209 research outputs found
Dynamical Image Charge Effect in Molecular Tunnel Junctions: Beyond Energy Level Alignment
When an electron tunnels between two metal contacts it temporarily induces an
image charge (IC) in the electrodes which acts back on the tunneling electron.
It is usually assumed that the IC forms instantaneously such that a static
model for the image potential applies. Here we investigate how the finite IC
formation time affects charge transport through a molecule suspended between
two electrodes. For a single level model, an analytical treatment shows that
the conductance is suppressed by a factor (compared to the static IC
approximation) where is the quasiparticle renormalization factor. We show
that can be expressed either in terms of the plasma frequency of the
electrode or as the overlap between the ground states of the electrode with and
without an electron on the molecule. First-principles GW calculations for
benzene-diamine connected to gold electrodes show that the dynamical
corrections can reduce the conductance by more than a factor of two.Comment: 5 pages, 3 figure
Renormalization of Molecular Quasiparticle Levels at Metal-Molecule Interfaces: Trends Across Binding Regimes
When an electron or a hole is added into an orbital of an adsorbed molecule
the substrate electrons will rearrange in order to screen the added charge.
This results in a reduction of the electron addition/removal energies as
compared to the free molecule case. In this work we use a simple model to
illustrate the universal trends of this renormalization mechanism as a function
of the microscopic key parameters. Insight of both fundamental and practical
importance is obtained by comparing GW quasiparticle energies with Hartree-Fock
and Kohn-Sham calculations. We identify two different polarization mechanisms:
(i) polarization of the metal (image charge formation) and (ii) polarization of
the molecule via charge transfer across the interface. The importance of (i)
and (ii) is found to increase with the metal density of states at the Fermi
level and metal-molecule coupling strength, respectively.Comment: 4 pages, 3 figure
Extending the random-phase approximation for electronic correlation energies: The renormalized adiabatic local density approximation
The adiabatic connection fluctuation-dissipation theorem with the random
phase approximation (RPA) has recently been applied with success to obtain
correlation energies of a variety of chemical and solid state systems. The main
merit of this approach is the improved description of dispersive forces while
chemical bond strengths and absolute correlation energies are systematically
underestimated. In this work we extend the RPA by including a parameter-free
renormalized version of the adiabatic local density (ALDA) exchange-correlation
kernel. The renormalization consists of a (local) truncation of the ALDA kernel
for wave vectors , which is found to yield excellent results for the
homogeneous electron gas. In addition, the kernel significantly improves both
the absolute correlation energies and atomization energies of small molecules
over RPA and ALDA. The renormalization can be straightforwardly applied to
other adiabatic local kernels.Comment: 5 page
Static correlation beyond the random phase approximation: Dissociating H2 with the Bethe-Salpeter equation and time-dependent GW
We investigate various approximations to the correlation energy of a H
molecule in the dissociation limit, where the ground state is poorly described
by a single Slater determinant. The correlation energies are derived from the
density response function and it is shown that response functions derived from
Hedin's equations (Random Phase Approximation (RPA), Time-dependent
Hartree-Fock (TDHF), Bethe-Salpeter equation (BSE), and Time-Dependent GW
(TDGW)) all reproduce the correct dissociation limit. We also show that the BSE
improves the correlation energies obtained within RPA and TDHF significantly
for intermediate binding distances. A Hubbard model for the dimer allow us to
obtain exact analytical results for the various approximations, which is
readily compared with the exact diagonalization of the model. Moreover, the
model is shown to reproduce all the qualitative results from the \textit{ab
initio} calculations and confirms that BSE greatly improves the RPA and TDHF
results despite the fact that the BSE excitation spectrum breaks down in the
dissociation limit. In contrast, Second Order Screened Exchange (SOSEX) gives a
poor description of the dissociation limit, which can be attributed to the fact
that it cannot be derived from an irreducible response function
Adiabatic-connection fluctuation-dissipation DFT for the structural properties of solids-the renormalized ALDA and electron gas kernels
We present calculations of the correlation energies of crystalline solids and
isolated systems within the adiabatic-connection fluctuation-dissipation
formulation of density-functional theory. We perform a quantitative comparison
of a set of model exchange-correlation kernels originally derived for the
homogeneous electron gas (HEG), including the recently-introduced renormalized
adiabatic local-density approximation (rALDA) and also kernels which (a)
satisfy known exact limits of the HEG, (b) carry a frequency dependence or (c)
display a 1/ divergence for small wavevectors. After generalizing the
