53 research outputs found
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
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
Long-lived submicrometric bubbles in very diluted alkali halide water solutions
Solutions of LiCl and of NaCl in ultrapure water were studied through
Rayleigh/Brillouin scattering as a function of the concentration (molarity, M)
of dissolved salt from 0.2M to extremely low concentration (2.10^-17 M ). The
Landau-Placzek ratio, R/B, of the Rayleigh scattering intensity over the total
Brillouin, was measured thanks to the dynamically controlled stability of the
used Fabry-Perot interferometer. It was observed that the R/B ratio follows two
stages as a function of increasing dilution rate: after a strong decrease
between 0.2M and 2.10^-5 M, it increases to reach a maximum between 10^-9 M and
10^-16 M. The first stage corresponds to the decrease of the Rayleigh
scattering by the ion concentration fluctuations with the decrease of salt
concentration. The second stage, at lower concentrations, is consistent with
the increase of the Rayleigh scattering by long-lived sub-microscopic bubbles
with the decrease of ion concentration. The origin of these sub-microscopic
bubbles is the shaking of the solutions which was carried out after each
centesimal dilution. The very long lifetime of the sub-microscopic bubbles and
the effects of aging originate in the electric charge of bubbles. The increase
of R/B with the decrease of the low salt concentration corresponds to the
increase of the sub-microscopic bubble size with the decrease of concentration,
that is imposed by the bubble stability due to the covering of the surface
bubble by negative ions.Comment: 6 figures. To be published in Physical Chemistry and Chemical Physics
(PCCP
Susceptibility functions for slow relaxation processes in supercooled liquids and the search for universal relaxation patterns
In order to describe the slow response of a glass former we discuss some
distribution of correlation times, e.g., the generalized gamma distribution
(GG) and an extension thereof (GGE), the latter allowing to reproduce a simple
peak susceptibility such as of Cole-Davidson type as well as a susceptibility
exhibiting an additional high frequency power law contribution (excess wing).
Applying the GGE distribution to the dielectric spectra of glass formers
exhibiting no beta-process peak (glycerol, propylene carbonate and picoline) we
are able to reproduce the salient features of the slow response (1e-6 Hz - 1e9
Hz). A line shape analysis is carried out either in the time or frequency
domain and in both cases an excess wing can be identified. The latter evolves
in a universal way while cooling and shows up for correlation times tau_alpha >
1e-8 s. It appears that its first emergence marks the break down of the high
temperature scenario of mode coupling theory. - In order to describe a glass
former exhibiting a beta-process peak we have introduced a distribution
function which is compatible with assuming a thermally activated process in
contrast to some commonly used fit functions. Together with the GGE
distribution this function allows in the frame of the Williams-Watts approach
to completely interpolate the spectra, e.g. of fluoro aniline (1e-6 Hz - 1e9
Hz). The parameters obtained indicate an emergence of both the excess wing and
the beta-process again at tau_alpha > 1e-8s.Comment: 22 pages, 12 figure
Glassy dynamics in mono-, di-, and tri-propylene glycol: From the alpha- to the fast beta-relaxation
We present a thorough characterization of the glassy dynamics of three
propylene glycols (mono-, di- and trimer) by broadband dielectric spectroscopy.
By covering a frequency range of more than 15 decades, we have access to the
entire variety of dynamic processes typical for glassy dynamics. These results
add three more molecular glass formers to the sparse list of materials for
which real broadband spectra, including the region of the fast beta-process,
are available. Some first analyses of the various observed dynamic processes
are provided
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
- …