41,478 research outputs found
Observing the evaporation transition in vibro-fluidized granular matter
By shaking a sand box the grains on the top start to jump giving the picture
of evaporating a sand bulk, and a gaseous transition starts at the surface
granular matter (GM) bed. Moreover the mixture of the grains in the whole bed
starts to move in a cooperative way which is far away from a Brownian
description. In a previous work we have shown that the key element to describe
the statistics of this behavior is the exclusion of volume principle, whereby
the system obeys a Fermi configurational approach. Even though the experiment
involves an archetypal non-equilibrium system, we succeeded in defining a
global temperature, as the quantity associated to the Lagrange parameter in a
maximum entropic statistical description. In fact in order to close our
approach we had to generalize the equipartition theorem for dissipative
systems. Therefore we postulated, found and measured a fundamental dissipative
parameter, written in terms of pumping and gravitational energies, linking the
configurational entropy to the collective response for the expansion of the
centre of mass (c.m.) of the granular bed. Here we present a kinetic approach
to describe the experimental velocity distribution function (VDF) of this
non-Maxwellian gas of macroscopic Fermi-like particles (mFp). The evaporation
transition occurs mainly by jumping balls governed by the excluded volume
principle. Surprisingly in the whole range of low temperatures that we measured
this description reveals a lattice-gas, leading to a packing factor, which is
independent of the external parameters. In addition we measure the mean free
path, as a function of the driving frequency, and corroborate our prediction
from the present kinetic theory.Comment: 6 pages, 4 figures, submitted for publication September 1st, 200
Zenithal bistability in a nematic liquid crystal device with a monostable surface condition
The ground-state director configurations in a grating-aligned, zenithally bistable nematic device are calculated in two dimensions using a Q tensor approach. The director profiles generated are well described by a one-dimensional variation of the director across the width of the device, with the distorted region near the grating replaced by an effective surface anchoring energy. This work shows that device bistability can in fact be achieved by using a monostable surface term in the one-dimensional model. This implies that is should be possible to construct a device showing zenithal bistability without the need for a micropatterned surface
Exotic and excited-state radiative transitions in charmonium from lattice QCD
We compute, for the first time using lattice QCD methods, radiative
transition rates involving excited charmonium states, states of high spin and
exotics. Utilizing a large basis of interpolating fields we are able to project
out various excited state contributions to three-point correlators computed on
quenched anisotropic lattices. In the first lattice QCD calculation of the
exotic 1-+ eta_c1 radiative decay, we find a large partial width Gamma(eta_c1
-> J/psi gamma) ~ 100 keV. We find clear signals for electric dipole and
magnetic quadrupole transition form factors in chi_c2 -> J/psi gamma,
calculated for the first time in this framework, and study transitions
involving excited psi and chi_c1,2 states. We calculate hindered magnetic
dipole transition widths without the sensitivity to assumptions made in model
studies and find statistically significant signals, including a non-exotic
vector hybrid candidate Y_hyb? -> eta_c gamma. As well as comparison to
experimental data, we discuss in some detail the phenomenology suggested by our
results and the extent to which it mirrors that of quark potential models and
make suggestions for the interpretation of our results involving exotic quantum
numbered states
Spiral vortices traveling between two rotating defects in the Taylor-Couette system
Numerical calculations of vortex flows in Taylor-Couette systems with counter
rotating cylinders are presented. The full, time dependent Navier-Stokes
equations are solved with a combination of a finite difference and a Galerkin
method. Annular gaps of radius ratio and of several heights are
simulated. They are closed by nonrotating lids that produce localized Ekman
vortices in their vicinity and that prevent axial phase propagation of spiral
vortices. Existence and spatio temporal properties of rotating defects, of
modulated Ekman vortices, and of the spiral vortex structures in the bulk are
elucidated in quantitative detail.Comment: 9 pages, 9 figure
Are Topological Charge Fluctuations in QCD Instanton Dominated?
We consider a recent proposal by Horv\'ath {\em et al.} to address the
question whether topological charge fluctuations in QCD are instanton dominated
via the response of fermions using lattice fermions with exact chiral symmetry,
the overlap fermions. Considering several volumes and lattice spacings we find
strong evidence for chirality of a finite density of low-lying eigenvectors of
the overlap-Dirac operator in the regions where these modes are peaked. This
result suggests instanton dominance of topological charge fluctuations in
quenched QCD.Comment: LaTeX, 15 pages, 8 postscript figures, minor improvements, version to
appear in PR
Space VLBI Observations of 3C 279 at 1.6 and 5 GHz
We present the first VLBI Space Observatory Programme (VSOP) observations of
the gamma-ray blazar 3C 279 at 1.6 and 5 GHz. The combination of the VSOP and
VLBA-only images at these two frequencies maps the jet structure on scales from
1 to 100 mas. On small angular scales the structure is dominated by the quasar
core and the bright secondary component `C4' located 3 milliarcseconds from the
core (at this epoch). On larger angular scales the structure is dominated by a
jet extending to the southwest, which at the largest scale seen in these images
connects with the smallest scale structure seen in VLA images. We have
exploited two of the main strengths of VSOP: the ability to obtain
matched-resolution images to ground-based images at higher frequencies and the
ability to measure high brightness temperatures. A spectral index map was made
by combining the VSOP 1.6 GHz image with a matched-resolution VLBA-only image
at 5 GHz from our VSOP observation on the following day. The spectral index map
shows the core to have a highly inverted spectrum, with some areas having a
spectral index approaching the limiting value for synchrotron self-absorbed
radiation of 2.5. Gaussian model fits to the VSOP visibilities revealed high
brightness temperatures (>10^{12} K) that are difficult to measure with
ground-only arrays. An extensive error analysis was performed on the brightness
temperature measurements. Most components did not have measurable brightness
temperature upper limits, but lower limits were measured as high as 5x10^{12}
K. This lower limit is significantly above both the nominal inverse Compton and
equipartition brightness temperature limits. The derived Doppler factor,
Lorentz factor, and angle to the line-of-sight in the case of the equipartition
limit are at the upper end of the range of expected values for EGRET blazars.Comment: 11 pages, 6 figures, emulateapj.sty, To be published in The
Astrophysical Journal, v537, Jul 1, 200
Dispersion of Ripplons in Superfluid 4he
A detailed study of the dispersion law of surface excitations in liquid \hef
at zero temperature is presented, with special emphasis to the short wave
length region. The hybridization mechanism between surface and bulk modes is
discussed on a general basis, investigating the scattering of slow rotons from
the surface. An accurate density functional, accounting for backflow effects,
is then used to determine the dispersion of both bulk and surface excitations.
The numerical results are close to the experimental data obtained on thick
films and explicitly reveal the occurrence of important hybridization effects
between ripplons and rotons.Comment: 23 pages, REVTEX 3.0, 11 figures upon request, UTF-326/9
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