14,260 research outputs found
Formation and structural characterization of Ni nanoparticles embedded in SiO₂
Face-centered cubic Ni nanoparticles were formed in SiO₂ by ion implantation and thermal annealing. Small-angle x-ray scattering in conjunction with transmission electron microscopy was used to determine the nanoparticle size as a function of annealing temperature, whereas the local atomic structure was measured with x-ray absorption spectroscopy. The influence of finite-size effects on the nanoparticle structural properties was readily apparent and included a decrease in coordination number and bond length and an increase in structural disorder for decreasing nanoparticle size. Such results are consistent with the non-negligible surface-to-volume ratio characteristic of nanoparticles. In addition, temperature-dependent x-ray absorption spectroscopy measurements showed the mean vibrational frequency (as obtained from the Einstein temperature) decreased with decreasing nanoparticle size. This reduction was attributed to the greater influence of the loosely bound, under-coordinated surface atoms prevailing over the effects of capillary pressure, the former enhancing the low frequency modes of the vibrational density of statesThis work was financially supported by the Australian
Synchrotron and the Australian Research Council with access to equipment provided by the Australian Nanofabrication
Facility
Phonon self-energy corrections to non-zero wavevector phonon modes in single-layer graphene
Phonon self-energy corrections have mostly been studied theoretically and
experimentally for phonon modes with zone-center (q = 0) wave-vectors. Here,
gate-modulated Raman scattering is used to study phonons of a single layer of
graphene (1LG) in the frequency range from 2350 to 2750 cm-1, which shows the
G* and the G'-band features originating from a double-resonant Raman process
with q \not= 0. The observed phonon renormalization effects are different from
what is observed for the zone-center q = 0 case. To explain our experimental
findings, we explored the phonon self-energy for the phonons with non-zero
wave-vectors (q \not= 0) in 1LG in which the frequencies and decay widths are
expected to behave oppositely to the behavior observed in the corresponding
zone-center q = 0 processes. Within this framework, we resolve the
identification of the phonon modes contributing to the G* Raman feature at 2450
cm-1 to include the iTO+LA combination modes with q \not= 0 and the 2iTO
overtone modes with q = 0, showing both to be associated with wave-vectors near
the high symmetry point K in the Brillouin zone
The Limits on Cosmological Anisotropies and Inhomogeneities from COBE Data
Assuming that the cosmological principle holds, Maartens, Ellis and Stoeger
(MES) recently constructed a detailed scheme linking anisotropies in the cosmic
background radiation (CMB) with anisotropies and inhomogeneities in the large
scale structure of the universe and showed how to place limits on those
anisotropies and inhomogeneities simply by using CMB quadrupole and octupole
limits. First we indicate and discuss the connection between the covariant
multipole moments of the temperature anisotropy used in the MES scheme and the
quadrupole and octupole results from COBE. Then we introduce those results into
the MES limit equations to obtain definite quantitative limits on the complete
set of cosmological measures of anisotropy and inhomogeneity.
We find that all the anisotropy measures are less than 10^{-4} in the case of
those not affected by the expansion rate H, and less than 10^{-6} Mpc^{-1} in
the case of those which are. These results quantitatively demonstrate that the
observable universe is indeed close to Friedmann-Lemaitre-Robertson-Walker
(FLRW) on the largest scales, and can be adequately modelled by an almost-FLRW
model -- that is, the anisotropies and inhomogeneities characterizing the
observable universe on the largest scales are not too large to be considered
perturbations to FLRW.Comment: Original paper with corrections. ApJ 476 435 (1997) erratum to appear
ApJ Sept 199
Avaliação dos impactos ambientais do sistema de produção integrada de uva de mesa na região do Submédio São Francisco.
O objetivo deste estudo foi analisar os impactos ambientais do sistema de produção integrada de uva na região do Submédio São Francisco, que é o principal pólo de exportação desta fruta no país
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