273 research outputs found
Mechanisms of resonant low frequency Raman scattering from metallic nanoparticle Lamb modes
International audienceThe low frequency Raman scattering from gold nanoparticle bimodal assemblies with controlled size distributions has been studied. Special care has been paid to determining the size dependence of the Raman intensity corresponding to the quadrupolar Lamb mode. Existing models based on a microscopic description of the scattering mechanism in small particles (bond polarizability, dipole induced dipole models) predict, for any Raman-active Lamb modes, an inelastic intensity scaling as the volume of the nanoparticle. Surprisingly experimental intensity ratios are found to be anomalously much greater than theoretical ones, calling into question this scaling law. To explain these discrepancies, a simple mechanism of Raman scattering, based on the density fluctuations in the nanoparticles induced by the Lamb modes, is introduced. This modeling, in which the nanoparticle is described as an elastic isotropic continuous medium-as in Lamb theory, successfully explains the major features exhibited by low frequency Raman modes. Moreover this model provides a unified picture for any material, suitable for handling both small and large size ranges, as well as non-resonant and resonant excitation conditions in the case of metallic species. Published by AIP Publishing
Effect of physical aging on the low-frequency vibrational density of states of a glassy polymer
The effects of the physical aging on the vibrational density of states (VDOS)
of a polymeric glass is studied. The VDOS of a poly(methyl methacrylate) glass
at low-energy (<15 meV), was determined from inelastic neutron scattering at
low-temperature for two different physical thermodynamical states. One sample
was annealed during a long time at temperature lower than Tg, and another was
quenched from a temperature higher than Tg. It was found that the VDOS around
the boson peak, relatively to the one at higher energy, decreases with the
annealing at lower temperature than Tg, i.e., with the physical aging.Comment: To be published in Europhys. Let
Continuum elastic sphere vibrations as a model for low-lying optical modes in icosahedral quasicrystals
The nearly dispersionless, so-called "optical" vibrational modes observed by
inelastic neutron scattering from icosahedral Al-Pd-Mn and Zn-Mg-Y
quasicrystals are found to correspond well to modes of a continuum elastic
sphere that has the same diameter as the corresponding icosahedral basic units
of the quasicrystal. When the sphere is considered as free, most of the
experimentally found modes can be accounted for, in both systems. Taking into
account the mechanical connection between the clusters and the remainder of the
quasicrystal allows a complete assignment of all optical modes in the case of
Al-Pd-Mn. This approach provides support to the relevance of clusters in the
vibrational properties of quasicrystals.Comment: 9 pages without figure
High frequency longitudinal and transverse dynamics in water
High-resolution, inelastic x-ray scattering measurements of the dynamic
structure factor S(Q,\omega) of liquid water have been performed for wave
vectors Q between 4 and 30 nm^-1 in distinctly different thermodynamic
conditions (T= 263 - 420 K ; at, or close to, ambient pressure and at P = 2
kbar). In agreement with previous inelastic x-ray and neutron studies, the
presence of two inelastic contributions (one dispersing with Q and the other
almost non-dispersive) is confirmed. The study of their temperature- and
Q-dependence provides strong support for a dynamics of liquid water controlled
by the structural relaxation process. A viscoelastic analysis of the
Q-dispersing mode, associated with the longitudinal dynamics, reveals that the
sound velocity undergoes the complete transition from the adiabatic sound
velocity (c_0) (viscous limit) to the infinite frequency sound velocity
(c_\infinity) (elastic limit). On decreasing Q, as the transition regime is
approached from the elastic side, we observe a decrease of the intensity of the
second, weakly dispersing feature, which completely disappears when the viscous
regime is reached. These findings unambiguously identify the second excitation
to be a signature of the transverse dynamics with a longitudinal symmetry
component, which becomes visible in the S(Q,\omega) as soon as the purely
viscous regime is left.Comment: 28 pages, 12 figure
Contribution of simulation and gaming to natural resource management issues: An introduction
The Raman coupling function in amorphous silica and the nature of the long wavelength excitations in disordered systems
New Raman and incoherent neutron scattering data at various temperatures and
molecular dynamic simulations in amorphous silica, are compared to obtain the
Raman coupling coefficient and, in particular, its low frequency
limit. This study indicates that in the limit
extrapolates to a non vanishing value, giving important indications on the
characteristics of the vibrational modes in disordered materials; in particular
our results indicate that even in the limit of very long wavelength the local
disorder implies non-regular local atomic displacements.Comment: Revtex, 4 ps figure
Clock-dependent chromatin topology modulates circadian transcription and behavior.
The circadian clock in animals orchestrates widespread oscillatory gene expression programs, which underlie 24-h rhythms in behavior and physiology. Several studies have shown the possible roles of transcription factors and chromatin marks in controlling cyclic gene expression. However, how daily active enhancers modulate rhythmic gene transcription in mammalian tissues is not known. Using circular chromosome conformation capture (4C) combined with sequencing (4C-seq), we discovered oscillatory promoter-enhancer interactions along the 24-h cycle in the mouse liver and kidney. Rhythms in chromatin interactions were abolished in arrhythmic <i>Bmal1</i> knockout mice. Deleting a contacted intronic enhancer element in the <i>Cryptochrome 1</i> ( <i>Cry1</i> ) gene was sufficient to compromise the rhythmic chromatin contacts in tissues. Moreover, the deletion reduced the daily dynamics of <i>Cry1</i> transcriptional burst frequency and, remarkably, shortened the circadian period of locomotor activity rhythms. Our results establish oscillating and clock-controlled promoter-enhancer looping as a regulatory layer underlying circadian transcription and behavior
Multiple-scattering effects on incoherent neutron scattering in glasses and viscous liquids
Incoherent neutron scattering experiments are simulated for simple dynamic
models: a glass (with a smooth distribution of harmonic vibrations) and a
viscous liquid (described by schematic mode-coupling equations). In most
situations multiple scattering has little influence upon spectral
distributions, but it completely distorts the wavenumber-dependent amplitudes.
This explains an anomaly observed in recent experiments
Elastic constant dishomogeneity and dependence of the broadening of the dynamical structure factor in disordered systems
We propose an explanation for the quadratic dependence on the momentum ,
of the broadening of the acoustic excitation peak recently found in the study
of the dynamic structure factor of many real and simulated glasses. We ascribe
the observed law to the spatial fluctuations of the local wavelength of
the collective vibrational modes, in turn produced by the dishomegeneity of the
inter-particle elastic constants. This explanation is analitically shown to
hold for 1-dimensional disordered chains and satisfatorily numerically tested
in both 1 and 3 dimensions.Comment: 4 pages, RevTeX, 5 postscript figure
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