4,696 research outputs found
H(II) centers in natural silica under repeated UV laser irradiations
We investigated the kinetics of H(II) centers (=Ge'-H) in natural silica
under repeated 266nm UV irradiations performed by a Nd:YAG pulsed laser. UV
photons temporarily destroy these paramagnetic defects, their reduction being
complete within 250 pulses. After re-irradiation, H(II) centers grow again, and
the observed recovery kinetics depends on the irradiation dose; multiple 2000
pulses re-irradiations induce the same post-irradiation kinetics of H(II)
centers after each exposure cycle. The analysis of these effects allows us to
achieve a deeper understanding of the dynamics of the centers during and after
laser irradiation.Comment: Submitted to Journal of Non Crystalline Solid
On the balanced condition for the Eguchi-Hanson metric
Let be the Eguchi-Hanson metric on the blow-up of at
the origin. In this paper we show that is not balanced for any
positive integer .Comment: 9 pages, 1 figure, to appear in Journal of Geometry and Physic
Character of the reaction between molecular hydrogen and silicon dangling bond in amorphous SiO_2
The passivation by diffusing H2 of silicon dangling bond defects (E' centers,
induced by laser irradiation in amorphous SiO_2 (silica), is investigated in
situ at several temperatures. It is found that the reaction between E' center
and H_2 requires an activation energy of 0.38eV, and that its kinetics is not
diffusion-limited. The results are compared with previous findings on the other
fundamental paramagnetic point defect in silica, the non bridging oxygen hole
center, which features completely different reaction properties with H_2.
Besides, a comparison is proposed with literature data on the reaction
properties of surface E' centers, of E' centers embedded in silica films, and
with theoretical calculations. In particular, the close agreement with the
reaction properties of surface E' centers with H_2 leads to conclude that the
bulk and surface E' varieties are indistinguishable from their reaction
properties with molecular hydrogen.Comment: 15 pages, 3 figures, submitted to J. Phys. Chem.
Prediction of the relationship between body weight and body condition score in sheep
During the whole production cycle it is important to monitor the energy balance and to quantify body reserve changes of the ewes. This can be done, both in experimental settings and in the field, by estimating the body condition score (BCS) of the ewes and its variations. However, if this tool is used to balance the diets it is necessary to know the relationship between BCS and body weight (BW),
which varies depending on the mature size of the breed and of the population considered within each breed. The relationship between BW and BCS has been studied only for some sheep breeds and populations.
For this reason, this research aimed to develop a prediction model of this relationship in ewes for any breed or population
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.
Bleaching of optical activity induced by UV Laser exposure in natural silica
We report experimental data on two types of natural silica, differing for
their OH content, irradiated with UV photons (4.66 eV) from a pulsed Nd:YAG
laser. Irradiation induces a reduction of the absorption band at 5.12eV and of
the associated emissions at 3.14eV and 4.28eV, ascribed to twofold coordinated
Ge (=Ge'') centers pre-existing in our samples. The bleaching is mainly due to
the post-irradiation conversion of =Ge'' into the paramagnetic H(II) center via
trapping of a H atom. Comparison with literature data points out the
peculiarities of silica with a low Ge concentration as regards UV induced
transformations.Comment: Accepted for publication on Journal of Non crystalline solid
Stability of E' centers induced by 4.7eV laser radiation in SiO2
The kinetics of E' centers (silicon dangling bonds) induced by 4.7eV pulsed
laser irradiation in dry fused silica was investigated by in situ optical
absorption spectroscopy. The stability of the defects, conditioned by reaction
with mobile hydrogen of radiolytic origin, is discussed and compared to results
of similar experiments performed on wet fused silica. A portion of E' and
hydrogen are most likely generated by laser-induced breaking of Si-H
precursors, while an additional fraction of the paramagnetic centers arise from
another formation mechanism. Both typologies of E' participate to the reaction
with H_2 leading to the post-irradiation decay of the defects. This annealing
process is slowed down on decreasing temperature and is frozen at T=200K,
consistently with the diffusion properties of H_2 in silica.Comment: 12 pages, 3 figures, in press on J. Non cryst. solids (2007
Non glassy ground-state in a long-range antiferromagnetic frustrated model in the hypercubic cell
We analize the statistical mechanics of a long-range antiferromagnetic model
defined on a D-dimensional hypercube, both at zero and finite temperatures. The
associated Hamiltonian is derived from a recently proposed complexity measure
of Boolean functions, in the context of neural networks learning processes. We
show that, depending of the value of D, the system either presents a low
temperature antiferromagnetic stable phase or the global antiferromagnetic
order disappears at any temperature. In the last case the ground state is an
infinitely degenerated non-glassy one, composed by two equal size anti-aligned
antiferromagnetic domains. We also present some results for the ferromagnetic
version of the model.Comment: 8 pages, 5 figure
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