3,421 research outputs found
Theoretical analysis of STM-derived lifetimes of excitations in the Shockley surface state band of Ag(111)
We present a quantitative many-body analysis using the GW approximation of
the decay rate due to electron-electron scattering of excitations in
the Shockley surface state band of Ag(111), as measured using the scanning
tunnelling microscope (STM). The calculations include the perturbing influence
of the STM, which causes a Stark-shift of the surface state energy and
concomitant increase in . We find varies more rapidly with
than recently found for image potential states, where the STM has been shown to
significantly affect measured lifetimes. For the Shockley states, the
Stark-shifts that occur under normal tunnelling conditions are relatively small
and previous STM-derived lifetimes need not be corrected.Comment: 4 pages, 3 figure
Ab initio calculation of the binding energy of impurities in semiconductors: Application to Si nanowires
We discuss the binding energy E_b of impurities in semiconductors within
density functional theory (DFT) and the GW approximation, focusing on donors in
nanowires as an example. We show that DFT succeeds in the calculation of E_b
from the Kohn-Sham (KS) hamiltonian of the ionized impurity, but fails in the
calculation of E_b from the KS hamiltonian of the neutral impurity, as it
misses most of the interaction of the bound electron with the surface
polarization charges of the donor. We trace this deficiency back to the lack of
screened exchange in the present functionals
GW quasi-particle spectra from occupied states only
We introduce a method that allows for the calculation of quasi-particle
spectra in the GW approximation, yet avoiding any explicit reference to empty
one-electron states. This is achieved by expressing the irreducible
polarizability operator and the self-energy operator through a set of linear
response equations, which are solved using a Lanczos-chain algorithm. We first
validate our approach by calculating the vertical ionization energies of the
benzene molecule and then show its potential by addressing the spectrum of a
large molecule such as free-base tetraphenylporphyrin.Comment: 4 pages, 3 figure
A theoretical analysis of the chemical bonding and electronic structure of graphene interacting with Group IA and Group VIIA elements
We propose a new class of materials, which can be viewed as graphene
derivatives involving Group IA or Group VIIA elements, forming what we refer to
as graphXene. We show that in several cases large band gaps can be found to
open up, whereas in other cases a semimetallic behavior is found. Formation
energies indicate that under ambient conditions, sp and mixed sp/sp
systems will form. The results presented allow us to propose that by careful
tuning of the relative concentration of the adsorbed atoms, it should be
possible to tune the band gap of graphXene to take any value between 0 and 6.4
eV.Comment: 5 pages, 4 figures. Transferred to PR
Lifetimes of Shockley electrons and holes at the Cu(111) surface
A theoretical many-body analysis is presented of the electron-electron
inelastic lifetimes of Shockley electrons and holes at the (111) surface of Cu.
For a description of the decay of Shockley states both below and above the
Fermi level, single-particle wave functions have been obtained by solving the
Schr\"odinger equation with the use of an approximate one-dimensional
pseudopotential fitted to reproduce the correct bulk energy bands and
surface-state dispersion. A comparison with previous calculations and
experiment indicates that inelastic lifetimes are very sensitive to the actual
shape of the surface-state single-particle orbitals beyond the
() point, which controls the coupling between the Shockley
electrons and holes.Comment: 4 pages, 3 figures, to appear in Phys. Rev.
Self-energy and lifetime of Shockley and image states on Cu(100) and Cu(111): Beyond the GW approximation of many-body theory
We report many-body calculations of the self-energy and lifetime of Shockley
and image states on the (100) and (111) surfaces of Cu that go beyond the
approximation of many-body theory. The self-energy is computed in the framework
of the GW\Gamma approximation by including short-range exchange-correlation
(XC) effects both in the screened interaction W (beyond the random-phase
approximation) and in the expansion of the self-energy in terms of W (beyond
the GW approximation). Exchange-correlation effects are described within
time-dependent density-functional theory from the knowledge of an adiabatic
nonlocal XC kernel that goes beyond the local-density approximation.Comment: 8 pages, 5 figures, to appear in Phys. Rev.
Many-Body Approximation Scheme Beyond GW
We explore the combination of the extended dynamical mean field theory
(EDMFT) with the GW approximation (GWA); the former sums the local
contributions to the self-energies to infinite order in closed form and the
latter handles the non-local ones to lowest order. We investigate the different
levels of self-consistency that can be implemented within this method by
comparing to the exact QMC solution of a finite-size model Hamiltonian. We find
that using the EDMFT solution for the local self-energies as input to the GWA
for the non-local self-energies gives the best result.Comment: 4 pages, 8 figure
The Effective Particle-Hole Interaction and the Optical Response of Simple Metal Clusters
Following Sham and Rice [L. J. Sham, T. M. Rice, Phys. Rev. 144 (1966) 708]
the correlated motion of particle-hole pairs is studied, starting from the
general two-particle Greens function. In this way we derive a matrix equation
for eigenvalues and wave functions, respectively, of the general type of
collective excitation of a N-particle system. The interplay between excitons
and plasmons is fully described by this new set of equations. As a by-product
we obtain - at least a-posteriori - a justification for the use of the TDLDA
for simple-metal clusters.Comment: RevTeX, 15 pages, 5 figures in uufiles format, 1 figure avaible from
[email protected]
Frequency-dependent local interactions and low-energy effective models from electronic structure calculations
We propose a systematic procedure for constructing effective models of
strongly correlated materials. The parameters, in particular the on-site
screened Coulomb interaction U, are calculated from first principles, using the
GW approximation. We derive an expression for the frequency-dependent U and
show that its high frequency part has significant influence on the spectral
functions. We propose a scheme for taking into account the energy dependence of
U, so that a model with an energy-independent local interaction can still be
used for low-energy properties.Comment: 16 pages, 5 figure
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