411 research outputs found
Signature of small rings in the Raman spectra of normal and compressed amorphous silica: A combined classical and ab initio study
We calculate the parallel (VV) and perpendicular (VH) polarized Raman spectra
of amorphous silica. Model SiO2 glasses, uncompressed and compressed, were
generated by a combination of classical and ab initio molecular-dynamics
simulations and their dynamical matrices were computed within the framework of
the density functional theory. The Raman scattering intensities were determined
using the bond-polarizability model and a good agreement with experimental
spectra was found. We confirm that the modes associated to the fourfold and
threefold rings produce most of the Raman intensity of the D1 and D2 peaks,
respectively, in the VV Raman spectra. Modifications of the Raman spectra upon
compression are found to be in agreement with experimental data. We show that
the modes associated to the fourfold rings still exist upon compression but do
not produce a strong Raman intensity, whereas the ones associated to the
threefold rings do. This result strongly suggests that the area under the D1
and D2 peaks is not directly proportional to the concentration of small rings
in amorphous SiO2.Comment: 21 pages, 8 figures. Phys. Rev. B, in pres
Rings and rigidity transitions in network glasses
Three elastic phases of covalent networks, (I) floppy, (II) isostatically
rigid and (III) stressed-rigid have now been identified in glasses at specific
degrees of cross-linking (or chemical composition) both in theory and
experiments. Here we use size-increasing cluster combinatorics and constraint
counting algorithms to study analytically possible consequences of
self-organization. In the presence of small rings that can be locally I, II or
III, we obtain two transitions instead of the previously reported single
percolative transition at the mean coordination number , one from a
floppy to an isostatic rigid phase, and a second one from an isostatic to a
stressed rigid phase. The width of the intermediate phase and the
order of the phase transitions depend on the nature of medium range order
(relative ring fractions). We compare the results to the Group IV
chalcogenides, such as Ge-Se and Si-Se, for which evidence of an intermediate
phase has been obtained, and for which estimates of ring fractions can be made
from structures of high T crystalline phases.Comment: 29 pages, revtex, 7 eps figure
Frequency dependent specific heat of viscous silica
We apply the Mori-Zwanzig projection operator formalism to obtain an
expression for the frequency dependent specific heat c(z) of a liquid. By using
an exact transformation formula due to Lebowitz et al., we derive a relation
between c(z) and K(t), the autocorrelation function of temperature fluctuations
in the microcanonical ensemble. This connection thus allows to determine c(z)
from computer simulations in equilibrium, i.e. without an external
perturbation. By considering the generalization of K(t) to finite wave-vectors,
we derive an expression to determine the thermal conductivity \lambda from such
simulations. We present the results of extensive computer simulations in which
we use the derived relations to determine c(z) over eight decades in frequency,
as well as \lambda. The system investigated is a simple but realistic model for
amorphous silica. We find that at high frequencies the real part of c(z) has
the value of an ideal gas. c'(\omega) increases quickly at those frequencies
which correspond to the vibrational excitations of the system. At low
temperatures c'(\omega) shows a second step. The frequency at which this step
is observed is comparable to the one at which the \alpha-relaxation peak is
observed in the intermediate scattering function. Also the temperature
dependence of the location of this second step is the same as the one of the
peak, thus showing that these quantities are intimately connected to
each other. From c'(\omega) we estimate the temperature dependence of the
vibrational and configurational part of the specific heat. We find that the
static value of c(z) as well as \lambda are in good agreement with experimental
data.Comment: 27 pages of Latex, 8 figure
Family-based pediatric weight management interventions in US primary care settings targeting children ages 6-12 years old: A systematic review guided by the RE-AIM framework.
