137,798 research outputs found
Breathing mode in an improved transport approach
The nuclear breathing-mode giant monopole resonance is studied within an
improved relativistic Boltzmann-Uehling-Uhlenbeck (BUU) transport approach. As
a new feature, the numerical treatment of ground state nuclei and their
phase-space evolution is realized with the same semiclassical energy density
functional. With this new method a very good stability of ground state nuclei
in BUU simulations is achieved. This is important in extracting clear
breathing-mode signals for the excitation energy and, in particular, for the
lifetime from transport theoretical studies including mean-field and
collisional effects.Comment: 33 pages, 11 figures, accepted for publication in Phys. Rev.
Collision energy dependence of elliptic flow splitting between particles and their antiparticles from an extended multiphase transport model
Based on an extended multiphase transport model, which includes mean-field
potentials in both the partonic and hadronic phases, uses the mix-event
coalescence, and respects charge conservation during the hadronic evolution, we
have studied the collision energy dependence of the elliptic flow splitting
between particles and their antiparticles. This extended transport model
reproduces reasonably well the experimental data at lower collision energies
but only describes qualitatively the elliptic flow splitting at higher beam
energies. The present study thus indicates the existence of other mechanisms
for the elliptic flow splitting besides the mean-field potentials and the need
of further improvements of the multiphase transport model.Comment: 8 pages, 6 figure
Transport Properties of the Quark-Gluon Plasma -- A Lattice QCD Perspective
Transport properties of a thermal medium determine how its conserved charge
densities (for instance the electric charge, energy or momentum) evolve as a
function of time and eventually relax back to their equilibrium values. Here
the transport properties of the quark-gluon plasma are reviewed from a
theoretical perspective. The latter play a key role in the description of
heavy-ion collisions, and are an important ingredient in constraining particle
production processes in the early universe. We place particular emphasis on
lattice QCD calculations of conserved current correlators. These Euclidean
correlators are related by an integral transform to spectral functions, whose
small-frequency form determines the transport properties via Kubo formulae. The
universal hydrodynamic predictions for the small-frequency pole structure of
spectral functions are summarized. The viability of a quasiparticle description
implies the presence of additional characteristic features in the spectral
functions. These features are in stark contrast with the functional form that
is found in strongly coupled plasmas via the gauge/gravity duality. A central
goal is therefore to determine which of these dynamical regimes the quark-gluon
plasma is qualitatively closer to as a function of temperature. We review the
analysis of lattice correlators in relation to transport properties, and
tentatively estimate what computational effort is required to make decisive
progress in this field.Comment: 54 pages, 37 figures, review written for EPJA and APPN; one parag.
added end of section 3.4, and one at the end of section 3.2.2; some Refs.
added, and some other minor change
Reversible Jump Metropolis Light Transport using Inverse Mappings
We study Markov Chain Monte Carlo (MCMC) methods operating in primary sample
space and their interactions with multiple sampling techniques. We observe that
incorporating the sampling technique into the state of the Markov Chain, as
done in Multiplexed Metropolis Light Transport (MMLT), impedes the ability of
the chain to properly explore the path space, as transitions between sampling
techniques lead to disruptive alterations of path samples. To address this
issue, we reformulate Multiplexed MLT in the Reversible Jump MCMC framework
(RJMCMC) and introduce inverse sampling techniques that turn light paths into
the random numbers that would produce them. This allows us to formulate a novel
perturbation that can locally transition between sampling techniques without
changing the geometry of the path, and we derive the correct acceptance
probability using RJMCMC. We investigate how to generalize this concept to
non-invertible sampling techniques commonly found in practice, and introduce
probabilistic inverses that extend our perturbation to cover most sampling
methods found in light transport simulations. Our theory reconciles the
inverses with RJMCMC yielding an unbiased algorithm, which we call Reversible
Jump MLT (RJMLT). We verify the correctness of our implementation in canonical
and practical scenarios and demonstrate improved temporal coherence, decrease
in structured artifacts, and faster convergence on a wide variety of scenes
Fermi Surface Nesting and Nanoscale Fluctuating Charge/Orbital Ordering in Colossal Magnetoresistive Oxides
We used high resolution angle-resolved photoemission spectroscopy to reveal
the Fermi surface and key transport parameters of the metallic state of the
layered Colossal Magnetoresistive (CMR) oxide La1.2Sr1.8Mn2O7. With these
parameters the calculated in-plane conductivity is nearly one order of
magnitude larger than the measured DC conductivity. This discrepancy can be
accounted for by including the pseudogap which removes at least 90% of the
spectral weight at the Fermi energy. Key to the pseudogap and many other
properties are the parallel straight Fermi surface sections which are highly
susceptible to nesting instabilities. These nesting instabilities produce
nanoscale fluctuating charge/orbital modulations which cooperate with
Jahn-Teller distortions and compete with the electron itinerancy favored by
double exchange
Artificial dielectric devices for variable polarization compensation at millimeter and submillimeter wavelengths
Variable polarization compensation has been demonstrated at 100 GHz. The device consists of two interlocking V-groove artificial dielectric gratings that produce a birefringence that varies with the separation distance. A maximum retardance of 74/spl deg/ has been obtained experimentally in a silicon device, in good agreement with rigorous coupled-wave computer simulations. Further simulations predict that adding quarter wave dielectric antireflection (AR) coatings to the outer surfaces of the device can reduce the insertion loss to below 4 dB. The use of rectangular grooved gratings provides increased retardance and reduced loss. It is predicted that a coupled device with rectangular grooved gratings will be capable of maximum retardance in excess of 180/spl deg/, with low insertion loss (<0.6 dB). The sensitivity of the wave retardation as a function of mechanical separation has a peak value of 485/spl deg//mm. The design and micromachining fabrication techniques scale for operation at submillimeter wavelengths
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