536 research outputs found
Applicability of Taylor's hypothesis in thermally driven turbulence
In this paper, we show that in the presence of large-scale circulation (LSC),
Taylor's hypothesis can be invoked to deduce the energy spectrum in thermal
convection using real space probes, a popular experimental tool. We perform
numerical simulation of turbulent convection in a cube and observe that the
velocity field follows Kolmogorov's spectrum (). We also record the
velocity time series using real space probes near the lateral walls. The
corresponding frequency spectrum exhibits Kolmogorov's spectrum (),
thus validating Taylor's hypothesis with the steady LSC playing the role of a
mean velocity field. The aforementioned findings based on real space probes
provide valuable inputs for experimental measurements used for studying the
spectrum of convective turbulence
Excitation Functions of Stopping Power and Flow in Relativistic Heavy-Ion Collisions
Using a relativistic transport (ART) model, we study the stopping power, the
formation of superdense hadronic matter as well as the strength of transverse
and radial flow in central Au+Au collisions at beam momentum from 2 to 12 GeV/c
per nucleon. We find that complete stopping is achieved in the whole beam
momentum range. In particular, the proton rapidity distribution scaled by the
beam rapidity is independent of the beam momentum, and this is in agreement
with the experimental findings. Also, a large volume of superdense hadronic
matter with a local energy density exceeding that expected for the transition
of a hadronic matter to the quark-gluon plasma is formed in collisions at beam
momenta greater than 8 GeV/c per nucleon. Furthermore, it is found that the
transverse flow in these collisions is sensitive to the nuclear equation of
state and decreases with increasing beam momentum. On the other hand, the
radial flow is insensitive to the equation of state, and its strength increases
with beam momentum.Comment: Talk given at NN97, Gatlinburg, Tennessee June 2-6,1997. To appear in
the proc. in Nucl. Phys.
Superradiant Solid in Cavity QED Coupled to a Lattice of Rydberg Gas
We study an optical cavity coupled to a lattice of Rydberg atoms, which can
be represented by a generalized Dicke model. We show that the competition
between the atomic interaction and atom-light coupling induces a rich phase
diagram. A novel "superradiant solid" (SRS) phase is found, where both the
superradiance and crystalline orders coexist. Different from the normal second
order superradiance (SR) transition, here both the Solid-1/2 and SRS to SR
phase transitions are first order. These results are confirmed by the large
scale quantum Monte Carlo simulations.Comment: 5 pages,4 figure
NEW TOPOLOGIES IN THE PHASE DIAGRAM OF THE SEMI-INFINITE BLUME-CAPEL MODEL
The phase diagram of the Blume--Capel model on a semi--infinite simple cubic
lattice with a (100) free surface is studied in the pair approximation of the
cluster variation method. Six main topologies are found, of which two are new,
due to the occurrence of a first order surface transition in the phase with
ordered bulk, separating two phases with large and small surface order
parameters. The latter is a new phase and is studied in some detail, giving the
behaviour of the order parameter profiles in two typical cases. A comparison is
made with the results of a low temperature expansion, where these are
available, showing a great increase in accuracy with respect to the mean field
approximation.Comment: RevTeX, 13 pages + 7 uuencoded PostScript figures (substituted raw
with encoded PostScript
Strongly anisotropic media: the THz perspectives of left-handed materials
We demonstrate that non-magnetic () left-handed materials can
be effectively used for waveguide imaging systems. We also propose a specific
THz realization of the non-magnetic left-handed material based on homogeneous,
naturally-occurring media
Exact wave-packet decoherence dynamics in a discrete spectrum environment
We find an exact analytical solution of the reduced density matrix from the
Feynman-Vernon influence functional theory for a wave packet influenced by an
environment containing a few discrete modes. We obtain two intrinsic energy
scales relating to the time scales of the system and the environment. Different
relationship between these two scales alters the overall form of the solution
of the system. We also introduce a decoherence measure for a single wave packet
which is defined as the ratio of Schr\"odinger uncertainty over the
delocalization extension of the wave packet and characterizes the
time-evolution behavior of the off-diagonal reduced density matrix element. We