Obesity is a pandemic that disproportionately affects children from vulnerable populations in the USA. Current treatment approaches in primary care settings in the USA have been reported to be insufficient at managing pediatric obesity, primarily due to implementation challenges for healthcare systems and barriers for families. While the literature has examined the efficacy of pediatric obesity interventions focused on internal validity, it lacks sufficient reporting and analysis of external validity necessary for successful translation to primary care settings. We conducted a systematic review of the primary-care-setting literature from January 2007 to March 2020 on family-based pediatric weight management interventions in both English and/or Spanish for children ages 6-12 years in the USA using the Reach, Efficacy/Effectiveness, Adoption, Implementation, Maintenance (RE-AIM) framework. A literature search, using PRISMA guidelines, was conducted in January 2022 using the following electronic databases: Medline Ovid, Embase, and Cochrane Library. 22 270 records were screened, and 376 articles were reviewed in full. 184 studies were included. The most commonly reported dimensions of the RE-AIM framework were Reach (65%), Efficacy/Effectiveness (64%), and Adoption (64%), while Implementation (47%) and Maintenance (42%) were less often reported. The prevalence of reporting RE-AIM construct indicators ranged greatly, from 1% to 100%. This systematic review underscores the need for more focus on external validity to guide the development, implementation, and dissemination of future pediatric obesity interventions based in primary care settings. It also suggests conducting additional research on sustainable financing for pediatric obesity interventions
Synthesis of Glass Nanofibers Using Femtosecond Laser Radiation Under Ambient Condition
We report the unique growth of nanofibers in silica and borosilicate glass using femtosecond laser radiation at 8 MHz repetition rate and a pulse width of 214 fs in air at atmospheric pressure. The nanofibers are grown perpendicular to the substrate surface from the molten material in laser-drilled microvias where they intertwine and bundle up above the surface. The fibers are few tens of nanometers in thickness and up to several millimeters in length. Further, it is found that at some places nanoparticles are attached to the fiber surface along its length. Nanofiber growth is explained by the process of nanojets formed in the molten liquid due to pressure gradient induced from the laser pulses and subsequently drawn into fibers by the intense plasma pressure. The attachment of nanoparticles is due to the condensation of vapor in the plasma
Raman scattering study of the a-GeTe structure and possible mechanism for the amorphous-to-crystal transition
We report on an inelastic (Raman) light scattering study of the local
structure of amorphous GeTe films. A detailed analysis of the
temperature-reduced Raman spectra has shown that appreciable structural changes
occur as a function of temperature. These changes involve modifications of
atomic arrangements such as to facilitate the rapid amorphous-to-crystal
transformation, which is the major advantage of phase-change materials used in
optical data storage media. A particular structural model, supported by
polarization analysis, is proposed being compatible with the experimental data
as regards both the structure of a-GeTe and the crystallization transition. The
remarkable difference between the Raman spectrum of the crystal and the glass
can thus naturally be accounted for.Comment: Published in: J. Phys. Condens. Matter. 18, 965-979 (2006
Isoelectronic series of oxygen deficient centers in silica: experimental estimation of homogeneous and inhomogeneous spectral widths
We report nanosecond time-resolved photoluminescence measurements on the
isoelectronic series of oxygen deficient centers in amorphous silica,
Si-ODC(II), Ge-ODC(II) and Sn-ODC(II) which are responsible of fluorescence
activities at 4 eV under excitation at 5 eV. The dependence of the
first moment of their emission band on time, and that of the radiative decay
lifetime on emission energy are analyzed within a theoretical model able to
describe the effects introduced by disorder on the optical properties of the
defects. We obtain separate estimates of the homogeneous and inhomogeneous
contributions to the measured emission linewidth and we derive homogeneous
spectroscopic features of the investigated point defects (Huangh-Rhys factor,
homogeneous width, oscillator strength, vibrational frequency). The results
point to a picture in which an oxygen deficient center localized on a heavier
atom features a higher degree of inhomogeneity due to stronger local distortion
of the surrounding matrix. For Si, Ge, Sn related defects the parameter
, able to quantify inhomogeneity, results to be 65, 78 and 90$%,
respectively
On the analysis of the vibrational Boson peak and low-energy excitations in glasses
Implications of reduction procedures applied to the low energy part of the
vibrational density of states in glasses and supercooled liquids are considered
by advancing a detailed comparison between the excess - over the Debye limit -
vibrational density of states g(w) and the frequency-reduced representation
g(w)/w^2 usually referred to as the Boson peak. Analyzing representative
experimental data from inelastic neutron and Raman scattering we show that
reduction procedures distort to a great extent the otherwise symmetric excess
density of states. The frequency of the maximum and the intensity of the excess
experience dramatic changes; the former is reduced while the latter increases.
The frequency and the intensity of the Boson peak are also sensitive to the
distribution of the excess. In the light of the critical appraisal between the
two forms of the density of states (i.e. the excess and the frequency-reduced
one) we discuss changes of the Boson peak spectral features that are induced
under the presence of external stimuli such as temperature (quenching rate,
annealing), pressure, and irradiation. The majority of the Boson peak changes
induced by the presence of those stimuli can be reasonably traced back to
simple and expected modifications of the excess density of states and can be
quite satisfactorily accounted for the Euclidean random matrix theory.
Parallels to the heat capacity Boson peak are also briefly discussed.Comment: To appear in J. Non-Cryst. Solids (Proceedings of the 5th IDMRCS,
Lille, July 2005
- …