320 research outputs found
The magnetization-driven random field Ising model at T=0
We study the hysteretic evolution of the random field Ising model (RFIM) at
T=0 when the magnetization M is controlled externally and the magnetic field H
becomes the output variable. The dynamics is a simple modification of the
single-spin-flip dynamics used in the H-driven situation and consists in
flipping successively the spins with the largest local field. This allows to
perform a detailed comparison between the microscopic trajectories followed by
the system with the two protocols. Simulations are performed on random graphs
with connectivity z=4 (Bethe lattice) and on the 3-D cubic lattice. The same
internal energy U(M)is found with the two protocols when there is no
macroscopic avalanche and it does not depend on whether the microscopic states
are stable or not. On the Bethe lattice, the energy inside the macroscopic
avalanche also coincides with the one that is computed analytically with the
H-driven algorithm along the unstable branch of the hysteresis loop. The output
field, defined here as dU/dM, exhibits very large fluctuations with the
magnetization and is not self-averaging. Relation to the experimental situation
is discussed.Comment: 11 pages, 13 figure
Statistical similarity between the compression of a porous material and earthquakes
It has been long stated that there are profound analogies between fracture
experiments and earthquakes; however, few works attempt a complete
characterization of the parallelisms between these so separate phenomena. We
study the Acoustic Emission events produced during the compression of Vycor
(SiO2). The Gutenberg-Richter law, the modified Omori's law, and the law of
aftershock productivity are found to hold for a minimum of 5 decades, are
independent of the compression rate, and keep stationary for all the duration
of the experiments. The waiting-time distribution fulfills a unified scaling
law with a power-law exponent close to 2.45 for long times, which is explained
in terms of the temporal variations of the activity rate.Comment: 4 pages and a bit more, 4 figure
Influence of the driving mechanism on the response of systems with athermal dynamics: the example of the random-field Ising model
We investigate the influence of the driving mechanism on the hysteretic
response of systems with athermal dynamics. In the framework of local-mean
field theory at finite temperature (but neglecting thermallly activated
processes), we compare the rate-independent hysteresis loops obtained in the
random field Ising model (RFIM) when controlling either the external magnetic
field or the extensive magnetization . Two distinct behaviors are
observed, depending on disorder strength. At large disorder, the -driven and
-driven protocols yield identical hysteresis loops in the thermodynamic
limit. At low disorder, when the -driven magnetization curve is
discontinuous (due to the presence of a macroscopic avalanche), the -driven
loop is re-entrant while the induced field exhibits strong intermittent
fluctuations and is only weakly self-averaging. The relevance of these results
to the experimental observations in ferromagnetic materials, shape memory
alloys, and other disordered systems is discussed.Comment: 11 pages, 11 figure
Multidisciplinary investigation on the catfish parasite Hamatopeduncularia Yamaguti, 1953 (Monogenoidea: Dactylogyridae): description of two new species from India, and phylogenetic considerations
Hamatopeduncularia was erected with Hamatopeduncularis arii as the type species. This genus comprises monogenoidean species mostly found as ectoparasites of marine catfishes belonging to the Ariidae. There is a significant taxonomic ambiguity among Hamatopeduncularia species due to their morphological similarity, but so far only a few morphological studies have succeeded in addressing interspecific variation and relationships. Moreover, little molecular data is available for this genus. A multidisciplinary, integrated study consisting of morphological, morphometric and molecular analyses was conducted on different species of Hamatopeduncularia recovered from the gills of two marine catfishes, Arius jella Day and Plicofollis dussumieri (Valenciennes). Five species of Hamatopeduncularia, two of which represent new species, were investigated: H. arii, H. elongatum, H. thalassini, H. madhaviae sp. nov. and H. bifida sp. nov. Phylogenetic analysis was performed using the 18S rDNA sequence as a molecular marker. The most important results of the present work are: (1) the multidisciplinary description of two novel species; (2) the multidisciplinary redescription of two species and of the type species of the genus; (3) the first molecular characterisation of 18S rDNA sequences of five species of genus Hamatopeduncularia; and (4) molecular support for the monophyly of the genus. http://zoobank.org/urn:lsid:zoobank.org:act:1333F4CC-E497-4D0A-AD7D-276D44AE6413 http://zoobank.org/urn:lsid:zoobank.org:act:43D18F75-6F4A-4F9B-8C00-6234E5BA652
ProtoDESI: First On-Sky Technology Demonstration for the Dark Energy Spectroscopic Instrument
The Dark Energy Spectroscopic Instrument (DESI) is under construction to
measure the expansion history of the universe using the baryon acoustic
oscillations technique. The spectra of 35 million galaxies and quasars over
14,000 square degrees will be measured during a 5-year survey. A new prime
focus corrector for the Mayall telescope at Kitt Peak National Observatory will
deliver light to 5,000 individually targeted fiber-fed robotic positioners. The
fibers in turn feed ten broadband multi-object spectrographs. We describe the
ProtoDESI experiment, that was installed and commissioned on the 4-m Mayall
telescope from August 14 to September 30, 2016. ProtoDESI was an on-sky
technology demonstration with the goal to reduce technical risks associated
with aligning optical fibers with targets using robotic fiber positioners and
maintaining the stability required to operate DESI. The ProtoDESI prime focus
instrument, consisting of three fiber positioners, illuminated fiducials, and a
guide camera, was installed behind the existing Mosaic corrector on the Mayall
telescope. A Fiber View Camera was mounted in the Cassegrain cage of the
telescope and provided feedback metrology for positioning the fibers. ProtoDESI
also provided a platform for early integration of hardware with the DESI
Instrument Control System that controls the subsystems, provides communication
with the Telescope Control System, and collects instrument telemetry data.
