2,773 research outputs found
Work distributions in the T=0 Random Field Ising Model
We perform a numerical study of the three-dimensional Random Field
Ising Model at T=0. We compare work distributions along metastable
trajectories obtained with the single-spin flip dynamics with the distribution
of the internal energy change along equilibrium trajectories. The goal is to
investigate the possibility of extending the Crooks fluctuation theorem to zero
temperature when, instead of the standard ensemble statistics, one considers
the ensemble generated by the quenched disorder. We show that a simple
extension of Crooks fails close to the disordered induced equilibrium phase
transition due to the fact that work and internal energy distributions are very
asymmetric
Maximum entropy approach to power-law distributions in coupled dynamic-stochastic systems
Statistical properties of coupled dynamic-stochastic systems are studied
within a combination of the maximum information principle and the
superstatistical approach. The conditions at which the Shannon entropy
functional leads to a power-law statistics are investigated. It is demonstrated
that, from a quite general point of view, the power-law dependencies may appear
as a consequence of "global" constraints restricting both the dynamic phase
space and the stochastic fluctuations. As a result, at sufficiently long
observation times the dynamic counterpart is driven into a non-equilibrium
steady state whose deviation from the usual exponential statistics is given by
the distance from the conventional equilibrium
EAGLE ISS - A modular twin-channel integral-field near-IR spectrograph
The ISS (Integral-field Spectrograph System) has been designed as part of the
EAGLE Phase A Instrument Study for the E-ELT. It consists of two input channels
of 1.65x1.65 arcsec field-of-view, each reconfigured spatially by an
image-slicing integral-field unit to feed a single near-IR spectrograph using
cryogenic volume-phase-holographic (VPH) gratings to disperse the image
spectrally. A 4k x 4k array detector array records the dispersed images. The
optical design employs anamorphic magnification, image slicing, VPH gratings
scanned with a novel cryo-mechanism and a three-lens camera. The mechanical
implementation features IFU optics in Zerodur, a modular bench structure and a
number of high-precision cryo-mechanisms.Comment: 12 pages, to be published in Proc SPIE 7735: Ground-based & Airborne
Instrumentation for Astronomy II
Magnetic hysteresis in the Cu-Al-Mn intermetallic alloy: experiments and modeling
We study isothermal magnetization processes in the Cu-Al-Mn intermetallic
alloy. Hysteresis is observed at temperatures below the spin-freezing of the
system. The characteristics of the hysteresis cycles as a function of
temperature and Mn content (magnetic element) are obtained. At low temperature
(5 K) a change from smooth to sharp cycles is observed with increasing Mn
content, which is related to the decrease of configurational disorder. We also
study a zero-temperature site-diluted Ising model, suitable for the description
of this Cu-Al-Mn system. The model reproduces the main features of the
hysteresis loops observed experimentally. It exhibits a disorder-induced
critical line separating a disordered phase from an incipient ferromagnetic
ground-state. The comparison between the model and the experiments allows to
conclude that the observed change in the experimental hysteresis loops can be
understood within the framework of the theory of disorder-induced criticality
in fluctuationless first-order phase transitions.Comment: 30 pages, 15 eps figures, 2 tables. To appear Phys. Rev. B 59 (June
1999
Hysteresis and Avalanches in the Random Anisotropy Ising Model
The behaviour of the Random Anisotropy Ising model at T=0 under local
relaxation dynamics is studied. The model includes a dominant ferromagnetic
interaction and assumes an infinite anisotropy at each site along local
anisotropy axes which are randomly aligned. Two different random distributions
of anisotropy axes have been studied. Both are characterized by a parameter
that allows control of the degree of disorder in the system. By using numerical
simulations we analyze the hysteresis loop properties and characterize the
statistical distribution of avalanches occuring during the metastable evolution
of the system driven by an external field. A disorder-induced critical point is
found in which the hysteresis loop changes from displaying a typical
ferromagnetic magnetization jump to a rather smooth loop exhibiting only tiny
avalanches. The critical point is characterized by a set of critical exponents,
which are consistent with the universal values proposed from the study of other
simpler models.Comment: 40 pages, 21 figures, Accepted for publication in Phys. Rev.
Magnetic hysteresis in the Cu-Al-Mn intermetallic alloy: experiments and modeling
We study isothermal magnetization processes in the Cu-Al-Mn intermetallic
alloy. Hysteresis is observed at temperatures below the spin-freezing of the
system. The characteristics of the hysteresis cycles as a function of
temperature and Mn content (magnetic element) are obtained. At low temperature
(5 K) a change from smooth to sharp cycles is observed with increasing Mn
content, which is related to the decrease of configurational disorder. We also
study a zero-temperature site-diluted Ising model, suitable for the description
of this Cu-Al-Mn system. The model reproduces the main features of the
hysteresis loops observed experimentally. It exhibits a disorder-induced
critical line separating a disordered phase from an incipient ferromagnetic
ground-state. The comparison between the model and the experiments allows to
conclude that the observed change in the experimental hysteresis loops can be
understood within the framework of the theory of disorder-induced criticality
in fluctuationless first-order phase transitions.Comment: 30 pages, 15 eps figures, 2 tables. To appear Phys. Rev. B 59 (June
1999
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
Cold Positrons from Decaying Dark Matter
Many models of dark matter contain more than one new particle beyond those in
the Standard Model. Often heavier particles decay into the lightest dark matter
particle as the Universe evolves. Here we explore the possibilities that arise
if one of the products in a (Heavy Particle) (Dark Matter) decay
is a positron, and the lifetime is shorter than the age of the Universe. The
positrons cool down by scattering off the cosmic microwave background and
eventually annihilate when they fall into Galactic potential wells. The
resulting 511 keV flux not only places constraints on this class of models but
might even be consistent with that observed by the INTEGRAL satellite.Comment: 20 pages, 7 figure
On the athermal character of structural phase transitions
The significance of thermal fluctuations on nucleation in structural
first-order phase transitions has been examined. The prototype case of
martensitic transitions has been experimentally investigated by means of
acoustic emission techniques. We propose a model based on the mean
first-passage time to account for the experimental observations. Our study
provides a unified framework to establish the conditions for isothermal and
athermal transitions to be observed.Comment: 5 pages, 4 figures, accepted in Phys. Rev. Let
The effect of progressive muscle relaxation on daily cortisol secretion
Abbreviated progressive muscle relaxation (APMR) is a much used stress-management technique. Its efficacy relevant to placebo control is already established in the literature and the primary aim of the present study was to ascertain whether its proven impact on psychological stress measures is matched by a decrease in prevailing levels of the stress-associated hormone cortisol, using accurate and robust measurement based on multiple sampling of full diurnal cortisol secretion profiles. First-year university students can face significant stress in adjustment to academic demands and immersion in a novel social network and provided a convenient study population. One hundred and one first-year students completed APMR with prevailing stress levels assessed a week before and after intervention. Both cortisol and self-report measures were significantly reduced post-intervention by 8% and 10%, respectively. The efficacy of the intervention was independent of, and not modulated by neuroticism, gender, age and smoking status. We also demonstrated that cortisol reduction was unlikely to have been a consequence of adaptation to any initial cortisol elevation prompted by the challenge of the demanding saliva collection protocol. We conclude that the efficacy of APMR in this population extends to reduction in biologically expressed stress levels as well as levels based solely on self-report
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