775 research outputs found
Interacting Random Walkers and Non-Equilibrium Fluctuations
We introduce a model of interacting Random Walk, whose hopping amplitude
depends on the number of walkers/particles on the link. The mesoscopic
counterpart of such a microscopic dynamics is a diffusing system whose
diffusivity depends on the particle density. A non-equilibrium stationary flux
can be induced by suitable boundary conditions, and we show indeed that it is
mesoscopically described by a Fourier equation with a density dependent
diffusivity. A simple mean-field description predicts a critical diffusivity if
the hopping amplitude vanishes for a certain walker density. Actually, we
evidence that, even if the density equals this pseudo-critical value, the
system does not present any criticality but only a dynamical slowing down. This
property is confirmed by the fact that, in spite of interaction, the particle
distribution at equilibrium is simply described in terms of a product of
Poissonians. For mesoscopic systems with a stationary flux, a very effect of
interaction among particles consists in the amplification of fluctuations,
which is especially relevant close to the pseudo-critical density. This agrees
with analogous results obtained for Ising models, clarifying that larger
fluctuations are induced by the dynamical slowing down and not by a genuine
criticality. The consistency of this amplification effect with altered coloured
noise in time series is also proved.Comment: 8 pages, 7 figure
Supporting sustainable chainsaw milling through multi-stakeholder dialogue in Guyana
The Chainsaw Milling Project is using multi-stakeholder dialogue (MSD) as its key strategy for achieving its objectives. In Guyana the dialogue process aims to achieve a shared understanding of chainsaw milling practices and associated opportunities for economic development at local level. It aims to build consensus among stakeholders to reduce the level of conflict and illegality related to chainsaw milling by local communities; and review regulatory frameworks in order to strengthen governance in the forestry sector. So far the MSD in Guyana has focused largely on addressing chainsaw milling opportunities and challenges at local and regional level. To maximize its effectiveness the dialogue is in the process of being upscaled to a national level. Narratives, personal experiences and lessons from the MSD at the local level were felt to be useful in support of this endeavour. To that effect project staff and other stakeholders have written “stories” that show the different dimensions of their work but with a common message: this multi-stakeholder dialogue is of key importance for those who make a living out of the forest, for now and in the future. This book contains their stories
Wang-Landau study of the 3D Ising model with bond disorder
We implement a two-stage approach of the Wang-Landau algorithm to investigate
the critical properties of the 3D Ising model with quenched bond randomness. In
particular, we consider the case where disorder couples to the nearest-neighbor
ferromagnetic interaction, in terms of a bimodal distribution of strong versus
weak bonds. Our simulations are carried out for large ensembles of disorder
realizations and lattices with linear sizes in the range . We apply
well-established finite-size scaling techniques and concepts from the scaling
theory of disordered systems to describe the nature of the phase transition of
the disordered model, departing gradually from the fixed point of the pure
system. Our analysis (based on the determination of the critical exponents)
shows that the 3D random-bond Ising model belongs to the same universality
class with the site- and bond-dilution models, providing a single universality
class for the 3D Ising model with these three types of quenched uncorrelated
disorder.Comment: 7 pages, 7 figures, to be published in Eur. Phys. J.
Superluminal optical pulse propagation in nonlinear coherent media
The propagation of light-pulse with negative group-velocity in a nonlinear
medium is studied theoretically. We show that the necessary conditions for
these effects to be observable are realized in a three-level -system
interacting with a linearly polarized laser beam in the presence of a static
magnetic field. In low power regime, when all other nonlinear processes are
negligible, the light-induced Zeeman coherence cancels the resonant absorption
of the medium almost completely, but preserves the dispersion anomalous and
very high. As a result, a superluminal light pulse propagation can be observed
in the sense that the peak of the transmitted pulse exits the medium before the
peak of the incident pulse enters. There is no violation of causality and
energy conservation. Moreover, the superluminal effects are prominently
manifested in the reshaping of pulse, which is caused by the
intensity-dependent pulse velocity. Unlike the shock wave formation in a
nonlinear medium with normal dispersion, here, the self-steepening of the pulse
trailing edge takes place due to the fact that the more intense parts of the
pulse travel slower. The predicted effect can be easily observed in the well
known schemes employed for studying of nonlinear magneto-optical rotation. The
upper bound of sample length is found from the criterion that the pulse
self-steepening and group-advance time are observable without pulse distortion
caused by the group-velocity dispersion.Comment: 16 pages, 7 figure
The Casimir Problem of Spherical Dielectrics: Numerical Evaluation for General Permittivities
The Casimir mutual free energy F for a system of two dielectric concentric
nonmagnetic spherical bodies is calculated, at arbitrary temperatures. The
present paper is a continuation of an earlier investigation [Phys. Rev. E {\bf
63}, 051101 (2001)], in which F was evaluated in full only for the case of
ideal metals (refractive index n=infinity). Here, analogous results are
presented for dielectrics, for some chosen values of n. Our basic calculational
method stems from quantum statistical mechanics. The Debye expansions for the
Riccati-Bessel functions when carried out to a high order are found to be very
useful in practice (thereby overflow/underflow problems are easily avoided),
and also to give accurate results even for the lowest values of l down to l=1.
