6,490 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
Electronic correlations in double ionization of atoms in pump-probe experiments
The ionization dynamics of a two-electron atom in an attosecond XUV-infrared
pump-probe experiment is simulated by solving the time-dependent two-electron
Schr\"odinger equation. A dramatic change of the double ionization (DI) yield
with variation of the pump-probe delay is reported and the governing role of
electron-electron correlations is shown. The results allow for a direct control
of the DI yield and of the relative strength of double and single ionization
C in intense femtosecond laser pulses: nonlinear dipole response and ionization
We study the interaction of strong femtosecond laser pulses with the C
molecule employing time-dependent density functional theory with the ionic
background treated in a jellium approximation. The laser intensities considered
are below the threshold of strong fragmentation but too high for perturbative
treatments such as linear response. The nonlinear response of the model to
excitations by short pulses of frequencies up to 45eV is presented and analyzed
with the help of Kohn-Sham orbital resolved dipole spectra. In femtosecond
laser pulses of 800nm wavelength ionization is found to occur multiphoton-like
rather than via excitation of a ``giant'' resonance.Comment: 14 pages, including 1 table, 5 figure
Delayed differentiation of epidermal cells walls can underlie pedomorphosis in plants: the case of pedomorphic petals in the hummingbird-pollinated Caiophora hibiscifolia (Loasaceae, subfam. Loasoideae) species
Understanding the relationship between macroevolutionary diversity and variation in organism development is an important goal of evolutionary biology. Variation in the morphology of several plant and animal lineages is attributed to pedomorphosis, a case of heterochrony, where an ancestral juvenile shape is retained in an adult descendant. Pedomorphosis facilitated morphological adaptation in different plant lineages, but its cellular and molecular basis needs further exploration. Plant development differs from animal development in that cells are enclosed by cell walls and do not migrate. Moreover, in many plant lineages, the differentiated epidermis of leaves, and leaf-derived structures, such as petals, limits organ growth. We, therefore, proposed that pedomorphosis in leaves, and in leaf-derived structures, results from delayed differentiation of epidermal cells with respect to reproductive maturity. This idea was explored for petal evolution, given the importance of corolla morphology for angiosperm reproductive success
EUV ionization of pure He nanodroplets: Mass-correlated photoelectron imaging, Penning ionization and electron energy-loss spectra
The ionization dynamics of pure He nanodroplets irradiated by EUV radiation
is studied using Velocity-Map Imaging PhotoElectron-PhotoIon COincidence
(VMI-PEPICO) spectroscopy. We present photoelectron energy spectra and angular
distributions measured in coincidence with the most abundant ions He+, He2+,
and He3+. Surprisingly, below the autoionization threshold of He droplets we
find indications for multiple excitation and subsequent ionization of the
droplets by a Penning-like process. At high photon energies we evidence
inelastic collisions of photoelectrons with the surrounding He atoms in the
droplets
Rab11 regulates recycling through the pericentriolar recycling endosome
Small GTPases of the rab family are crucial elements of the machinery that controls membrane traffic. In the present study, we examined the distribution and function of rab11. Rab11 was shown by confocal immunofluorescence microscopy and EM to colocalize with internalized transferrin in the pericentriolar recycling compartment of CHO and BHK cells. Expression of rab11 mutants that are preferentially in the GTP- or GDP-bound state caused opposite effects on the distribution of transferrin-containing elements; rab11-GTP expression caused accumulation of labeled elements in the perinuclear area of the cell, whereas rab11-GDP caused a dispersion of the transferrin labeling. Functional studies showed that the early steps of uptake and recycling for transferrin were not affected by overexpression of rab11 proteins. However, recycling from the later recycling endosome was inhibited in cells overexpressing the rab11-GDP mutant. Rab5, which regulates early endocytic trafficking, acted before rab11 in the transferrin-recycling pathway as expression of rab5-GTP prevented transport to the rab11-positive. recycling endosome. These results suggest a novel role for rab11 in controlling traffic through the recycling endosome
Comparison of theoretical and absolute experimental fully differential cross sections for ion-atom impact ionization
