1,337 research outputs found
Hydrogen-Related Conversion Processes of Ge-Related Point Defects in Silica Triggered by UV Laser Irradiation
The conversion processes of Ge-related point defects triggered in amorphous
SiO2 by 4.7eV laser exposure were investigated. Our study has focused on the
interplay between the (=Ge•-H) H(II) center and the twofold coordinated
Ge defect (=Ge••). The former is generated in the post-irradiation
stage, while the latter decays both during and after exposure. The
post-irradiation decay kinetics of =Ge•• is isolated and found to
be anti-correlated to the growth of H(II), at least at short times. From this
finding it is suggested that both processes are due to trapping of radiolytic
H0 at the diamagnetic defect site. Furthermore, the anti-correlated behavior is
preserved also under repeated irradiation: light at 4.7eV destroys the already
formed H(II) centers and restore their precursors =Ge••. This
process leads to repeatability of the post-irradiation kinetics of the two
species after multiple laser exposures. A comprehensive scheme of chemical
reactions explaining the observed post-irradiation processes is proposed and
tested against experimental data.Comment: 25 pages, 7 figures, submitted to Phys. Rev.
Anisotropy-based mechanism for zigzag striped patterns in magnetic thin films
In this work we studied a two dimensional ferromagnetic system using Monte
Carlo simulations. Our model includes exchange and dipolar interactions, a
cubic anisotropy term, and uniaxial out-of-plane and in-plane ones. According
to the set of parameters chosen, the model including uniaxial out-of-plane
anisotropy has a ground-state which consists of a canted state with stripes of
opposite out-of-plane magnetization. When the cubic anisotropy is introduced
zigzag patterns appear in the stripes at fields close to the remanence. An
analysis of the anisotropy terms of the model shows that this configuration is
related to specific values of the ratio between the cubic and the effective
uniaxial anisotropy. The mechanism behind this effect is related to particular
features of the anisotropy's energy landscape, since a global minima transition
as a function of the applied field is required in the anisotropy terms. This
new mechanism for zigzags formation could be present in monocrystal
ferromagnetic thin films in a given range of thicknesses.Comment: 910 pages, 10 figure
The exchange bias phenomenon in uncompensated interfaces: Theory and Monte Carlo simulations
We performed Monte Carlo simulations in a bilayer system composed by two thin
films, one ferromagnetic (FM) and the other antiferromagnetic (AFM). Two
lattice structures for the films were considered: simple cubic (sc) and a body
center cubic (bcc). In both lattices structures we imposed an uncompensated
interfacial spin structure, in particular we emulated a FeF2-FM system in the
case of the (bcc) lattice. Our analysis focused on the incidence of the
interfacial strength interactions between the films J_eb and the effect of
thermal fluctuations on the bias field H_EB. We first performed Monte Carlo
simulations on a microscopic model based on classical Heisenberg spin
variables. To analyze the simulation results we also introduced a simplified
model that assumes coherent rotation of spins located on the same layer
parallel to the interface. We found that, depending on the AFM film anisotropy
to exchange ratio, the bias field is either controlled by the intrinsic pinning
of a domain wall parallel to the interface or by the stability of the first AFM
layer (quasi domain wall) near the interface.Comment: 18 pages, 11 figure
Evidence of exactness of the mean field theory in the nonextensive regime of long-range spin models
The q-state Potts model with long-range interactions that decay as 1/r^alpha
subjected to an uniform magnetic field on d-dimensional lattices is analized
for different values of q in the nonextensive regime (alpha between 0 and d).