Another virtue of the Debye expansions is that the limiting case of metals
becomes quite amenable to an analytical treatment in spherical geometry. We
first discuss the zero-frequency TE mode problem from a mathematical viewpoint
and then, as a physical input, invoke the actual dispersion relations. The
result of our analysis, based upon the adoption of the Drude dispersion
relation at low frequencies, is that the zero-frequency TE mode does not
contribute for a real metal. Accordingly, F turns out in this case to be only
one half of the conventional value at high temperatures. The applicability of
the Drude model in this context has however been questioned recently, and we do
not aim at a complete discussion of this issue here. Existing experiments are
low-temperature experiments, and are so far not accurate enough to distinguish
between the different predictions. We also calculate explicitly the
contribution from the zero-frequency mode for a dielectric. For a dielectric,
this zero-frequency problem is absent.Comment: 23 pages, LaTeX, 7 ps figures; expanded discussion, especially in
Sec. 5. To appear in Phys. Rev.
Gluon dipole penguin contributions to and CP violation in Hyperon decays in the Standard Model
We consider the gluon dipole penguin operator contributions to
and CP violation in hyperon decays. It has been proposed
by Bertolini et al. that the contribution to may be
significant. We show that there is a cancellation in the leading order
contribution and this contribution is actually suppressed by a factor of order
O(. We find that the same operator also contributes
to CP violation in hyperon decays where it is not suppressed. The gluon dipole
penguin operator can enhance CP violation in hyperon decays by as much as 25\%.Comment: 11pages, Revte
Criticality in the randomness-induced second-order phase transition of the triangular Ising antiferromagnet with nearest- and next-nearest-neighbor interactions
Using a Wang-Landau entropic sampling scheme, we investigate the effects of
quenched bond randomness on a particular case of a triangular Ising model with
nearest- () and next-nearest-neighbor () antiferromagnetic
interactions. We consider the case , for which the pure
model is known to have a columnar ground state where rows of nearest-neighbor
spins up and down alternate and undergoes a weak first-order phase transition
from the ordered to the paramagnetic state. With the introduction of quenched
bond randomness we observe the effects signaling the expected conversion of the
first-order phase transition to a second-order phase transition and using the
Lee-Kosterlitz method, we quantitatively verify this conversion. The emerging,
under random bonds, continuous transition shows a strongly saturating specific
heat behavior, corresponding to a negative exponent , and belongs to a
new distinctive universality class with , ,
and . Thus, our results for the critical exponents support
an extensive but weak universality and the emerged continuous transition has
the same magnetic critical exponent (but a different thermal critical exponent)
as a wide variety of two-dimensional (2d) systems without and with quenched
disorder.Comment: 17 pages, 6 figures, accepted for publication in Physica
4H-SiC Schottky diode arrays for X-ray detection
Five SiC Schottky photodiodes for X-ray detection have been electrically characterized at room temperature.
One representative diode was also electrically characterized over the temperature range 20°C to 140 °C. The performance at 30 °C of all five X-ray detectors, in both current mode and for photon counting X-ray spectroscopy was investigated. The diodes were fabricated in an array form such that they could be operated as either a 2×2 or 1×3 pixel array. Although the devices showed double barrier heights, high ideality factors and higher than expected leakage current at room temperature (12 nA/cm2 at an internal electric field of 105 kV/ cm), they operated as spectroscopic photon counting soft X-ray detectors uncooled at 30 °C. The measured energy resolution (FWHM at 17.4 keV, Mo Kα) varied from 1.36 to 1.68 keV among different diodes
Scaling and self-averaging in the three-dimensional random-field Ising model
We investigate, by means of extensive Monte Carlo simulations, the magnetic
critical behavior of the three-dimensional bimodal random-field Ising model at
the strong disorder regime. We present results in favor of the two-exponent
scaling scenario, , where and are the
critical exponents describing the power-law decay of the connected and
disconnected correlation functions and we illustrate, using various finite-size
measures and properly defined noise to signal ratios, the strong violation of
self-averaging of the model in the ordered phase.Comment: 8 pages, 6 figures, to be published in Eur. Phys. J.
Higgs-Boson Production Induced by Bottom Quarks
Bottom quark-induced processes are responsible for a large fraction of the
LHC discovery potential, in particular for supersymmetric Higgs bosons.
Recently, the discrepancy between exclusive and inclusive Higgs boson
production rates has been linked to the choice of an appropriate bottom
factorization scale. We investigate the process kinematics at hadron colliders
and show that it leads to a considerable decrease in the bottom factorization
scale. This effect is the missing piece needed to understand the corresponding
higher order results. Our results hold generally for charged and for neutral
Higgs boson production at the LHC as well as at the Tevatron. The situation is
different for single top quark production, where we find no sizeable
suppression of the factorization scale. Turning the argument around, we can
specify how large the collinear logarithms are, which can be resummed using the
bottom parton picture.Comment: 18 page
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