Lacking a spectrograph, ProtoDESI monitored the output of the fibers using a
Fiber Photometry Camera mounted on the prime focus instrument. ProtoDESI was
successful in acquiring targets with the robotically positioned fibers and
demonstrated that the DESI guiding requirements can be met.Comment: Accepted versio
Avalanches in compressed porous SiO2-based materials
The failure dynamics in SiO2-based porous materials under compression, namely the synthetic glass Gelsil and three natural sandstones, has been studied for slowly increasing compressive uniaxial stress with rates between 0.2 and 2.8 kPa/s. The measured collapsed dynamics is similar to Vycor, which is another synthetic porous SiO2 glass similar to Gelsil but with a different porous mesostructure. Compression occurs by jerks of strain release and a major collapse at the failure point. The acoustic emission and shrinking of the samples during jerks are measured and analyzed. The energy of acoustic emission events, its duration, and waiting times between events show that the failure process follows avalanche criticality with power law statistics over ca. 4 decades with a power law exponent ε 1.4 for the energy distribution. This exponent is consistent with the mean-field value for the collapse of granular media. Besides the absence of length, energy, and time scales, we demonstrate the existence of aftershock correlations during the failure process
Statistical Similarity between the Compression of a Porous Material and Earthquakes
It has long been stated that there are profound analogies between fracture experiments and earthquakes; however, few works attempt a complete characterization of the parallels between these so separate phenomena. We study the acoustic emission events produced during the compression of Vycor (SiO2). The Gutenberg-Richter law, the modified Omori's law, and the law of aftershock productivity hold for a minimum of 5 decades, are independent of the compression rate, and keep stationary for all the duration of the experiments. The waiting-time distribution fulfills a unified scaling law with a power-law exponent close to 2.45 for long times, which is explained in terms of the temporal variations of the activity rate
Metastable Random Field Ising model with exchange enhancement: a simple model for Exchange Bias
We present a simple model that allows hysteresis loops with exchange bias to
be reproduced. The model is a modification of the T=0 random field Ising model
driven by an external field and with synchronous local relaxation dynamics. The
main novelty of the model is that a certain fraction f of the exchange
constants between neighbouring spins is enhanced to a very large value J_E. The
model allows the dependence of the exchange bias and other properties of the
hysteresis loops to be analyzed as a function of the parameters of the model:
the fraction f of enhanced bonds, the amount of the enhancement J_E and the
amount of disorder which is controlled by the width sigma of the Gaussian
distribution of the random fields.Comment: 8 pages, 11 figure
Statistical similarity between the compression of a porous material and earthquakes
It has been long stated that there are profound analogies between fracture experiments and earthquakes; however, few works attempt a complete characterization of the parallelisms between these so separate phenomena. We study the Acoustic Emission events produced during the compression of Vycor (SiO&sub&2&/sub&). The Gutenberg-Richter law, the modified Omori's law, and the law of aftershock productivity hold for a minimum of 5 decades, are independent of the compression rate, and keep stationary for all the duration of the experiments. The waiting-time distribution fulfills a unified scaling law with a power-law exponent close to 2.45 for long times, which is explained in terms of the temporal variations of the activity rate
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