13,630 research outputs found
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
Covariant statistical mechanics and the stress-energy tensor
After recapitulating the covariant formalism of equilibrium statistical
mechanics in special relativity and extending it to the case of a non-vanishing
spin tensor, we show that the relativistic stress-energy tensor at
thermodynamical equilibrium can be obtained from a functional derivative of the
partition function with respect to the inverse temperature four-vector \beta.
For usual thermodynamical equilibrium, the stress-energy tensor turns out to be
the derivative of the relativistic thermodynamic potential current with respect
to the four-vector \beta, i.e. T^{\mu \nu} = - \partial \Phi^\mu/\partial
\beta_\nu. This formula establishes a relation between stress-energy tensor and
entropy current at equilibrium possibly extendable to non-equilibrium
hydrodynamics.Comment: 4 pages. Final version accepted for publication in Phys. Rev. Let
Controlling quasiparticle excitations in a trapped Bose-Einstein condensate
We describe an approach to quantum control of the quasiparticle excitations
in a trapped Bose-Einstein condensate based on adiabatic and diabatic changes
in the trap anisotropy. We describe our approach in the context of Landau-Zener
transition at the avoided crossings in the quasiparticle excitation spectrum.
We show that there can be population oscillation between different modes at the
specific aspect ratios of the trapping potential at which the mode energies are
almost degenerate. These effects may have implications in the expansion of an
excited condensate as well as the dynamics of a moving condensate in an atomic
wave guide with a varying width
Resonance Damping in Ferromagnets and Ferroelectrics
The phenomenological equations of motion for the relaxation of ordered phases
of magnetized and polarized crystal phases can be developed in close analogy
with one another. For the case of magnetized systems, the driving magnetic
field intensity toward relaxation was developed by Gilbert. For the case of
polarized systems, the driving electric field intensity toward relaxation was
developed by Khalatnikov. The transport times for relaxation into thermal
equilibrium can be attributed to viscous sound wave damping via
magnetostriction for the magnetic case and electrostriction for the
polarization case.Comment: 5 pages no figures ReVTeX
Non-linear effects in the cyclotron resonance of a massless quasi-particle in graphene
We consider the classical motion of a massless quasi-particle in a magnetic
field and under a weak electromagnetic radiation with the frequency .
Due to the non-parabolic, linear energy dispersion, the particle responds not
only at the frequency but generates a broad frequency spectrum around
it. The linewidth of the cyclotron resonance turns out to be very broad even in
a perfectly pure material which allows one to explain recent experimental data
in graphene. It is concluded that the linear response theory does not work in
graphene in finite magnetic fields.Comment: 5 pages, 4 figure
Ringing the Randall-Sundrum braneworld: metastable gravity wave bound states
In the Randall-Sundrum scenario, our universe is a 4-dimensional `brane'
living in a 5-dimensional bulk spacetime. By studying the scattering of bulk
gravity waves, we show that this brane rings with a characteristic set of
complex quasinormal frequencies, much like a black hole. To a bulk observer
these modes are interpreted as metastable gravity wave bound states, while a
brane observer views them as a discrete spectrum of decaying massive gravitons.
Potential implications of these scattering resonances are discussed.Comment: References and misc. comments added. "Implications" section expanded.
REVTeX4, 5 pages, 4 figure
Remarks on transient photon production in heavy ion collisions
In this note, we discuss the derivation of a formula that has been used in
the literature in order to compute the number of photons emitted by a hot or
dense system during a finite time. Our derivation is based on a variation of
the standard operator-based -matrix approach. The shortcomings of this
formula are then emphasized, which leads to a negative conclusion concerning
the possibility of using it to predict transient effects for the photon rate.Comment: 13 page
Technology as an economic catalyst in rural and depressed places in Massachusetts
This paper uses case studies, including two cities (Lynn and New Bedford), a sub-city district (Roxbury) and two towns in rural Franklin County (Greenfield and Orange), to examine the role of technology as a potential economic catalyst in rural and depressed places in Massachusetts. Though the five target areas vary in size, density, geographic area, demographic characteristics and economic resources, each exhibits chronic patterns of economic distress related to the decline of manufacturing, construction and other key industries
Disappearance of Schwinger's string at the charge - monopole "molecule"
An equivalence of total momentum operator of charge - monopole system to the
momentum operator of a symmetrical quantum top is observed. This explicitly
shows the string independence of Dirac's quantization condition leading to
disappearance of Schwinger's string and reveals some properties of diatomic
molecule for this system.Comment: 9 page
Asymptotic energy profile of a wavepacket in disordered chains
We investigate the long time behavior of a wavepacket initially localized at
a single site in translationally invariant harmonic and anharmonic chains
with random interactions. In the harmonic case, the energy profile averaged on time and disorder decays for large as a power
law where and 3/2 for
initial displacement and momentum excitations, respectively. The prefactor
depends on the probability distribution of the harmonic coupling constants and
diverges in the limit of weak disorder. As a consequence, the moments of the energy distribution averaged with respect to disorder
diverge in time as for , where
for . Molecular dynamics simulations yield good agreement with
these theoretical predictions. Therefore, in this system, the second moment of
the wavepacket diverges as a function of time despite the wavepacket is not
spreading. Thus, this only criteria often considered earlier as proving the
spreading of a wave packet, cannot be considered as sufficient in any model.
The anharmonic case is investigated numerically. It is found for intermediate
disorder, that the tail of the energy profile becomes very close to those of
the harmonic case. For weak and strong disorder, our results suggest that the
crossover to the harmonic behavior occurs at much larger and larger
time.Comment: To appear in Phys. Rev.
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