kernels to inhomogeneous systems through a reciprocal-space averaging
procedure, we calculate the lattice constants and bulk moduli of a test set of
10 solids consisting of tetrahedrally-bonded semiconductors (C, Si, SiC), ionic
compounds (MgO, LiCl, LiF) and metals (Al, Na, Cu, Pd). We also consider the
atomization energy of the H molecule. We compare the results calculated
with different kernels to those obtained from the random-phase approximation
(RPA) and to experimental measurements. We demonstrate that the model kernels
correct the RPA's tendency to overestimate the magnitude of the correlation
energy whilst maintaining a high-accuracy description of structural properties.Comment: 41 pages, 7 figure
Excitons in van der Waals heterostructures: The important role of dielectric screening
The existence of strongly bound excitons is one of the hallmarks of the newly
discovered atomically thin semi-conductors. While it is understood that the
large binding energy is mainly due to the weak dielectric screening in two
dimensions (2D), a systematic investigation of the role of screening on 2D
excitons is still lacking. Here we provide a critical assessment of a widely
used 2D hydrogenic exciton model which assumes a dielectric function of the
form {\epsilon}(q) = 1 + 2{\pi}{\alpha}q, and we develop a quasi-2D model with
a much broader applicability. Within the quasi-2D picture, electrons and holes
are described as in-plane point charges with a finite extension in the
perpendicular direction and their interaction is screened by a dielectric
function with a non-linear q-dependence which is computed ab-initio. The
screened interaction is used in a generalized Mott-Wannier model to calculate
exciton binding energies in both isolated and supported 2D materials. For
isolated 2D materials, the quasi-2D treatment yields results almost identical
to those of the strict 2D model and both are in good agreement with ab-initio
many-body calculations. On the other hand, for more complex structures such as
supported layers or layers embedded in a van der Waals heterostructure, the
size of the exciton in reciprocal space extends well beyond the linear regime
of the dielectric function and a quasi-2D description has to replace the 2D
one. Our methodology has the merit of providing a seamless connection between
the strict 2D limit of isolated monolayer materials and the more bulk-like
screening characteristics of supported 2D materials or van der Waals
heterostructures.Comment: 14 pages, 13 figure
Non-equilibrium GW approach to quantum transport in nano-scale contacts
Correlation effects within the GW approximation have been incorporated into
the Keldysh non-equilibrium transport formalism. We show that GW describes the
Kondo effect and the zero-temperature transport properties of the Anderson
model fairly well. Combining the GW scheme with density functional theory and a
Wannier function basis set, we illustrate the impact of correlations by
computing the I-V characteristics of a hydrogen molecule between two Pt chains.
Our results indicate that self-consistency is fundamental for the calculated
currents, but that it tends to wash out satellite structures in the spectral
function.Comment: 5 pages, 4 figure
Quantifying Transition Voltage Spectroscopy of Molecular Junctions
Transition voltage spectroscopy (TVS) has recently been introduced as a
spectroscopic tool for molecular junctions where it offers the possibility to
probe molecular level energies at relatively low bias voltages. In this work we
perform extensive ab-initio calculations of the non-linear current voltage
relations for a broad class of single-molecule transport junctions in order to
assess the applicability and limitations of TVS. We find, that in order to
fully utilize TVS as a quantitative spectroscopic tool, it is important to
consider asymmetries in the coupling of the molecule to the two electrodes.
When this is taken properly into account, the relation between the transition
voltage and the energy of the molecular orbital closest to the Fermi level
closely follows the trend expected from a simple, analytical model.Comment: 5 pages, 4 figures. To appear in PR
Improving Transition Voltage Spectroscopy of Molecular Junctions
Transition voltage spectroscopy (TVS) is a promising spectroscopic tool for
molecular junctions. The principles in TVS is to find the minimum on a
Fowler-Nordheim plot where is plotted against and relate the
voltage at the minimum, , to the closest molecular level.
Importantly, , is approximately half the voltage required to see a
peak in the curve. Information about the molecular level position can
thus be obtained at relatively low voltages. In this work we show that the
molecular level position can be determined at even lower voltages, by finding the minimum of with .
On the basis of a simple Lorentzian transmission model we analyze theoretical
{\it ab initio} as well as experimental curves and show that the voltage
required to determine the molecular levels can be reduced by as
compared to conventional TVS. As for conventional TVS, the symmetry/asymmetry
of the molecular junction needs to be taken into account in order to gain
quantitative information. We show that the degree of asymmetry may be estimated
from a plot of vs. .Comment: 6 pages, 8 figure
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
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