1,830 research outputs found
Towards quantitative accuracy in first-principles transport calculations: The GW method applied to alkane/gold junctions
The calculation of electronic conductance of nano-scale junctions from first
principles is a long standing problem in molecular electronics. Here we
demonstrate excellent agreement with experiments for the transport properties
of the gold/alkanediamine benchmark system when electron-electron interactions
are described using the many-body GW approximation. The main difference from
standard density functional theory (DFT) calculations is a significant
reduction of the contact conductance, G_c, due an improved alignment of the
molecular energy levels with the metal Fermi energy. The molecular orbitals
involved in the tunneling process comprise states delocalized over the carbon
backbone and states localized on the amine end groups. We find that dynamical
screening effects renormalize the two types of states in qualitatively
different ways when the molecule is inserted in the junction. Consequently, the
GW transport results cannot be mimicked by DFT calculations employing a simple
scissors operator.Comment: 7 page
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
Renormalization of Optical Excitations in Molecules near a Metal Surface
The lowest electronic excitations of benzene and a set of donor-acceptor
molecular complexes are calculated for the gas phase and on the Al(111) surface
using the many-body Bethe-Salpeter equation (BSE). The energy of the
charge-transfer excitations obtained for the gas phase complexes are found to
be around 10% lower than the experimental values. When the molecules are placed
outside the surface, the enhanced screening from the metal reduces the exciton
binding energies by several eVs and the transition energies by up to 1 eV
depending on the size of the transition-generated dipole. As a striking
consequence we find that close to the metal surface the optical gap of benzene
can exceed its quasiparticle gap. A classical image charge model for the
screened Coulomb interaction can account for all these effects which, on the
other hand, are completely missed by standard time-dependent density functional
theory.Comment: 4 pages, 3 figures; revised versio
Unraveling the acoustic electron-phonon interaction in graphene
Using a first-principles approach we calculate the acoustic electron-phonon
couplings in graphene for the transverse (TA) and longitudinal (LA) acoustic
phonons. Analytic forms of the coupling matrix elements valid in the
long-wavelength limit are found to give an almost quantitative description of
the first-principles based matrix elements even at shorter wavelengths. Using
the analytic forms of the coupling matrix elements, we study the acoustic
phonon-limited carrier mobility for temperatures 0-200 K and high carrier
densities of 10^{12}-10^{13} cm^{-2}. We find that the intrinsic effective
acoustic deformation potential of graphene is \Xi_eff = 6.8 eV and that the
temperature dependence of the mobility \mu ~ T^{-\alpha} increases beyond an
\alpha = 4 dependence even in the absence of screening when the full coupling
matrix elements are considered. The large disagreement between our calculated
deformation potential and those extracted from experimental measurements (18-29
eV) indicates that additional or modified acoustic phonon-scattering mechanisms
are at play in experimental situations.Comment: 7 pages, 3 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
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
Assessment of the fiscal stance appropriate for the euro area in 2019
On 18 June 2018, the European Fiscal Board (EFB) has published its assessment of the general orientation of fiscal policy in the euro area. The report concludes that the favourable economic outlook offers a prime opportunity to rebuild fiscal buffers. Especially euro area Member States with a high government debt-to-GDP ratio need to do more than simply accrue the budgetary benefits of the economic expansion. Lest we repeat the mistakes of the past and rob ourselves of room to manoeuvre when the next crisis hits, this is the time to move towards a somewhat restrictive orientation of fiscal policy in the euro area. It is also the time to upgrade the EU's fiscal framework and prepare a capacity for joint stabilisation for the euro area
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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