16,862 research outputs found
Open shells in reduced-density-matrix-functional theory
Reduced-density-matrix-functional theory is applied to open-shell systems. We
introduce a spin-restricted formulation by appropriately expressing approximate
correlation-energy functionals in terms of spin-dependent occupation numbers
and spin-independent natural orbitals. We demonstrate that the additional
constraint of total-spin conservation is indispensable for the proper treatment
of open-shell systems. The formalism is applied to the first-row open-shell
atoms. The obtained ground-state energies are in very good agreement with the
exact values as well as other state of the art quantum chemistry calculationsComment: 4 pages, 2 figures, corrected typo
Competing spin transfer and dissipation at Co/Cu(001) interfaces on femtosecond timescales
By combining interface-sensitive non-linear magneto-optical experiments with
femtosecond time resolution and ab-initio time-dependent density functional
theory, we show that optically excited spin dynamics at Co/Cu(001) interfaces
proceeds via spin-dependent charge transfer and backtransfer between Co and Cu.
This ultrafast spin transfer competes with dissipation of spin angular momentum
mediated by spin-orbit coupling already on sub 100 fs timescales. We thereby
identify the fundamental microscopic processes during laser-induced spin
transfer at a model interface for technologically relevant ferromagnetic
heterostructures.Comment: 5 pages, 4 figure
COMPLETE SOLUTION OF THE XXZ-MODEL ON FINITE RINGS. DYNAMICAL STRUCTURE FACTORS AT ZERO TEMPERATURE.
The finite size effects of the dynamical structure factors in the XXZ-model
are studied in the euclidean time -representation. Away from the
critical momentum finite size effects turn out to be small except for
the large limit. The large finite size effects at the critical momentum
signal the emergence of infrared singularities in the spectral
-representation of the dynamical structure factors.Comment: PostScript file with 12 pages + 11 figures uuencoded compresse
N-glycans of human amniotic fluid transferrin stimulate progesterone production in human first trimester trophoblast cells in vitro
Aims: During pregnancy, the placenta produces a variety of steroid hormones and proteins. Several of these substances have been shown to exert immunomodulatory effects. Progesterone is thought to mediate some of these effects by regulating uterine responsiveness. The aim of this study was to clarify the effect of amniotic fluid transferrin and its N-glycans on the release of progesterone by first trimester trophoblast cells in vitro. Methods: Cytotrophoblast cells were prepared from human first trimester placentae by trypsin-DNAse dispersion of villous tissue followed by a percoll gradient centrifugation and depletion of CD45 positive cells by magnetic cell sorting. Trophoblasts were incubated with varying concentrations (50-300 mug/ml) of transferrin from human amniotic fluid and serum as well as with N-glycans obtained from amniotic fluid transferrin. Culture supernatants were assayed for progesterone by enzyme-immunometric methods. Results: The release of progesterone increased in amniotic fluid transferrin- and N-glycan-treated trophoblast cell cultures compared to untreated trophoblast cells. There was no stimulating effect of serum transferrin on the progesterone production of trophoblast cells. Conclusions: The results suggest that amnion-transferrin and especially its N-glycans modulate the endocrine function of trophoblasts in culture by up regulating progesterone secretion
Evaluating and expanding the European Union's protected-area network toward potential post-2020 coverage targets
The Convention on Biological Diversity's (CBD) strategic plan will expire in 2020, but biodiversity loss is ongoing. Scientists call for more ambitious targets in the next agreement. The nature-needs-half movement, for example, has advocated conserving half of Earth to solve the biodiversity crisis, which has been translated to protecting 50 of each ecoregion. We evaluated current protection levels of ecoregions in the territory of one of the CBD's signatories, the European Union (EU). We also explored the possible enlargement of the Natura 2000 network to implement 30 or 50 ecoregion coverage in the EU member states’ protected area (PA) network. Based on the most recent land-use data, we examined whether ecoregions have enough natural area left to reach such high coverage targets. We used a spatially explicit mixed integer programing model to estimate the least-cost expansion of the PA network based on 3 scenarios that put different emphasis on total conservation cost, ecological representation of ecosystems, or emphasize an equal share of the burden among member states. To realize 30 and 50 ecoregion coverage, the EU would need to add 6.6 and 24.2, respectively, of its terrestrial area to its PA network. For all 3 scenarios, the EU would need to designate most recommended new PAs in seminatural forests and other semi- or natural ecosystems. Because 15 ecoregions did not have enough natural area left to implement the ecoregion-coverage targets, some member states would also need to establish new PAs on productive land, allocating the largest share to arable land. Thirty percent ecoregion coverage was met by protecting remaining natural areas in all ecoregions except 3, where productive land would also need to be included. Our results support discussions of higher ecoregions protection targets for post-2020 biodiversity frameworks. © 2020 The Authors. Conservation Biology published by Wiley Periodicals, Inc. on behalf of Society for Conservation Biology
Polarization operator approach to electron-positron pair production in combined laser and Coulomb fields
The optical theorem is applied to the process of electron-positron pair
creation in the superposition of a nuclear Coulomb and a strong laser field. We
derive new representations for the total production rate as two-fold integrals,
both for circular laser polarization and for the general case of elliptic
polarization, which has not been treated before. Our approach allows us to
obtain by analytical means the asymptotic behaviour of the pair creation rate
for various limits of interest. In particular, we consider pair production by
two-photon absorption and show that, close to the energetic threshold of this
process, the rate obeys a power law in the laser frequency with different
exponents for linear and circular laser polarization. With the help of the
upcoming x-ray laser sources our results could be tested experimentally.Comment: 10 pages, 3 figure
Sigma Decay at Finite Temperature and Density
Sigma decay and its relation with chiral phase transition are discussed at
finite temperature and density in the framework of the Nambu-Jona-Lasinio
model. The decay rate for the process sigma -> 2 pions to first order in a
1/N_c expansion is calculated as a function of temperature T and baryon density
n_b. In particular, only when the chiral phase transition happens around the
tricritical point, the sigma decay results in a non-thermal enhancement of
pions in the final state distributions in relativistic heavy ion collisions.Comment: 6 pages, 3 Postscript figures, submitted to Chin. Phys. Let
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