8,201 research outputs found
On the problem of mass-dependence of the two-point function of the real scalar free massive field on the light cone
We investigate the generally assumed inconsistency in light cone quantum
field theory that the restriction of a massive, real, scalar, free field to the
nullplane is independent of mass \cite{LKS}, but the
restriction of the two-point function depends on it (see, e.g., \cite{NakYam77,
Yam97}). We resolve this inconsistency by showing that the two-point function
has no canonical restriction to in the sense of distribution theory.
Only the so-called tame restriction of the two-point function exists which we
have introduced in \cite{Ull04sub}. Furthermore, we show that this tame
restriction is indeed independent of mass. Hence the inconsistency appears only
by the erroneous assumption that the two-point function would have a
(canonical) restriction to .Comment: 10 pages, 2 figure
A minimal model for excitons within time-dependent density-functional theory
The accurate description of the optical spectra of insulators and
semiconductors remains an important challenge for time-dependent
density-functional theory (TDDFT). Evidence has been given in the literature
that TDDFT can produce bound as well as continuum excitons for specific
systems, but there are still many unresolved basic questions concerning the
role of dynamical exchange and correlation (xc). In particular, the role of the
long spatial range and the frequency dependence of the xc kernel
for excitonic binding are still not very well explored. We present a minimal
model for excitons in TDDFT, consisting of two bands from a one-dimensional
Kronig-Penney model and simple approximate xc kernels, which allows us to
address these questions in a transparent manner. Depending on the system, it is
found that adiabatic xc kernels can produce a single bound exciton, and
sometimes two bound excitons, where the long spatial range of is
not a necessary condition. It is shown how the Wannier model, featuring an
effective electron-hole interaction, emerges from TDDFT. The collective,
many-body nature of excitons is explicitly demonstrated.Comment: 12 pages, 11 figure
Ultra-nonlocality in density functional theory for photo-emission spectroscopy
We derive an exact expression for the photo-current of photo-emission
spectroscopy using time-dependent current density functional theory (TDCDFT).
This expression is given as an integral over the Kohn-Sham spectral function
renormalized by effective potentials that depend on the exchange-correlation
kernel of current density functional theory. We analyze in detail the physical
content of this expression by making a connection between the
density-functional expression and the diagrammatic expansion of the
photo-current within many-body perturbation theory. We further demonstrate that
the density functional expression does not provide us with information on the
kinetic energy distribution of the photo-electrons. Such information can, in
principle, be obtained from TDCDFT by exactly modeling the experiment in which
the photo-current is split into energy contributions by means of an external
electromagnetic field outside the sample, as is done in standard detectors. We
find, however, that this procedure produces very nonlocal correlations between
the exchange-correlation fields in the sample and the detector.Comment: 11 pages, 11 figure
Time-resolved X-ray microscopy of nanoparticle aggregates under oscillatory shear
Of all current detection techniques with nanometer resolution, only X-ray
microscopy allows imaging nanoparticles in suspension. Can it also be used to
investigate structural dynamics? When studying response to mechanical stimuli,
the challenge lies in applying them with precision comparable to spatial
resolution. In the first shear experiments performed in an X-ray microscope, we
accomplished this by inserting a piezo actuator driven shear cell into the
focal plane of a scanning transmission X-ray microscope (STXM). Thus
shear-induced reorganization of magnetite nanoparticle aggregates could be
demonstrated in suspension. As X-ray microscopy proves suitable for studying
structural change, new prospects open up in physics at small length scales.Comment: submitted to J. Synchrot. Radia
A systematic benchmark of the ab initio Bethe-Salpeter equation approach for low-lying optical excitations of small organic molecules
The predictive power of the ab initio Bethe-Salpeter equation (BSE) approach,
rigorously based on many-body Green's function theory but incorporating
information from density functional theory, has already been demonstrated for
the optical gaps and spectra of solid-state systems. Interest in photoactive
hybrid organic/inorganic systems has recently increased, and so has the use of
the BSE for computing neutral excitations of organic molecules. However, no
systematic benchmarks of the BSE for neutral electronic excitations of organic
molecules exist. Here, we study the performance of the BSE for the 28 small
molecules in Thiel's widely-used time-dependent density functional theory
benchmark set [M. Schreiber et al. J. Chem. Phys. 128, 134110 (2008)]. We
observe that the BSE produces results that depend critically on the mean-field
starting point employed in the perturbative approach. We find that this
starting point dependence is mainly introduced through the quasiparticle
energies obtained at the intermediate GW step, and that with a judicious choice
of starting mean-field, singlet excitation energies obtained from BSE are in
excellent quantitative agreement with higher-level wavefunction methods. The
quality of the triplet excitations is slightly less satisfactory
Photoelectron spectra of anionic sodium clusters from time-dependent density-functional theory in real-time
We calculate the excitation energies of small neutral sodium clusters in the
framework of time-dependent density-functional theory. In the presented
calculations, we extract these energies from the power spectra of the dipole
and quadrupole signals that result from a real-time and real-space propagation.
For comparison with measured photoelectron spectra, we use the ionic
configurations of the corresponding single-charged anions. Our calculations
clearly improve on earlier results for photoelectron spectra obtained from
static Kohn-Sham eigenvalues
Semiclassical description of the kinematically complete experiments
Based on the semiclassical, impact parameter method a theoretical model is
constructed to calculate totally differential cross sections for single
ionization of helium by impact with fast C ions. Good agreement with the
experiment is achieved in the scattering plane, while in the perpendicular
plane a similar structure to that observed experimentally is obtained. The
contribution of different partial waves to the cross section is also
investigated.Comment: 9 pages, 6 figure
Semiclassical model for calculating fully differential ionization cross sections of the H molecule
Fully differential cross sections are calculated for the ionization of H
by fast charged projectiles using a semiclassical model developed previously
for the ionization of atoms. The method is tested in case of 4 keV electron and
6 MeV proton projectiles. The obtained results show good agreement with the
available experimental data. Interference effects due to the two-center
character of the target are also observed and analyzed.Comment: 11 pages, 4 figure
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