We report fully differential cross section (FDCS) calculations and absolute measurements for ion-atom impact ionization. Using the COLTRIMS (cold target recoil ion momentum spectroscopy) method we have obtained absolute FDCS both in the scattering plane as well as out of the scattering plane for 100 MeV amu(- 1) C6+ ionization of helium FDCS results are presented for different projectile scattering angles and ejected electron energies. The measurements are compared with a theoretical calculation employing an asymptotically exact three body final state wavefunction that contains all active two particle subsystem interactions to infinite order in perturbation theory. For the active electron a Hartree-Fock (HF) bound state wavefunction is used for the initial state and numerical continuum state eigenfunctions of a HF potential for the ion are used for the final state In the scattering plane these theoretical results are in very good agreement with experiment for small and intermediate momentum transfer. However some significant discrepancies are found for large momentum transfer and outside the scattering plane. These discrepancies disappear upon comparison with successively less differential cross sections
Spiritual connectedness through prayer as a mediator of the relationship between Indigenous language use and positive mental health
Objective: The objective of this study is to understand how Indigenous language and spirituality revitalization efforts may affect mental health within Indigenous communities. Although Indigenous communities experience disproportionate rates of mental health problems, research supporting language and spiritualityâs role in improving mental health is under-researched and poorly understood. Method: Data for this study are from a Community-based Participatory Research Project involving five Anishinaabe tribes in Minnesota and Wisconsin. Participants were sampled from clinic records of adults with a diagnosis of type 2 diabetes, living on or near the reservation, and self-identifying as American Indian (mean age = 46.3; n = 191). Result: Structural equation modeling illustrates that language use in the home is associated with positive mental health through spiritual connectedness. Conclusion: Results support tribal community expressions of the positive effects of cultural involvement for Indigenous wellbeing, and improve what is known about the interconnectedness of language and spirituality.Sociolog
A two-dimensional, two-electron model atom in a laser pulse: exact treatment, single active electron-analysis, time-dependent density functional theory, classical calculations, and non-sequential ionization
Owing to its numerical simplicity, a two-dimensional two-electron model atom,
with each electron moving in one direction, is an ideal system to study
non-perturbatively a fully correlated atom exposed to a laser field. Frequently
made assumptions, such as the ``single active electron''- approach and
calculational approximations, e.g. time dependent density functional theory or
(semi-) classical techniques, can be tested. In this paper we examine the
multiphoton short pulse-regime. We observe ``non-sequential'' ionization, i.e.\
double ionization at lower field strengths as expected from a sequential,
single active electron-point of view. Since we find non-sequential ionization
also in purely classical simulations, we are able to clarify the mechanism
behind this effect in terms of single particle trajectories. PACS Number(s):
32.80.RmComment: 10 pages, 16 figures (gzipped postscript), see also
http://www.physik.tu-darmstadt.de/tqe
Time-dependent electron transport through a strongly correlated quantum dot: multiple-probe open boundary conditions approach
We present a time-dependent study of electron transport through a strongly
correlated quantum dot. The time-dependent current is obtained with the
multiple-probe battery method, while adiabatic lattice density functional
theory in the Bethe ansatz local-density approximation to the Hubbard model
describes the dot electronic structure. We show that for a certain range of
voltages the quantum dot can be driven into a dynamical state characterized by
regular current oscillations. This is a manifestation of a recently proposed
dynamical picture of Coulomb blockade. Furthermore, we investigate how the
various approximations to the electron-electron interaction affect the
line-shapes of the Coulomb peaks and the I-V characteristics. We show that the
presence of the derivative discontinuity in the approximate
exchange-correlation potential leads to significantly different results
compared to those obtained at the simpler Hartree level of description. In
particular, a negative differential conductance (NDC) in the I-V
characteristics is observed at large bias voltages and large Coulomb
interaction strengths. We demonstrate that such NDC originates from the
combined effect of electron-electron interaction in the dot and the finite
bandwidth of the electrodes.Comment: 10 pages, 7 figure
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