12,350 research outputs found
Relativistic Nucleus-Nucleus Collisions: Zone of Reactions and Space-Time Structure of a Fireball
A zone of reactions is determined and then exploited as a tool in studying
the space-time structure of an interacting system formed in a collision of
relativistic nuclei. The time dependence of the reaction rates integrated over
spatial coordinates is also considered. Evaluations are made with the help of
the microscopic transport model UrQMD. The relation of the boundaries of
different zones of reactions and the hypersurfaces of sharp chemical and
kinetic freeze-outs is discussed.Comment: 6 pages, 5 figure
Anisotropic Flow and Viscous Hydrodynamics
We report part of our recent work on viscous hydrodynamics with consistent
phase space distribution f(x,\p) for freeze out. We develop the gradient
expansion formalism based on kinetic theory, and with the constraints from the
comparison between hydrodynamics and kinetic theory, viscous corrections to
f(x,\p) can be consistently determined order by order. Then with the obtained
f(x,\p), second order viscous hydrodynamical calculations are carried out for
elliptic flow .Comment: 8 pages, 2 figures. Proceedings for the 28th Winter Workshop on
Nuclear Dynamics, Dorado Del Mar, Puerto Rico, United States Of America, 7 -
14 Apr 201
Power law tails of time correlations in a mesoscopic fluid model
In a quenched mesoscopic fluid, modelling transport processes at high
densities, we perform computer simulations of the single particle energy
autocorrelation function C_e(t), which is essentially a return probability.
This is done to test the predictions for power law tails, obtained from mode
coupling theory. We study both off and on-lattice systems in one- and
two-dimensions. The predicted long time tail ~ t^{-d/2} is in excellent
agreement with the results of computer simulations. We also account for finite
size effects, such that smaller systems are fully covered by the present theory
as well.Comment: 11 pages, 12 figure
Dissipative effects from transport and viscous hydrodynamics
We compare 2->2 covariant transport theory and causal Israel-Stewart
hydrodynamics in 2+1D longitudinally boost invariant geometry with RHIC-like
initial conditions and a conformal e = 3p equation of state. The pressure
evolution in the center of the collision zone and the final differential
elliptic flow v2(pT) from the two theories agree remarkably well for a small
shear viscosity to entropy density ratio eta/s ~ 1/(4 pi), and also for a large
cross section sigma ~ 50 mb. A key to this agreement is keeping ALL terms in
the Israel-Stewart equations of motion. Our results indicate promising
prospects for the applicability of Israel-Stewart dissipative hydrodynamics at
RHIC, provided the shear viscosity of hot and dense quark-gluon matter is
indeed very small for the relevant temperatures T ~ 200-500 MeV.Comment: Presentation at Quark Matter 2008. 4 pages, 3 figure
Momentum of an electromagnetic wave in dielectric media
Almost a hundred years ago, two different expressions were proposed for the
energy--momentum tensor of an electromagnetic wave in a dielectric. Minkowski's
tensor predicted an increase in the linear momentum of the wave on entering a
dielectric medium, whereas Abraham's tensor predicted its decrease. Theoretical
arguments were advanced in favour of both sides, and experiments proved
incapable of distinguishing between the two. Yet more forms were proposed, each
with their advocates who considered the form that they were proposing to be the
one true tensor. This paper reviews the debate and its eventual conclusion:
that no electromagnetic wave energy--momentum tensor is complete on its own.
When the appropriate accompanying energy--momentum tensor for the material
medium is also considered, experimental predictions of all the various proposed
tensors will always be the same, and the preferred form is therefore
effectively a matter of personal choice.Comment: 23 pages, 3 figures, RevTeX 4. Removed erroneous factor of mu/mu_0
from Eq.(44
Scattering Theory of Charge-Current Induced Magnetization Dynamics
In ferromagnets, charge currents can excite magnons via the spin-orbit
coupling. We develop a novel and general scattering theory of charge current
induced macrospin magnetization torques in normal metalferromagnetnormal
metal layers. We apply the formalism to a dirty GaAs(Ga,Mn)AsGaAs system.
