37,353 research outputs found
Velocity statistics in excited granular media
We present an experimental study of velocity statistics for a partial layer
of inelastic colliding beads driven by a vertically oscillating boundary. Over
a wide range of parameters (accelerations 3-8 times the gravitational
acceleration), the probability distribution P(v) deviates measurably from a
Gaussian for the two horizontal velocity components. It can be described by
P(v) ~ exp(-|v/v_c|^1.5), in agreement with a recent theory. The characteristic
velocity v_c is proportional to the peak velocity of the boundary. The granular
temperature, defined as the mean square particle velocity, varies with particle
density and exhibits a maximum at intermediate densities. On the other hand,
for free cooling in the absence of excitation, we find an exponential velocity
distribution. Finally, we examine the sharing of energy between particles of
different mass. The more massive particles are found to have greater kinetic
energy.Comment: 27 pages, 13 figures, to appear in Chaos, September 99, revised 3
figures and tex
Study of a signal processor employing a synthetic phase isolator
Evaluation of signal data processor employing synthetic phase isolator techniqu
Non-equilibrium dynamics in quantum field theory at high density: the tsunami
The dynamics of a dense relativistic quantum fluid out of thermodynamic
equilibrium is studied in the framework of the Phi^4 scalar field theory in the
large N limit. The time evolution of a particle distribution in momentum space
(the tsunami) is computed. The effective mass felt by the particles in such a
high density medium equals the tree level mass plus the expectation value of
the squared field. The case of negative tree level squared mass is particularly
interesting. In such case dynamical symmetry restoration as well as dynamical
symmetry breaking can happen. Furthermore, the symmetry may stay broken with
vanishing asymptotic squared mass showing the presence of out of equilibrium
Goldstone bosons. We study these phenomena and identify the set of initial
conditions that lead to each case. We compute the equation of state which turns
to depend on the initial state. Although the system does not thermalize, the
equation of state for asymptotically broken symmetry is of radiation type. We
compute the correlation functions at equal times. The two point correlator for
late times is the sum of different terms. One stems from the initial particle
distribution. Another term accounts for the out of equilibrium Goldstone bosons
created by spinodal unstabilities when the symmetry is asymptotically
broken.Both terms are of the order of the inverse of the coupling for distances
where causal signals can connect the two points. The contribution of the out of
equilibrium Goldstones exhibits scaling behaviour in a generalized sense.Comment: LaTex, 49 pages, 15 .ps figure
Entanglement enhanced atomic gyroscope
The advent of increasingly precise gyroscopes has played a key role in the
technological development of navigation systems. Ring-laser and fibre-optic
gyroscopes, for example, are widely used in modern inertial guidance systems
and rely on the interference of unentangled photons to measure mechanical
rotation. The sensitivity of these devices scales with the number of particles
used as . Here we demonstrate how, by using sources of entangled
particles, it is possible to do better and even achieve the ultimate limit
allowed by quantum mechanics where the precision scales as 1/N. We propose a
gyroscope scheme that uses ultra-cold atoms trapped in an optical ring
potential.Comment: 19 pages, 2 figure
Microwave resonances of the bubble phases in 1/4 and 3/4 filled higher Landau levels
We have measured the diagonal conductivity in the microwave regime of an
ultrahigh mobility two dimensional electron system. We find a sharp resonance
in Re[sigma_{xx}] versus frequency when nu > 4 and the partial filling of the
highest Landau level, nu^*, is ~ 1/4 or 3/4 and temperatures < 0.1 K. The
resonance appears for a range of nu^* from 0.20 to 0.37 and again from 0.62 to
0.82. the peak frequency, f_{pk} changes from ~ 500 to ~ 150 as nu^* = 1/2 is
approached. This range of f_{pk} shows no dependence on nu where the resonance
is observed. The quality factor, Q, of the resonance is maximum at ~ nu^* =
0.25 and 0.74. We interpret the resonance as due to a pinning mode of the
bubble phase crystal.Comment: revtex 4, 3 figures, minor corrections made. Accepted by pr
Raman modes of the deformed single-wall carbon nanotubes
With the empirical bond polarizability model, the nonresonant Raman spectra
of the chiral and achiral single-wall carbon nanotubes (SWCNTs) under uniaxial
and torsional strains have been systematically studied by \textit{ab initio}
method. It is found that both the frequencies and the intensities of the
low-frequency Raman active modes almost do not change in the deformed
nanotubes, while their high-frequency part shifts obviously. Especially, the
high-frequency part shifts linearly with the uniaxial tensile strain, and two
kinds of different shift slopes are found for any kind of SWCNTs. More
interestingly, new Raman peaks are found in the nonresonant Raman spectra under
torsional strain, which are explained by a) the symmetry breaking and b) the
effect of bond rotation and the anisotropy of the polarizability induced by
bond stretching
Schwinger-Dyson approach to non-equilibrium classical field theory
In this paper we discuss a Schwinger-Dyson [SD] approach for determining the
time evolution of the unequal time correlation functions of a non-equilibrium
classical field theory, where the classical system is described by an initial
density matrix at time . We focus on field theory in 1+1
space time dimensions where we can perform exact numerical simulations by
sampling an ensemble of initial conditions specified by the initial density
matrix. We discuss two approaches. The first, the bare vertex approximation
[BVA], is based on ignoring vertex corrections to the SD equations in the
auxiliary field formalism relevant for 1/N expansions. The second approximation
is a related approximation made to the SD equations of the original formulation
in terms of alone. We compare these SD approximations as well as a
Hartree approximation with exact numerical simulations. We find that both
approximations based on the SD equations yield good agreement with exact
numerical simulations and cure the late time oscillation problem of the Hartree
approximation. We also discuss the relationship between the quantum and
classical SD equations.Comment: 36 pages, 5 figure
Are there localized saddles behind the heterogeneous dynamics of supercooled liquids?
We numerically study the interplay between heterogeneous dynamics and
properties of negatively curved regions of the potential energy surface in a
model glassy system. We find that the unstable modes of saddles and
quasi-saddles undergo a localization transition close to the Mode-Coupling
critical temperature. We also find evidence of a positive spatial correlation
between clusters of particles having large displacements in the unstable modes
and dynamical heterogeneities.Comment: 7 pages, 3 figures, submitted to Europhys. Let
Neutral pion production in solar flares
The Gamma-Ray Spectrometer (GRS) on SMM has detected more than 130 flares with emission approx 300 keV. More than 10 of these flares were detected at photon energies 10 MeV. Although the majority of the emission at 10 MeV must be from electron bremsstrahlung, at least two of the flares have spectral properties 40 MeV that require gamma rays from the decay of neutral pions. It is found that pion production can occur early in the impulsive phase as defined by hard X-rays near 100 keV. It is also found in one of these flares that a significant portion of this high-energy emission is produced well after the impulsive phase. This extended production phase, most clearly observed at high energies, may be a signature of the acceleration process which produces solar energetic particles (SEP's) in space
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