11,648 research outputs found
A Reversibility Parameter for a Markovian Stepper
Recent experimental studies on the stepwize motion of biological molecular
motors have revealed that the ``characteristic distance'' of a step is usually
less than the actual step size. This observation implies that the
detailed-balance condition for kinetic rates of steps is violated in these
motors. In this letter, in order to clarify the significance of the
characteristic distance, we study a Langevin model of a molecular motor with a
hidden degree of freedom. We find that the ratio of the characteristic distance
to the step size is equal to unity if the dominant paths in the state space are
one dimensional, while it deviates from unity if the dominant paths are
branched. Therefore, this parameter can be utilized to determine the
reversibility of a motor even under a restricted observation.Comment: 6 pages, 2 figures - minor revision
A perturbation theory for large deviation functionals in fluctuating hydrodynamics
We study a large deviation functional of density fluctuation by analyzing
stochastic non-linear diffusion equations driven by the difference between the
densities fixed at the boundaries. By using a fundamental equality that yields
the fluctuation theorem, we first relate the large deviation functional with a
minimization problem. We then develop a perturbation method for solving the
problem. In particular, by performing an expansion with respect to the average
current, we derive the lowest order expression for the deviation from the local
equilibrium part. This expression implies that the deviation is written as the
space-time integration of the excess entropy production rate during the most
probable process of generating the fluctuation that corresponds to the argument
of the large deviation functional.Comment: 12page
Gluon propagators and center vortices in gluon plasma
We study electric and magnetic components of the gluon propagators in
quark-gluon plasma in terms of center vortices by using a quenched simulation
of SU(2) lattice theory. In the Landau gauge, the magnetic components of the
propagators are strongly affected in the infrared region by removal of the
center vortices, while the electric components are almost unchanged by this
procedure. In the Coulomb gauge, the time-time correlators, including an
instantaneous interaction, also have an essential contribution from the center
vortices. As a result, one finds that magnetic degrees of freedom in the
infrared region couple strongly to the center vortices in the deconfinement
phase.Comment: 12 pages, 11 figure
Roundabout relaxation: collective excitation requires a detour to equilibrium
Relaxation to equilibrium after strong and collective excitation is studied,
by using a Hamiltonian dynamical system of one dimensional XY model. After an
excitation of a domain of elements, the excitation is concentrated to fewer
elements, which are made farther away from equilibrium, and the excitation
intensity increases logarithmically with . Equilibrium is reached only after
taking this ``roundabout'' route, with the time for relaxation diverging
asymptotically as with .Comment: 4 pages, 5 figure
Experimental evidence of shock mitigation in a Hertzian tapered chain
We present an experimental study of the mechanical impulse propagation
through a horizontal alignment of elastic spheres of progressively decreasing
diameter , namely a tapered chain. Experimentally, the diameters of
spheres which interact via the Hertz potential are selected to keep as close as
possible to an exponential decrease, , where the
experimental tapering factor is either ~% or ~%.