We also consider the two dimensional antiferromagnetic Ising model with the
same type of interactions. The mean field solution and Monte Carlo calculations
for the equations of state for these models are compared. We show that, using a
derived scaling which properly describes the nonextensive thermodynamic
behaviour, both types of calculations show an excellent agreement in all the
cases here considered, except for alpha=d. These results allow us to extend to
nonextensive magnetic models a previous conjecture which states that the mean
field theory is exact for the Ising one.Comment: 10 pages, 4 figure
Long-range interactions and non-extensivity in ferromagnetic spin models
The Ising model with ferromagnetic interactions that decay as is
analyzed in the non-extensive regime , where the
thermodynamic limit is not defined. In order to study the asymptotic properties
of the model in the limit ( being the number of spins)
we propose a generalization of the Curie-Weiss model, for which the
limit is well defined for all . We
conjecture that mean field theory is {\it exact} in the last model for all
. This conjecture is supported by Monte Carlo heat bath
simulations in the case. Moreover, we confirm a recently conjectured
scaling (Tsallis\cite{Tsallis}) which allows for a unification of extensive
() and non-extensive () regimes.Comment: RevTex, 12 pages, 1 eps figur
Photoluminescence dispersion as a probe of structural inhomogeneity in silica
We report time-resolved photoluminescence spectra of point defects in
amorphous silicon dioxide (silica), in particular the decay kinetics of the
emission signals of extrinsic Oxygen Deficient Centres of the second type from
singlet and directly-excited triplet states are measured and used as a probe of
structural inhomogeneity. Luminescence activity in sapphire
(-AlO) is studied as well and used as a model system to compare
the optical properties of defects in silica with those of defects embedded in a
crystalline matrix. Only for defects in silica, we observe a variation of the
decay lifetimes with emission energy and a time dependence of the first moment
of the emission bands. These features are analyzed within a theoretical model
with explicit hypothesis about the effect introduced by the disorder of
vitreous systems. Separate estimations of the homogenous and inhomogeneous
contributions to the measured emission linewidth are obtained: it is found that
inhomogeneous effects strongly condition both the triplet and singlet
luminescence activities of oxygen deficient centres in silica, although the
degree of inhomogeneity of the triplet emission turns out to be lower than that
of the singlet emission. Inhomogeneous effects appear to be negligible in
sapphire
The Lie-Poisson structure of the reduced n-body problem
The classical n-body problem in d-dimensional space is invariant under the
Galilean symmetry group. We reduce by this symmetry group using the method of
polynomial invariants. As a result we obtain a reduced system with a
Lie-Poisson structure which is isomorphic to sp(2n-2), independently of d. The
reduction preserves the natural form of the Hamiltonian as a sum of kinetic
energy that depends on velocities only and a potential that depends on
positions only. Hence we proceed to construct a Poisson integrator for the
reduced n-body problem using a splitting method.Comment: 26 pages, 2 figure
Deep Image Prior Amplitude SAR Image Anonymization
This paper presents an extensive evaluation of the Deep Image Prior (DIP) technique for image inpainting on Synthetic Aperture Radar (SAR) images. SAR images are gaining popularity in various applications, but there may be a need to conceal certain regions of them. Image inpainting provides a solution for this. However, not all inpainting techniques are designed to work on SAR images. Some are intended for use on photographs, while others have to be specifically trained on top of a huge set of images. In this work, we evaluate the performance of the DIP technique that is capable of addressing these challenges: it can adapt to the image under analysis including SAR imagery; it does not require any training. Our results demonstrate that the DIP method achieves great performance in terms of objective and semantic metrics. This indicates that the DIP method is a promising approach for inpainting SAR images, and can provide high-quality results that meet the requirements of various applications
Nonresident fathers' voice : marginalized disempowered and silenced
Nonresident fathers, following separation/divorce, are more likely to experience multiple forms of family types simultaneously than any other sociodemographic group. Although there is considerable writing on the factors and issues surrounding nonresident fathers from academics, the Family court, the Child Support Agency, and women’s and welfare groups, the voice of nonresident fathers themselves is rarely heard. This is due to nonresident fathers being marginalized, disempowered, and silenced by these same entities. The voice of nonresident fathers is routinely minimized, dismissed, and labeled as anti-feminist or a backlash to feminism. This opinion piece argues that there is a need for qualitative research to be undertaken to investigate, document, and explore nonresident fathers’ voices from their own perspective to hear what they have to say of themselves so that a better understanding of the dynamics that impact and influence them can be achieved. This would mean that actions can be identified and undertaken to better understand nonresident fathers’ situation while providing insights for the development of social policies by Government and Welfare agencies together with support care for nonresident fathers highlighting their desires and needs
Poisson Geometry in Constrained Systems
Constrained Hamiltonian systems fall into the realm of presymplectic
geometry. We show, however, that also Poisson geometry is of use in this
context.
For the case that the constraints form a closed algebra, there are two
natural Poisson manifolds associated to the system, forming a symplectic dual
pair with respect to the original, unconstrained phase space. We provide
sufficient conditions so that the reduced phase space of the constrained system
may be identified with a symplectic leaf in one of those. In the second class
case the original constrained system may be reformulated equivalently as an
abelian first class system in an extended phase space by these methods.
Inspired by the relation of the Dirac bracket of a general second class
constrained system to the original unconstrained phase space, we address the
question of whether a regular Poisson manifold permits a leafwise symplectic
embedding into a symplectic manifold. Necessary and sufficient for this is the
vanishing of the characteristic form-class of the Poisson tensor, a certain
element of the third relative cohomology.Comment: 41 pages, more detailed abstract in paper; v2: minor corrections and
an additional referenc
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