By computing the charge current induced magnetization torques and solving the
Landau-Lifshitz-Gilbert equation, we find magnetization switching for current
densities as low as ~A/cm. Our results are in agreement
with a recent experimental observation of charge-current induced magnetization
switching in (Ga,Mn)As.Comment: Final version accepted by EP
Transport in a highly asymmetric binary fluid mixture
We present molecular dynamics calculations of the thermal conductivity and
viscosities of a model colloidal suspension with colloidal particles roughly
one order of magnitude larger than the suspending liquid molecules. The results
are compared with estimates based on the Enskog transport theory and effective
medium theories (EMT) for thermal and viscous transport. We find, in
particular, that EMT remains well applicable for predicting both the shear
viscosity and thermal conductivity of such suspensions when the colloidal
particles have a ``typical'' mass, i.e. much larger than the liquid molecules.
Very light colloidal particles on the other hand yield higher thermal
conductivities, in disagreement with EMT. We also discuss the consequences of
these results to some proposed mechanisms for thermal conduction in
nanocolloidal suspensions.Comment: 13 pages, 6 figures, to appear in Physical Review E (2007
Transport of heat and mass in a two-phase mixture. From a continuous to a discontinuous description
We present a theory which describes the transport properties of the
interfacial region with respect to heat and mass transfer. Postulating the
local Gibbs relation for a continuous description inside the interfacial
region, we derive the description of the Gibbs surface in terms of excess
densities and fluxes along the surface. We introduce overall interfacial
resistances and conductances as the coefficients in the force-flux relations
for the Gibbs surface. We derive relations between the local resistivities for
the continuous description inside the interfacial region and the overall
resistances of the surface for transport between the two phases for a mixture.
It is shown that interfacial resistances depend among other things on the
enthalpy profile across the interface. Since this variation is substantial the
coupling between heat and mass flow across the surface are also substantial. In
particular, the surface puts up much more resistance to the heat and mass
transfer then the homogeneous phases over a distance comparable to the
thickness of the surface. This is the case not only for the pure heat
conduction and diffusion but also for the cross effects like thermal diffusion.
For the excess fluxes along the surface and the corresponding thermodynamic
forces we derive expressions for excess conductances as integrals over the
local conductivities along the surface. We also show that the curvature of the
surface affects only the overall resistances for transport across the surface
and not the excess conductivities along the surface.Comment: 25 pages, 2 figure
Relativistic viscoelastic fluid mechanics
A detailed study is carried out for the relativistic theory of
viscoelasticity which was recently constructed on the basis of Onsager's linear
nonequilibrium thermodynamics. After rederiving the theory using a local
argument with the entropy current, we show that this theory universally reduces
to the standard relativistic Navier-Stokes fluid mechanics in the long time
limit. Since effects of elasticity are taken into account, the dynamics at
short time scales is modified from that given by the Navier-Stokes equations,
so that acausal problems intrinsic to relativistic Navier-Stokes fluids are
significantly remedied. We in particular show that the wave equations for the
propagation of disturbance around a hydrostatic equilibrium in Minkowski
spacetime become symmetric hyperbolic for some range of parameters, so that the
model is free of acausality problems. This observation suggests that the
relativistic viscoelastic model with such parameters can be regarded as a
causal completion of relativistic Navier-Stokes fluid mechanics. By adjusting
parameters to various values, this theory can treat a wide variety of materials
including elastic materials, Maxwell materials, Kelvin-Voigt materials, and (a
nonlinearly generalized version of) simplified Israel-Stewart fluids, and thus
we expect the theory to be the most universal description of single-component
relativistic continuum materials. We also show that the presence of strains and
the corresponding change in temperature are naturally unified through the
Tolman law in a generally covariant description of continuum mechanics.Comment: 52pages, 11figures; v2: minor corrections; v3: minor corrections, to
appear in Physical Review E; v4: minor change
Onsager approach to 1D solidification problem and its relation to phase field description
We give a general phenomenological description of the steady state 1D front
propagation problem in two cases: the solidification of a pure material and the
isothermal solidification of two component dilute alloys.
The solidification of a pure material is controlled by the heat transport in
the bulk and the interface kinetics.
The isothermal solidification of two component alloys is controlled by the
diffusion in the bulk and the interface kinetics.
We find that the condition of positive-definiteness of the symmetric Onsager
matrix of interface kinetic coefficients still allows an arbitrary sign of the
slope of the velocity-concentration line near the solidus in the alloy problem
or of the velocity-temperature line in the case of solidification of a pure
material. This result offers a very simple and elegant way to describe the
interesting phenomenon of a possible non-single-value behavior of velocity
versus concentration which has previously been discussed by different
approaches. We also discuss the relation of this Onsager approach to the thin
interface limit of the phase field description.Comment: 5 pages, 1 figure, submitted to Physical Review
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