17,634 research outputs found
On the relevance of q-distribution functions: The return time distribution of restricted random walker
There exist a large literature on the application of -statistics to the
out-of-equilibrium non-ergodic systems in which some degree of strong
correlations exists. Here we study the distribution of first return times to
zero, , of a random walk on the set of integers
with a position dependent transition probability given by . We find
that for all values of can be fitted by
-exponentials, but only for is given exactly by a
-exponential in the limit . This is a remarkable result
since the exact analytical solution of the corresponding continuum model
represents as a sum of Bessel functions with a smooth dependence on
from which we are unable to identify as of special significance.
However, from the high precision numerical iteration of the discrete Master
Equation, we do verify that only for is exactly a
-exponential and that a tiny departure from this parameter value makes the
distribution deviate from -exponential. Further research is certainly
required to identify the reason for this result and also the applicability of
-statistics and its domain.Comment: 14 pages, 3 figures. The replacement correct that two papers in the
reference list were not mentioned in the tex
Connectivity-Driven Coherence in Complex Networks
We study the emergence of coherence in complex networks of mutually coupled
non-identical elements. We uncover the precise dependence of the dynamical
coherence on the network connectivity, on the isolated dynamics of the elements
and the coupling function. These findings predict that in random graphs, the
enhancement of coherence is proportional to the mean degree. In locally
connected networks, coherence is no longer controlled by the mean degree, but
rather on how the mean degree scales with the network size. In these networks,
even when the coherence is absent, adding a fraction s of random connections
leads to an enhancement of coherence proportional to s. Our results provide a
way to control the emergent properties by the manipulation of the dynamics of
the elements and the network connectivity.Comment: 4 pages, 2 figure
The What, Why and How of openness in science
We give a short introduction to open science followed by an overview of Creative Commons and open source licenses
Ultrafast double magnetization switching in GdFeCo with two picosecond-delayed femtosecond pump pulses
The recently discovered thermally induced magnetization switching (TIMS) induced by single femtosecond laser pulses in ferrimagnetic GdFeCo alloys proceeds on the picosecond time-scale. The rate at which data can be changed for use of TIMS in technological devices is limited by the processes leading to thermal equilibrium. In the present work, we address the question of whether it is possible to further excite switching via TIMS well before thermal equilibrium between subsystems is reached. In particular, we investigate the conditions for double thermally induced magnetic switching by the application of two shortly delayed laser pulses. These conditions become relevant for potential applications as it sets both a limit to rewrite data and demonstrates the importance of spatial confinement of a heat pulse to bit size, as neighboring bits may be accidentally re-switched for spatially extended pulse spots. To demonstrate this effect, we theoretically study the switching behavior in a prototypical ferrimagnetic GdFeCo alloy as a function of composition. We use computer simulations based on thermal atomistic spin dynamics and demonstrate the possibility of inducing a second switching event well before thermal equilibrium is reached and define the conditions under which it can occur. Our theoretical findings could serve as a guidance for further understanding of TIMS as well as to act as a guide for future applications
The Redshift Distribution of the TOUGH Survey
We present the redshift results from a Very Large Telescope program aimed at
optimizing the legacy value of the Swift mission: to characterize a
homogeneous, X-ray selected, sample of 69 GRB host galaxies. 19 new redshifts
have been secured, resulting in a 83% (57/69) redshift completion, making the
survey the most comprehensive in terms of redshift completeness of any sample
to the full Swift depth, available to date. We present the cumulative redshift
distribution and derive a conservative, yet small, associated uncertainty. We
constrain the fraction of Swift GRBs at high redshift to a maximum of 10% (5%)
for z > 6 (z > 7). The mean redshift of the host sample is assessed to be >
2.2. Using this more complete sample, we confirm previous findings that the GRB
rate at high redshift (z > 3) appears to be in excess of predictions based on
assumptions that it should follow conventional determinations of the star
formation history of the universe, combined with an estimate of its likely
metallicity dependence. This suggests that either star formation at high
redshifts has been significantly underestimated, for example due to a dominant
contribution from faint, undetected galaxies, or that GRB production is
enhanced in the conditions of early star formation, beyond those usually
ascribed to lower metallicity.Comment: 7th Huntsville Gamma-Ray Burst Symposium, GRB 2013: paper 34 in eConf
Proceedings C130414
Critical behavior of a one-dimensional monomer-dimer reaction model with lateral interactions
A monomer-dimer reaction lattice model with lateral repulsion among the same
species is studied using a mean-field analysis and Monte Carlo simulations. For
weak repulsions, the model exhibits a first-order irreversible phase transition
between two absorbing states saturated by each different species. Increasing
the repulsion, a reactive stationary state appears in addition to the saturated
states. The irreversible phase transitions from the reactive phase to any of
the saturated states are continuous and belong to the directed percolation
universality class. However, a different critical behavior is found at the
point where the directed percolation phase boundaries meet. The values of the
critical exponents calculated at the bicritical point are in good agreement
with the exponents corresponding to the parity-conserving universality class.
