2,925 research outputs found
Quantum fluid-dynamics from density functional theory
A partial differential eigenvalue equation for the density displacement
fields associated with electronic excitations is derived in the framework of
density functional theory. Our quantum fluid-dynamical approach is based on a
variational principle and the Kohn-Sham ground-state energy functional, using
only the occupied Kohn-Sham orbitals. It allows for an intuitive interpretation
of electronic excitations in terms of intrinsic local currents that obey a
continuity equation. We demonstrate the capabilities of this non-empirical
approach by calculating the photoabsorption spectra of small sodium clusters.
The quantitative agreement between theoretical and experimental spectra shows
that even for the smallest clusters, the resonances observed experimentally at
low temperatures can be interpreted in terms of density vibrations.Comment: RevTeX file with 2 figures. Update on April 17 2001: Typos corrected,
references updated, larger axes labels on Fig. 1. Accepted for publication in
Phys. Rev.
Proximity Effect, Andreev Reflections, and Charge Transport in Mesoscopic Superconducting-Semiconducting Heterostructures
In the quasi-twodimensional (Q2D) electron gas of an InAs channel between an
AlSb substrate and superconducting Niobium layers the proximity effect induces
a pair potential so that a Q2D mesoscopic
superconducting-normal-superconducting (SNS) junction forms in the channel. The
pair potential is calculated with quasiclassical Green's functions in the clean
limit. For such a junction alternating Josephson currents and current-voltage
characteristics (CVCs) are computed, using the non-equilibrium quasiparticle
wavefunctions which solve the time-dependent Bogoliubov-de Gennes Equations.
The CVCs exhibit features found experimentally by the Kroemer group: A steep
rise of the current at small voltages ("foot") changes at a "corner current" to
a much slower increase of current with higher voltages, and the zero-bias
differential resistance increases with temperature. Phase-coherent multiple
Andreev reflections and the associated Cooper pair transfers are the physical
mechanisms responsible for the oscillating Josephson currents and the CVCs.
Additional experimental findings not reproduced by the theory require model
improvements, especially a consideration of the external current leads which
should give rise to hybrid quasiparticle/collective mode excitations.Comment: 8 pages, 4 figures (consisting of 5 .ps-files), added referenc
Collectivity in the optical response of small metal clusters
The question whether the linear absorption spectra of metal clusters can be
interpreted as density oscillations (collective ``plasmons'') or can only be
understood as transitions between distinct molecular states is still a matter
of debate for clusters with only a few electrons. We calculate the
photoabsorption spectra of Na2 and Na5+ comparing two different methods:
quantum fluid-dynamics and time-dependent density functional theory. The
changes in the electronic structure associated with particular excitations are
visualized in ``snapshots'' via transition densities. Our analysis shows that
even for the smallest clusters, the observed excitations can be interpreted as
intuitively understandable density oscillations. For Na5+, the importance of
self-interaction corrections to the adiabatic local density approximation is
demonstrated.Comment: 6 pages, 3 figures. To appear in special issue of Applied Physics B,
"Optical properties of Nanoparticles
In Silico Genome-Scale Reconstruction and Validation of the Staphylococcus aureus Metabolic Network
A genome-scale metabolic model of the Gram-positive, facultative anaerobic opportunistic pathogen Staphylococcus aureus N315 was constructed based on current genomic data, literature, and physiological information. The model comprises 774 metabolic processes representing approximately 23% of all protein-coding regions. The model was extensively validated against experimental observations and it correctly predicted main physiological properties of the wild-type strain, such as aerobic and anaerobic respiration and fermentation. Due to the frequent involvement of S. aureus in hospital-acquired bacterial infections combined with its increasing antibiotic resistance, we also investigated the clinically relevant phenotype of small colony variants and found that the model predictions agreed with recent findings of proteome analyses. This indicates that the model is useful in assisting future experiments to elucidate the interrelationship of bacterial metabolism and resistance. To help directing future studies for novel chemotherapeutic targets, we conducted a large-scale in silico gene deletion study that identified 158 essential intracellular reactions. A more detailed analysis showed that the biosynthesis of glycans and lipids is rather rigid with respect to circumventing gene deletions, which should make these areas particularly interesting for antibiotic development. The combination of this stoichiometric model with transcriptomic and proteomic data should allow a new quality in the analysis of clinically relevant organisms and a more rationalized system-level search for novel drug targets.
Piecewise linearity in the approximation for accurate quasiparticle energy predictions
We identify the deviation from the straight line error (DSLE) -- i.e., the
spurious non-linearity of the total energy as a function of fractional particle
number -- as the main source for the discrepancy between experimental vertical
ionization energies and theoretical quasiparticle energies, as obtained from
the and +SOSEX approximations to many-body perturbation theory (MBPT).
For self-consistent calculations, we show that suffers from a small DSLE.
Conversely, for perturbative and +SOSEX calculations the DSLE
depends on the starting point. We exploit this starting-point dependence to
reduce (or completely eliminate) the DSLE. We find that the agreement with
experiment increases as the DSLE reduces. DSLE-minimized schemes, thus, emerge
as promising avenues for future developments in MBPT
On the challenge to improve the density response with unusual gradient approximations
Certain excitations, especially ones of long-range charge transfer character,
are poorly described by time-dependent density functional theory (TDDFT) when
typical (semi-)local functionals are used. A proper description of these
excitations would require an exchange-correlation response differing
substantially from the usual (semi-)local one. It has recently been shown that
functionals of the generalized gradient approximation (GGA) type can yield
unusual potentials, mimicking features of the exact exchange derivative
discontinuity and showing divergences on orbital nodal surfaces. We here
investigate whether these unusual potential properties translate into
beneficial response properties. Using the Sternheimer formalism we closely
investigate the response obtained with the 2013 exchange approximation by
Armiento and K\"ummel (AK13) and the 1988 exchange approximation by Becke
(B88), both of which show divergences on orbital nodal planes. Numerical
calculations for Na2 as well as analytical and numerical calculations for the
hydrogen atom show that the response of AK13 behaves qualitatively different
from usual semi local functionals. However, the AK13 functional leads to
fundamental instabilities in the asymptotic region that prevent its practical
application in TDDFT. Our findings may help the development of future improved
functionals, and corroborate that the frequency-dependent Sternheimer formalism
is excellently suited for running and analyzing TDDFT calculations
Static Electric Dipole Polarizabilities of Na Clusters
The static electric dipole polarizability of clusters with
even N has been calculated in a collective, axially averaged and a
three-dimensional, finite-field approach for , including the
ionic structure of the clusters. The validity of a collective model for the
static response of small systems is demonstrated. Our density functional
calculations verify the trends and fine structure seen in a recent experiment.
A pseudopotential that reproduces the experimental bulk bond length and atomic
energy levels leads to a substantial increase in the calculated
polarizabilities, in better agreement with experiment. We relate remaining
differences in the magnitude of the theoretical and experimental
polarizabilities to the finite temperature present in the experiments.Comment: 7 pages, 3 figures, accepted for publication in the European Physical
Journal
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