36 research outputs found
High order vibration modes of glass embedded AgAu nanoparticles
High resolution low frequency Raman scattering measurements from embedded
AgAu nanoparticles unveil efficient scattering by harmonics of both the
quadrupolar and the spherical modes. Comparing the experimental data with
theoretical calculations that account for both the embedding medium and the
resonant Raman process enables a very complete description of the observed
multiple components in terms of harmonics of both the quadrupolar and spherical
modes, with a dominating Raman response from the former ones. It is found that
only selected harmonics of the quadrupolar mode contribute significantly to the
Raman spectra in agreement with earlier theoretical predictions.Comment: 11 pages, 4 figure
Crossover behavior and multi-step relaxation in a schematic model of the cut-off glass transition
We study a schematic mode-coupling model in which the ideal glass transition
is cut off by a decay of the quadratic coupling constant in the memory
function. (Such a decay, on a time scale tau_I, has been suggested as the
likely consequence of activated processes.) If this decay is complete, so that
only a linear coupling remains at late times, then the alpha relaxation shows a
temporal crossover from a relaxation typical of the unmodified schematic model
to a final strongly slower-than-exponential relaxation. This crossover, which
differs somewhat in form from previous schematic models of the cut-off glass
transition, resembles light-scattering experiments on colloidal systems, and
can exhibit a `slower-than-alpha' relaxation feature hinted at there. We also
consider what happens when a similar but incomplete decay occurs, so that a
significant level of quadratic coupling remains for t>>tau_I. In this case the
correlator acquires a third, weaker relaxation mode at intermediate times. This
empirically resembles the beta process seen in many molecular glass formers. It
disappears when the initial as well as the final quadratic coupling lies on the
liquid side of the glass transition, but remains present even when the final
coupling is only just inside the liquid (so that the alpha relaxation time is
finite, but too long to measure). Our results are suggestive of how, in a
cut-off glass, the underlying `ideal' glass transition predicted by
mode-coupling theory can remain detectable through qualitative features in
dynamics.Comment: 14 pages revtex inc 10 figs; submitted to pr
Spectral Shape of Relaxations in Silica Glass
Precise low-frequency light scattering experiments on silica glass are
presented, covering a broad temperature and frequency range (9 GHz < \nu < 2
THz). For the first time the spectral shape of relaxations is observed over
more than one decade in frequency. The spectra show a power-law low-frequency
wing of the relaxational part of the spectrum with an exponent
proportional to temperature in the range 30 K < T < 200 K. A comparison of our
results with those from acoustic attenuation experiments performed at different
frequencies shows that this power-law behaviour rather well describes
relaxations in silica over 9 orders of magnitude in frequency. These findings
can be explained by a model of thermally activated transitions in double well
potentials.Comment: 4 pages, 3 figures, submitted to Phys. Rev. Let
Dielectric and thermal relaxation in the energy landscape
We derive an energy landscape interpretation of dielectric relaxation times
in undercooled liquids, comparing it to the traditional Debye and
Gemant-DiMarzio-Bishop pictures. The interaction between different local
structural rearrangements in the energy landscape explains qualitatively the
recently observed splitting of the flow process into an initial and a final
stage. The initial mechanical relaxation stage is attributed to hopping
processes, the final thermal or structural relaxation stage to the decay of the
local double-well potentials. The energy landscape concept provides an
explanation for the equality of thermal and dielectric relaxation times. The
equality itself is once more demonstrated on the basis of literature data for
salol.Comment: 7 pages, 3 figures, 41 references, Workshop Disordered Systems,
Molveno 2006, submitted to Philosophical Magazin
Statistical mechanical approach to secondary processes and structural relaxation in glasses and glass formers
The interrelation of dynamic processes active on separated time-scales in
glasses and viscous liquids is investigated using a model displaying two
time-scale bifurcations both between fast and secondary relaxation and between
secondary and structural relaxation. The study of the dynamics allows for
predictions on the system relaxation above the temperature of dynamic arrest in
the mean-field approximation, that are compared with the outcomes of the
equations of motion directly derived within the Mode Coupling Theory (MCT) for
under-cooled viscous liquids. Varying the external thermodynamic parameters a
wide range of phenomenology can be represented, from a very clear separation of
structural and secondary peak in the susceptibility loss to excess wing
structures.Comment: 13 pages, 8 figure
Elastic Properties of Viruses
Viruses are compact biological nanoparticles whose elastic and dynamical properties are hardly known. Inelastic (Brillouin) light scattering was used to characterize these properties, from microcrystals of the Satellite Tobacco Mosaic Virus, a nearly spherical plant virus of 17-nm diameter. Longitudinal sound velocities in wet and dry Satellite Tobacco Mosaic Virus crystals were determined and compared to that of the well-known protein crystal, lysozyme. Localized vibrational modes of the viral particles (i.e., particle modes) were sought in the relevant frequency ranges, as derived assuming the viruses as full free nanospheres. Despite very favorable conditions, regarding virus concentration and expected low damping in dry microcrystals, no firm evidence of virus particle modes could be detected
On the origin of quasi-elastic light scattering in glasses
It is found that the polarised quasi-elastic light scattering
(QELS) in three investigated glasses, namely silica,
o-terphenyl and ethanol, exhibits a step-wise increase
of intensity when passing the frequency of the longitudinal
Brillouin line towards lower frequencies, keeping, however, the
same frequency dependence as the depolarised spectrum. The latter
shows no such excess QELS. The amplitude of this excess QELS in
polarised scattering is proportional to the ratio of the integral
intensities of the Brillouin line and the boson peak, while the
relaxation mechanisms are the same for both polarised and
depolarised spectra. This we take as a clear evidence that the
dominant contribution to QELS in glasses arises due to vibrational
relaxation