In agreement with recent numerical results, an impulse initiated in a
monodisperse chain (a chain of identical beads) propagates without shape
changes, and progressively transfer its energy and momentum to a propagating
tail when it further travels in a tapered chain. As a result, the front pulse
of this wave decreases in amplitude and accelerates. Both effects are
satisfactorily described by the hard spheres approximation, and basically, the
shock mitigation is due to partial transmissions, from one bead to the next, of
momentum and energy of the front pulse. In addition when small dissipation is
included, a better agreement with experiments is found. A close analysis of the
loading part of the experimental pulses demonstrates that the front wave adopts
itself a self similar solution as it propagates in the tapered chain. Finally,
our results corroborate the capability of these chains to thermalize
propagating impulses and thereby act as shock absorbing devices.Comment: ReVTeX, 7 pages with 6 eps, accepted for Phys. Rev. E (Related papers
on http://www.supmeca.fr/perso/jobs/
Laboratory Experiment of Checkerboard Pupil Mask Coronagraph
We present the results of the first laboratory experiment of checkerboard
shaped pupil binary mask coronagraphs using visible light, in the context of
the R&D activities for future mid-infrared space missions such as the 3.5 m
SPICA telescope. The primary aim of this work is to demonstrate the
coronagraphic performance of checkerboard masks down to a
peak-to-peak contrast, which is required to detect self-luminous extra-solar
planets in the mid-infrared region. Two masks, consisting of aluminum films on
a glass substrates, were manufactured using nano-fabrication techniques with
electron beam lithography: one mask was optimized for a pupil with a 30%
central obstruction and the other was for a pupil without obstruction. The
theoretical contrast for both masks was and no adaptive optics system
was employed. For both masks, the observed point spread functions were quite
consistent with the theoretical ones. The average contrast measured within the
dark regions was and . The
coronagraphic performance significantly outperformed the requirement
and almost reached the theoretical limit determined by the mask designs. We
discuss the potential application of checkerboard masks for mid-infrared
coronagraphy, and conclude that binary masks are promising for future
high-contrast space telescopes.Comment: 6 pages, 6 figure
Axions and the pulsation periods of variable white dwarfs revisited
Axions are the natural consequence of the introduction of the Peccei-Quinn
symmetry to solve the strong CP problem. All the efforts to detect such elusive
particles have failed up to now. Nevertheless, it has been recently shown that
the luminosity function of white dwarfs is best fitted if axions with a mass of
a few meV are included in the evolutionary calculations. Our aim is to show
that variable white dwarfs can provide additional and independent evidence
about the existence of axions. The evolution of a white dwarf is a slow cooling
process that translates into a secular increase of the pulsation periods of
some variable white dwarfs, the so-called DAV and DBV types. Since axions can
freely escape from such stars, their existence would increase the cooling rate
and, consequently, the rate of change of the periods as compared with the
standard ones. The present values of the rate of change of the pulsation period
of G117-B15A are compatible with the existence of axions with the masses
suggested by the luminosity function of white dwarfs, in contrast with previous
estimations. Furthermore, it is shown that if such axions indeed exist, the
drift of the periods of pulsation of DBV stars would be noticeably perturbed.Comment: Accepted for publication in Astronomy & Astrophysic
Defect Detection in Bonded Structures Using the Reverberant Wavefield
With the increasing use of adhesives in the automotive, aerospace, and manufacturing industries, there is a growing interest in developing nondestructive methods for locating defects in adhesive bonds. While conventional techniques which utilize ultrasonic waves and Lamb waves are likely candidates for obtaining high resolution images of defects, these methods may not be practical for assembly line applications where the time required to scan the bonds and the access to the bonds are often limited. The objective of this work is to develop an approach for detecting defects in bonds that requires only a limited number of measurements of the reverberant acoustic wavefield (i.e., waves that are multiply scattered off the boundaries of the structure) made over a band of frequencies
Energy Calibration of the JLab Bremsstrahlung Tagging System
In this report, we present the energy calibration of the Hall B
bremsstrahlung tagging system at the Thomas Jefferson National Accelerator
Facility. The calibration was performed using a magnetic pair spectrometer. The
tagged photon energy spectrum was measured in coincidence with pairs
as a function of the pair spectrometer magnetic field. Taking advantage of the
internal linearity of the pair spectrometer, the energy of the tagging system
was calibrated at the level of . The absolute energy scale
was determined using the rate measurements close to the end-point of
the photon spectrum. The energy variations across the full tagging range were
found to be MeV.Comment: 15 pages, 12 figure
Gravity-driven Dense Granular Flows
We report and analyze the results of numerical studies of dense granular
flows in two and three dimensions, using both linear damped springs and
Hertzian force laws between particles. Chute flow generically produces a
constant density profile that satisfies scaling relations suggestive of a
Bagnold grain inertia regime. The type of force law has little impact on the
behavior of the system. Bulk and surface flows differ in their failure criteria
and flow rheology, as evidenced by the change in principal stress directions
near the surface. Surface-only flows are not observed in this geometry.Comment: 4 pages, RevTeX 3.0, 4 PostScript figures (5 files) embedded with
eps
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