1,975 research outputs found
A structure-preserving surrogate model for the closure of the moment system of the Boltzmann equation using convex deep neural networks
Direct simulation of physical processes on a kinetic level is prohibitively
expensive in aerospace applications due to the extremely high dimension of the
solution spaces. In this paper, we consider the moment system of the Boltzmann
equation, which projects the kinetic physics onto the hydrodynamic scale. The
unclosed moment system can be solved in conjunction with the entropy closure
strategy. Using an entropy closure provides structural benefits to the physical
system of partial differential equations. Usually computing such closure of the
system spends the majority of the total computational cost, since one needs to
solve an ill-conditioned constrained optimization problem. Therefore, we build
a neural network surrogate model to close the moment system, which preserves
the structural properties of the system by design, but reduces the
computational cost significantly. Numerical experiments are conducted to
illustrate the performance of the current method in comparison to the
traditional closure.Comment: 17 pages, 6 figure
Structural investigations of CeIrIn and CeCoIn on macroscopic and atomic length scales
For any thorough investigation of complex physical properties, as encountered
in strongly correlated electron systems, not only single crystals of highest
quality but also a detailed knowledge of the structural properties of the
material are pivotal prerequisites. Here, we combine physical and chemical
investigations on the prototypical heavy fermion superconductors CeIrIn
and CeCoIn on atomic and macroscopic length scale to gain insight into
their precise structural properties. Our approach spans from enhanced
resolution X-ray diffraction experiments to atomic resolution by means of
Scanning Tunneling Microscopy (STM) and reveal a certain type of local features
(coexistence of minority and majority structural patterns) in the tetragonal
HoCoGa-type structure of both compounds.Comment: 8 pages, 5 figures, submitted to JPSJ (SCES 2013
Gravitino Dark Matter and Cosmological Constraints
The gravitino is a promising candidate for cold dark matter. We study
cosmological constraints on scenarios in which the gravitino is the lightest
supersymmetric particle and a charged slepton the next-to-lightest
supersymmetric particle (NLSP). We obtain new results for the hadronic
nucleosynthesis bounds by computing the 4-body decay of the NLSP slepton into
the gravitino, the associated lepton, and a quark-antiquark pair. The bounds
from the observed dark matter density are refined by taking into account
gravitinos from both late NLSP decays and thermal scattering in the early
Universe. We examine the present free-streaming velocity of gravitino dark
matter and the limits from observations and simulations of cosmic structures.
Assuming that the NLSP sleptons freeze out with a thermal abundance before
their decay, we derive new bounds on the slepton and gravitino masses. The
implications of the constraints for cosmology and collider phenomenology are
discussed and the potential insights from future experiments are outlined. We
propose a set of benchmark scenarios with gravitino dark matter and long-lived
charged NLSP sleptons and describe prospects for the Large Hadron Collider and
the International Linear Collider.Comment: 51 pages, 20 figures, revised version matches published version
(results unchanged, JHEP style used, figures replaced with new high-quality
figures, typos corrected, references added
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Properties of vacuum arcs generated by switching RMF contacts at different ignition positions
The influence of initiation behavior of the drawn arc on the arc motion, on arc characteristics during the active phase, as well as on the post-arc parameters, was studied. The study was focused on arc dynamics, determination of the anode surface temperature after current interruption, and diagnostics of metal vapor density after current zero crossing. Different optical diagnostics, namely high-speed camera video enhanced by narrow-band optical filters, near infrared spectroscopy, and optical absorption spectroscopy was applied. The initiation behavior of the drawn arc had a clear influence on arc parameters. Higher local electrode temperature occurs in case of the electrodes with ignition point near the outer electrode boundary. This further causes an enhanced density of chromium vapor, even in cases with lower arc duration. The results of this study are important for design development of switching RMF contacts for future green energy applications. © 2020 by the authors. Licensee MDPI, Basel, Switzerland
Hard Thermal Photon Production in Relativistic Heavy Ion Collisions
The recent status of hard thermal photon production in relativistic heavy ion
collisions is reviewed and the current rates are presented with emphasis on
corrected bremsstrahlung processes in the quark-gluon plasma (QGP) and
quark-hadron duality. Employing Bjorken hydrodynamics with an EOS supporting
the phase transition from QGP to hot hadron gas (HHG), thermal photon spectra
are computed. For SPS 158 GeV Pb+Pb collisions, comparison with other
theoretical results and the WA98 direct photon data indicates significant
contributions due to prompt photons. Extrapolating the presented approach to
RHIC and LHC experiments, predictions of the thermal photon spectrum show a QGP
outshining the HHG in the high-pT-region.Comment: 20 pages with 8 figures. v3: Erratum to [Phys. Lett. B 510 (2001) 98]
with correctly labeled Figs. 2, 4, and 5 adde
Size dependent exciton dynamics in one-dimensional perylene bisimide aggregates
The size dependent exciton dynamics of one-dimensional aggregates of
substituted perylene bisimides are studied by ultrafast transient absorption
spectroscopy and kinetic Monte-Carlo simulations in dependence on the
temperature and the excitation density. For low temperatures the aggregates can
be treated as infinite chains and the dynamics is dominated by diffusion driven
exciton-exciton annihilation. With increasing temperature the aggregates
decompose into small fragments consisting of very few monomers. This scenario
is also supported by the time dependent anisotropy deduced from polarization
dependent experiments
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