utilize the exact solution and the docherence measure to study the wave packet
decoherence dynamics. We further demonstrate how the dynamical diffusion of the
wave packet leads to non-Markovian decoherence in such a microscopic
environment.Comment: 12 pages, 2 figure
Constraining the Symmetry Energy: A Journey in the Isospin Physics from Coulomb Barrier to Deconfinement
Heavy Ion Collisions (HIC) represent a unique tool to probe the in-medium
nuclear interaction in regions away from saturation. In this work we present a
selection of reaction observables in dissipative collisions particularly
sensitive to the isovector part of the interaction, i.e. to the symmetry term
of the nuclear Equation of State (EoS). At low energies the behavior of the
symmetry energy around saturation influences dissipation and fragment
production mechanisms. We will first discuss the recently observed Dynamical
Dipole Radiation, due to a collective neutron-proton oscillation during the
charge equilibration in fusion and deep-inelastic collisions. Important Iso-EOS
effects are stressed. Reactions induced by unstable 132Sn beams appear to be
very promising tools to test the sub-saturation Isovector EoS. New Isospin
sensitive observables are also presented for deep-inelastic, fragmentation
collisions and Isospin equilibration measurements (Imbalance Ratios). The high
density symmetry term can be derived from isospin effects on heavy ion
reactions at relativistic energies (few AGeV range), that can even allow a
``direct'' study of the covariant structure of the isovector interaction in the
hadron medium. Rather sensitive observables are proposed from collective flows
and from pion/kaon production. The possibility of the transition to a mixed
hadron-quark phase, at high baryon and isospin density, is finally suggested.
Some signatures could come from an expected ``neutron trapping'' effect. The
importance of studying violent collisions with radioactive beams from low to
relativistic energies is finally stressed.Comment: 15 pages, 5 figures, Int.Workshop on Nuclear Dynamics in Heavy Ion
Reactions and Neutron Stars, Beijing Normal Univ. July 07, to appear in
Int.Journ.Modern Physics E (2008
The Phase Diagram of the Gonihedric 3d Ising Model via CVM
We use the cluster variation method (CVM) to investigate the phase structure
of the 3d gonihedric Ising actions defined by Savvidy and Wegner. The
geometrical spin cluster boundaries in these systems serve as models for the
string worldsheets of the gonihedric string embedded in . The models
are interesting from the statistical mechanical point of view because they have
a vanishing bare surface tension. As a result the action depends only on the
angles of the discrete surface and not on the area, which is the antithesis of
the standard 3d Ising model.
The results obtained with the CVM are in good agreement with Monte Carlo
simulations for the critical temperatures and the order of the transition as
the self-avoidance coupling is varied. The value of the magnetization
critical exponent , calculated with the cluster
variation--Pad\`e approximant method, is also close to the simulation results.Comment: 8 pages text (LaTex) + 3 eps figures bundled together with uufile
Nuclear fragmentation: sampling the instabilities of binary systems
We derive stability conditions of Asymmetric Nuclear Matter () and
discuss the relation to mechanical and chemical instabilities of general
two-component systems. We show that the chemical instability may appear as an
instability of the system against isoscalar-like rather than isovector-like
fluctuations if the interaction between the two constituent species has an
attractive character as in the case of . This leads to a new kind of
liquid-gas phase transition, of interest for fragmentation experiments with
radioactive beams.Comment: 4 pages (LATEX), 3 Postscript figures, improved version, added
reference
Excitation functions in central Au+Au collisions from SIS/GSI to AGS/Brookhaven
Using the relativistic transport model (ART), we predict the energy
dependence of the stopping power, maximum baryon and energy densities, the
population of resonance matter as well as the strength of the transverse and
radial flow for central Au+Au reactions at beam momentum from 2 to 12 GeV/c
available at Brookhaven's AGS. The maximum baryon and energy densities are
further compared to the predictions of relativistic hydrodynamics assuming the
formation of shock waves. We also discuss the Fermi-Landau scaling of the pion
multiplicity in these reactions.Comment: 20 pages, latex, 10 figures available upon request from the authors,
Nucl. Phys. A in pres
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