7,628 research outputs found
Formation of correlations and energy-conservation at short time scales
The formation of correlations due to collisions in an interacting nucleonic
system is investigated. Results from one-time kinetic equations are compared
with the Kadanoff and Baym two-time equation with collisions included in Born
approximation. A reasonable agreement is found for a proposed approximation of
the memory effects by a finite duration of collisions. This form of collision
integral is in agreement with intuitive estimates from Fermi's golden rule. The
formation of correlations and the build up time is calculated analytically for
the high temperature and the low temperature limit. Different approximate
expressions are compared with the numerical results. We present analytically
the time dependent interaction energy and the formation time for Gau\ss{}- and
Yukawa type of potentials.Comment: Europ. Lournal Physics A accepte
Correlations in Many-Body Systems with Two-time Green's Functions
The Kadanoff-Baym (KB) equations are solved numerically for infinite nuclear
matter. In particular we calculate correlation energies and correlation times.
Approximating the Green's functions in the KB collision kernel by the free
Green's functions the Levinson equation is obtained. This approximation is
valid for weak interactions and/or low densities. It relates to the extended
quasi-classical approximation for the spectral function. Comparing the
Levinson, Born and KB calculations allows for an estimate of higher order
spectral corrections to the correlations. A decrease in binding energy is
reported due to spectral correlations and off-shell parts in the reduced
density matrix
Simulating anthropogenic impacts to bird communities in tropical rain forests
We used an aggregated modelling approach to simulate the impacts ofanthropogenic disturbances on the long-term dynamics of faunal diversityin tropical rain forests. We restricted our study to bird communities eventhough the approach is more general. We developed a model calledBIODIV which simulated the establishment of hypothetical bird speciesin a forest. Our model was based on the results of a simple matrix modelwhich calculated the spatio-temporal dynamics of a tropical rain forest inMalaysia. We analysed the establishment of bird species in a secondaryforest succession and the impacts of 60 different logging scenarios on thediversity of the bird community. Of the three logging parameters(cycle length, method, intensity), logging intensity had the most servereimpact on the bird community. In the worst case the number of bird specieswas reduced to 23% of the species richness found in a primary forest
Lattice and polarizability mediated spin activity in EuTiO_3
EuTiO_3 is shown to exhibit novel strong spin-charge-lattice coupling deep in
the paramagnetic phase. Its existence is evidenced by an, until now, unknown
response of the paramagnetic susceptibility at temperatures exceeding the
structural phase transition temperature T_S = 282K. The "extra" features in the
susceptibility follow the rotational soft zone boundary mode temperature
dependence above and below T_S. The theoretical modeling consistently
reproduces this behavior and provides reasoning for the stabilization of the
soft optic mode other than quantum fluctuations.Comment: 8 pages, 4 figure
Si-induced superconductivity and structural transformations in DyRh4B4
DyRh4B4 has been known to crystallize in the primitive tetragonal
(pt)-structure and to exhibit a ferromagnetic transition at 12 K, the highest
magnetic transition temperature in the entire series of the RRh4B4 materials
[1]. We show here that our silicon-added samples of the nominal composition
DyRh4B4Si0.2 exhibit superconductivity below Tc ~ 4.5 K and an
antiferromagnetic transition below TN ~ 2.7 K. The 12 K transition observed in
the pt-DyRh4B4 is completely suppressed. Our annealed samples mainly consist of
domains of the chemical composition DyRh3.9B4.2Si0.08. These domains contain
two crystallographic phases belonging to the body-centred tetragonal
(bct)-structure and the orthorhombic (o)-structure. We have reasons to suggest
that superconductivity and antiferromagnetic ordering arise from bct- DyRh4B4
phase and, therefore, coexist below TN ~ 2.7 K.Comment: 11 pages, 6 figures, Accepted for publication in Journal of Alloys
and Compound
Magnetic field enhanced structural instability in EuTiO_{3}
EuTiO_{3} undergoes a structural phase transition from cubic to tetragonal at
T_S = 282 K which is not accompanied by any long range magnetic order. However,
it is related to the oxygen ocathedra rotation driven by a zone boundary
acoustic mode softening. Here we show that this displacive second order
structural phase transition can be shifted to higher temperatures by the
application of an external magnetic field (increased by 4 K for mu_{0}H = 9 T).
This observed field dependence is in agreement with theoretical predictions
based on a coupled spin-anharmonic-phonon interaction model.Comment: 4 pages, 4 figure
Low albedos of hot to ultra-hot Jupiters in the optical to near-infrared transition regime
The depth of a secondary eclipse contains information of both the thermally
emitted light component of a hot Jupiter and the reflected light component. If
the dayside atmosphere of the planet is assumed to be isothermal, it is
possible to disentangle both. In this work, we analyze 11 eclipse light curves
of the hot Jupiter HAT-P-32b obtained at 0.89 m in the z' band. We obtain
a null detection for the eclipse depth with state-of-the-art precision, -0.01
+- 0.10 ppt. We confirm previous studies showing that a non-inverted atmosphere
model is in disagreement to the measured emission spectrum of HAT-P-32b. We
derive an upper limit on the reflected light component, and thus, on the
planetary geometric albedo . The 97.5%-confidence upper limit is <
0.2. This is the first albedo constraint for HAT-P-32b, and the first z' band
albedo value for any exoplanet. It disfavors the influence of large-sized
silicate condensates on the planetary day side. We inferred z' band geometric
albedo limits from published eclipse measurements also for the ultra-hot
Jupiters WASP-12b, WASP-19b, WASP-103b, and WASP-121b, applying the same
method. These values consistently point to a low reflectivity in the optical to
near-infrared transition regime for hot to ultra-hot Jupiters.Comment: accepted for publication in A&
Aerosol-cloud-precipitation effects over Germany as simulated by a convective-scale numerical weather prediction model
Possible aerosol-cloud-precipitation effects over Germany are investigated using the COSMO model in a convection-permitting configuration close to the operational COSMO-DE. Aerosol effects on clouds and precipitation are modeled by using an advanced two-moment microphysical parameterization taking into account aerosol assumptions for cloud condensation nuclei (CCN) as well as ice nuclei (IN). Simulations of three summer seasons have been performed with various aerosol assumptions, and are analysed regarding surface precipitation, cloud properties, and the indirect aerosol effect on near-surface temperature. We find that the CCN and IN assumptions have a strong effect on cloud properties, like condensate amounts of cloud water, snow and rain as well as on the glaciation of the clouds, but the effects on surface precipitation are – when averaged over space and time – small. This robustness can only be understood by the combined action of microphysical and dynamical processes. On one hand, this shows that clouds can be interpreted as a buffered system where significant changes to environmental parameters, like aerosols, have little effect on the resulting surface precipitation. On the other hand, this buffering is not active for the radiative effects of clouds, and the changes in cloud properties due to aerosol perturbations may have a significant effect on radiation and near-surface temperature
Nonlocal Kinetic Equation and Simulations of Heavy Ion Reactions
A kinetic equation which combines the quasiparticle drift of Landau's
equation with a dissipation governed by a nonlocal and noninstantaneous
scattering integral in the spirit of Enskog corrections is discussed. Numerical
values of the off-shell contribution to the Wigner distribution, of the
collision duration and of the collision nonlocality are presented for different
realistic potentials. On preliminary results we show that simulations of
quantum molecular dynamics extended by the nonlocal treatment of collisions
leads to a broader proton distribution bringing the theoretical spectra closer
towards the experimental values than the local approach.Comment: Proceedings of the Erice School, published as Vol. 42 of "Progress in
Particle and Nuclear Physics" by ELSEVIE
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