: 19 pages, 8 figures, accepted for publication in MNRA
Conserving GW scheme for nonequilibrium quantum transport in molecular contacts
We give a detailed presentation of our recent scheme to include correlation
effects in molecular transport calculations using the GW approximation within
the non-equilibrium Keldysh formalism. We restrict the GW self-energy to the
central region, and describe the leads by density functional theory (DFT). A
minimal basis of maximally localized Wannier functions is applied both in the
central GW region and the leads. The importance of using a conserving, i.e.
fully self-consistent, GW self-energy is demonstrated both analytically and by
numerical examples. We introduce an effective spin-dependent interaction which
automatically reduces self-interaction errors to all orders in the interaction.
The scheme is applied to the Anderson model in- and out of equilibrium. In
equilibrium at zero temperature we find that GW describes the Kondo resonance
fairly well for intermediate interaction strengths. Out of equilibrium we
demonstrate that the one-shot G0W0 approximation can produce severe errors, in
particular at high bias. Finally, we consider a benzene molecule between
featureless leads. It is found that the molecule's HOMO-LUMO gap as calculated
in GW is significantly reduced as the coupling to the leads is increased,
reflecting the more efficient screening in the strongly coupled junction. For
the IV characteristics of the junction we find that HF and G0W0[G_HF] yield
results closer to GW than does DFT and G0W0[G_DFT]. This is explained in terms
of self-interaction effects and life-time reduction due to electron-electron
interactions.Comment: 23 pages, 16 figure
Hybrid Local-Order Mechanism for Inversion Symmetry Breaking
Using classical Monte Carlo simulations, we study a simple statistical
mechanical model of relevance to the emergence of polarisation from local
displacements on the square and cubic lattices. Our model contains two key
ingredients: a Kitaev-like orientation-dependent interaction between nearest
neighbours, and a steric term that acts between next-nearest neighbours. Taken
by themselves, each of these two ingredients is incapable of driving long-range
symmetry breaking, despite the presence of a broad feature in the corresponding
heat capacity functions. Instead each component results in a "hidden"
transition on cooling to a manifold of degenerate states, the two manifolds are
different in the sense that they reflect distinct types of local order.
Remarkably, their intersection---\emph{i.e.} the ground state when both
interaction terms are included in the Hamiltonian---supports a spontaneous
polarisation. In this way, our study demonstrates how local ordering mechanisms
might be combined to break global inversion symmetry in a manner conceptually
similar to that operating in the "hybrid" improper ferroelectrics. We discuss
the relevance of our analysis to the emergence of spontaneous polarisation in
well-studied ferroelectrics such as BaTiO and KNbO.Comment: 8 pages, 8 figure
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
Strong plasmon-phonon splitting and hybridization in 2D materials revealed through a self-energy approach
We reveal new aspects of the interaction between plasmons and phonons in 2D
materials that go beyond a mere shift and increase in plasmon width due to
coupling to either intrinsic vibrational modes of the material or phonons in a
supporting substrate. More precisely, we predict strong plasmon splitting due
to this coupling, resulting in a characteristic avoided crossing scheme. We
base our results on a computationally efficient approach consisting in
including many-body interactions through the electron self-energy. We specify
this formalism for a description of plasmons based upon a tight-binding
electron Hamiltonian combined with the random-phase approximation. This
approach is accurate provided vertex corrections can be neglected, as is is the
case in conventional plasmon-supporting metals and Dirac-fermion systems. We
illustrate our method by evaluating plasmonic spectra of doped graphene
nanotriangles with varied size, where we predict remarkable peak splittings and
other radical modifications in the spectra due to plasmons interactions with
intrinsic optical phonons. Our method is equally applicable to other 2D
materials and provides a simple approach for investigating coupling of plasmons
to phonons, excitons, and other excitations in hybrid thin nanostructures
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