1,347 research outputs found
Breakdown of hydrodynamics in the inelastic Maxwell model of granular gases
Both the right and left eigenfunctions and eigenvalues of the linearized
homogeneous Boltzmann equation for inelastic Maxwell molecules corresponding to
the hydrodynamic modes are calculated. Also, some non-hydrodynamic modes are
identified. It is shown that below a critical value of the parameter
characterizing the inelasticity, one of the kinetic modes decays slower than
one of the hydrodynamic ones. As a consequence, a closed hydrodynamic
description does not exist in that regime. Some implications of this behavior
on the formally computed Navier-Stokes transport coefficients are discussed.Comment: Submitted to PRL (13/04/10
FM/CW radar system
An FM/CW radar system is presented with improved noise discrimination in which the received signal is multiplied by a sample of the transmitted signal, and the product signal is employed to deflect a laser beam as a function of frequency. The position of the beam is thus indicative of a discrete frequency, and it is detected by the frequency encoded positions of an array of photodiodes. The outputs of the photodiodes are scanned, then threshold detected, and used to obtain the range and velocity of a target
Scaling and universality of critical fluctuations in granular gases
The global energy fluctuations of a low density gas granular gas in the
homogeneous cooling state near its clustering instability are studied by means
of molecular dynamics simulations. The relative dispersion of the fluctuations
is shown to exhibit a power-law divergent behavior. Moreover, the probability
distribution of the fluctuations presents data collapse as the system
approaches the instability, for different values of the inelasticity. The
function describing the collapse turns out to be the same as the one found in
several molecular equilibrium and non-equilibrium systems, except for the
change in the sign of the fluctuations
Critical Behavior of a Heavy Particle in a Granular Fluid
Behavior analogous to a second order phase transition is observed for the
homogeneous cooling state of a heavy impurity particle in a granular fluid. The
order parameter is the ratio of impurity mean square velocity to that
of the fluid, with a conjugate field proportional to the mass ratio. A
parameter , measuring the fluid cooling rate relative to the
impurity--fluid collision rate, is the analogue of the inverse temperature. For
the fluid is ``normal'' with at , as in the case of a
system with elastic collisions. For an ``ordered'' state with occurs at , representing an extreme breakdown of equipartition.
Critical slowing and qualitative changes in the velocity distribution function
for the impurity particle near the transition are notedComment: 4 pages (4 figures included
Transversal inhomogeneities in dilute vibrofluidized granular fluids
The spontaneous symmetry breaking taking place in the direction perpendicular
to the energy flux in a dilute vibrofluidized granular system is investigated,
using both a hydrodynamic description and simulation methods. The latter
include molecular dynamics and direct Monte Carlo simulation of the Boltzmann
equation. A marginal stability analysis of the hydrodynamic equations, carried
out in the WKB approximation, is shown to be in good agreement with the
simulation results. The shape of the hydrodynamic profiles beyond the
bifurcation is discussed
Solitonic Phase in Manganites
Whenever a symmetry in the ground state of a system is broken, topological
defects will exist. These defects are essential for understanding phase
transitions in low dimensional systems[1]. Excitingly in some unique condensed
matter systems the defects are also the low energy electric charge excitations.
This is the case of skyrmions in quantum Hall ferromagnets[2] and solitons in
polymers[3]. Orbital order present in several transitions metal compounds[4-6]
could give rise to topological defects. Here we argue that the topological
defects in orbital ordered half doped manganites are orbital solitons.
Surprisingly, these solitons carry a fractional charge of e/2, and
whenever extra charge is added to the system an array of solitons is formed and
an incommensurate solitonic phase occurs. The striking experimental asymmetry
in the phase diagram as electrons or holes are added to half doped
manganites[7-12], is explained by the energy difference between positive and
negative charged solitons. Contrary to existent models that explain coexistence
between phases in manganites as an extrinsic effect[13-14], the presence of
inhomogeneities is naturally explained by the existence of solitonic phases.
The occurrence and relevance of orbital solitons might be a general phenomena
in strongly correlated systems.Comment: 10 pages, 5 figures include
Velocity distribution of fluidized granular gases in presence of gravity
The velocity distribution of a fluidized dilute granular gas in the direction
perpendicular to the gravitational field is investigated by means of Molecular
Dynamics simulations. The results indicate that the velocity distribution can
be exactly described neither by a Gaussian nor by a stretched exponential law.
Moreover, it does not exhibit any kind of scaling. In fact, the actual shape of
the distribution depends on the number of monolayers at rest, on the
restitution coefficient and on the height at what it is measured. The role
played by the number of particle-particle collisions as compared with the
number of particle-wall collisions is discussed
Temperature Induced Spin Density Wave in Magnetic Doped Topological Insulators
We study the magnetic properties of topological insulators doped with
isoelectronic magnetic impurities. We obtain that at zero temperature the
impurities order ferromagnetically, but when raising the temperature the
topological insulator undergoes a first order phase transition to a spin
density wave phase before the system reaches the paramagnetic phase. The origin
of this phase is the non-trivial dependence of the topological insulator spin
susceptibility on the momentum. We analyze the coupling of the non-uniform
magnetic phase with the Dirac electronic system that occurs at the surfaces of
the topological insulators.Comment: 7 pages. Major changes. the message has been revised and expande
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