6,977 research outputs found
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
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
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
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
Electron-phonon interaction and transport properties of metallic bulk and monolayer transition metal dichalcogenide TaS
Transition metal dichalcogenides have recently emerged as promising
two-dimensional materials with intriguing electronic properties. Existing
calculations of intrinsic phonon-limited electronic transport so far have
concentrated on the semicondcucting members of this family. In this paper we
extend these studies by investigating the influence of electron-phonon coupling
on the electronic transport properties and band renormalization of prototype
inherent metallic bulk and monolayer TaS. Based on density functional
perturbation theory and semi-classical Boltzmann transport calculations,
promising room temperature mobilities and sheet conductances are found, which
can compete with other established 2D materials, leaving TaS as promising
material candidate for transparent conductors or as atomically thin
interconnects. Throughout the paper, the electronic and transport properties of
TaS are compared to those of its isoelectronic counterpart TaSe and
additional informations to the latter are given. We furthermore comment on the
conventional su- perconductivity in TaS, where no phonon-mediated
enhancement of TC in the monolayer compared to the bulk state was found.Comment: accepted in IOPscience 2D Materials, supplemental material is
available on the publishers pag
Computational 2D Materials Database: Electronic Structure of Transition-Metal Dichalcogenides and Oxides
We present a comprehensive first-principles study of the electronic structure
of 51 semiconducting monolayer transition metal dichalcogenides and -oxides in
the 2H and 1T hexagonal phases. The quasiparticle (QP) band structures with
spin-orbit coupling are calculated in the approximation and comparison
is made with different density functional theory (DFT) descriptions. Pitfalls
related to the convergence of calculations for 2D materials are discussed
together with possible solutions. The monolayer band edge positions relative to
vacuum are used to estimate the band alignment at various heterostructure
interfaces. The sensitivity of the band structures to the in-plane lattice
constant is analysed and rationalized in terms of the electronic structure.
Finally, the -dependent dielectric functions and effective electron/hole
masses are obtained from the QP band structure and used as input to a 2D
hydrogenic model to estimate exciton binding energies. Throughout the paper we
focus on trends and correlations in the electronic structure rather than
detailed analysis of specific materials. All the computed data is available in
an open database.Comment: 24 pages, 13 figures and 5 tables. J. Phys. Chem. C, Article ASAP,
Publication Date (Web): April 30, 201
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
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