Since the adsorption-reaction processes does not lead to a non-trivial local
parity-conserving dynamics, this result confirms that the twofold symmetry
between absorbing states plays a relevant role in determining the universality
class. The value of the exponent , which characterizes the
fluctuations of an interface at the bicritical point, supports the
Bassler-Brown's conjecture which states that this is a new exponent in the
parity-conserving universality class.Comment: 19 pages, 22 figures, to be published in Phys. Rev
Determining the fraction of reddened quasars in COSMOS with multiple selection techniques from X-ray to radio wavelengths
The sub-population of quasars reddened by intrinsic or intervening clouds of
dust are known to be underrepresented in optical quasar surveys. By defining a
complete parent sample of the brightest and spatially unresolved quasars in the
COSMOS field, we quantify to which extent this sub-population is fundamental to
our understanding of the true population of quasars. By using the available
multiwavelength data of various surveys in the COSMOS field, we built a parent
sample of 33 quasars brighter than mag, identified by reliable X-ray to
radio wavelength selection techniques. Spectroscopic follow-up with the
NOT/ALFOSC was carried out for four candidate quasars that had not been
targeted previously to obtain a 100\% redshift completeness of the sample. The
population of high quasars (HAQs), a specific sub-population of quasars
selected from optical/near-infrared photometry, is found to contribute
of the parent sample. The full population of bright spatially
unresolved quasars represented by our parent sample consists of
reddened quasars defined by having , and
of the sample having assuming the extinction
curve of the Small Magellanic Cloud. We show that the HAQ selection works well
for selecting reddened quasars, but some are missed because their optical
spectra are too blue to pass the color cut in the HAQ selection. This is
either due to a low degree of dust reddening or anomalous spectra. We find that
the fraction of quasars with contributing light from the host galaxy is most
dominant at . At higher redshifts the population of spatially
unresolved quasars selected by our parent sample is found to be representative
of the full population at mag. This work quantifies the bias against
reddened quasars in studies that are based solely on optical surveys.Comment: 22 pages, 10 figures, accepted for publication in A&A. The ArXiv
abstract has been shortened for it to be printabl
Mean-field model of the ferromagnetic ordering in the superconducting phase of ErNi_2B_2C
A mean-field model explaining most of the details in the magnetic phase
diagram of ErNi_2B_2C is presented. The low-temperature magnetic properties are
found to be dominated by the appearance of long-period commensurate structures.
The stable structure at low temperatures and zero field is found to have a
period of 40 layers along the a direction, and upon cooling it undergoes a
first-order transition at T_C = 2.3 K to a different 40-layered structure
having a net ferromagnetic component of about 0.4 mu_B/Er. The
neutron-diffraction patterns predicted by the two 40-layered structures, above
and below T_C, are in agreement with the observations of Choi et al.Comment: 4 pages, 3 figures (Revtex4
Effective temperature in driven vortex lattices with random pinning
We study numerically correlation and response functions in non-equilibrium
driven vortex lattices with random pinning. From a generalized
fluctuation-dissipation relation we calculate an effective transverse
temperature in the fluid moving phase. We find that the effective temperature
decreases with increasing driving force and becomes equal to the equilibrium
melting temperature when the dynamic transverse freezing occurs. We also
discuss how the effective temperature can be measured experimentally from a
generalized Kubo formula.Comment: 4 pages, 4 figure
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