78 research outputs found
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
Evidence of secondary relaxations in the dielectric spectra of ionic liquids
We investigated the dynamics of a series of room temperature ionic liquids
based on the same 1-butyl-3-methyl imidazolium cation and different anions by
means of broadband dielectric spectroscopy covering 15 decades in frequency
(10^(-6)-10^9 Hz), and in the temperature range from 400 K down to 35 K. An
ionic conductivity is observed above the glass transition temperature T_{g}
with a relaxation in the electric modulus representation. Below T_{g}, two
relaxation processes appear, with the same features as the secondary
relaxations typically observed in molecular glasses. The activation energy of
the secondary processes and their dependence on the anion are different. The
slower process shows the characteristics of an intrinsic Johari-Goldstein
relaxation, in particular an activation energy E_{beta}=24k_{B}T_{g} is found,
as observed in molecular glasses.Comment: Major revision, submitted to Phys. Rev. Let
Frequency-domain study of relaxation in a spin glass model for the structural glass transition
We have computed the time-dependent susceptibility for the finite-size
mean-field Random Orthogonal model (ROM). We find that for temperatures above
the mode-coupling temperature the imaginary part of the susceptibility
obeys the scaling forms proposed for glass-forming liquids.
Furthermore, as the temperature is lowered the peak frequency of
decreases following a Vogel-Fulcher law with a critical temperature remarkably
close to the known critical temperature where the configurational entropy
vanishes.Comment: 7 pages, 4 figures, epl LaTeX packag
Non-Arrhenius Behavior of Secondary Relaxation in Supercooled Liquids
Dielectric relaxation spectroscopy (1 Hz - 20 GHz) has been performed on
supercooled glass-formers from the temperature of glass transition (T_g) up to
that of melting. Precise measurements particularly in the frequencies of
MHz-order have revealed that the temperature dependences of secondary
beta-relaxation times deviate from the Arrhenius relation in well above T_g.
Consequently, our results indicate that the beta-process merges into the
primary alpha-mode around the melting temperature, and not at the dynamical
transition point T which is approximately equal to 1.2 T_g.Comment: 4 pages, 4 figures, revtex
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
Minimal model for beta relaxation in viscous liquids
Contrasts between beta relaxation in equilibrium viscous liquids and glasses
are rationalized in terms of a double-well potential model with
structure-dependent asymmetry, assuming structure is described by a single
order parameter. The model is tested for tripropylene glycol where it accounts
for the hysteresis of the dielectric beta loss peak frequency and magnitude
during cooling and reheating through the glass transition.Comment: Phys. Rev. Lett. (in press
Electrode Polarization Effects in Broadband Dielectric Spectroscopy
In the present work, we provide broadband dielectric spectra showing strong
electrode polarization effects for various materials, belonging to very
different material classes. This includes both ionic and electronic conductors
as, e.g., salt solutions, ionic liquids, human blood, and
colossal-dielectric-constant materials. These data are intended to provide a
broad data base enabling a critical test of the validity of phenomenological
and microscopic models for electrode polarization. In the present work, the
results are analyzed using a simple phenomenological equivalent-circuit
description, involving a distributed parallel RC circuit element for the
modeling of the weakly conducting regions close to the electrodes. Excellent
fits of the experimental data are achieved in this way, demonstrating the
universal applicability of this approach. In the investigated ionically
conducting materials, we find the universal appearance of a second dispersion
region due to electrode polarization, which is only revealed if measuring down
to sufficiently low frequencies. This indicates the presence of a second
charge-transport process in ionic conductors with blocking electrodes.Comment: 9 pages, 6 figures, experimental data are provided in electronic form
(see "Data Conservancy"
Broadband Dielectric Spectroscopy on Glass-Forming Propylene Carbonate
Dielectric spectroscopy covering more than 18 decades of frequency has been
performed on propylene carbonate in its liquid and supercooled-liquid state.
Using quasi-optic submillimeter and far-infrared spectroscopy the dielectric
response was investigated up to frequencies well into the microscopic regime.
We discuss the alpha-process whose characteristic timescale is observed over 14
decades of frequency and the excess wing showing up at frequencies some three
decades above the peak frequency. Special attention is given to the
high-frequency response of the dielectric loss in the crossover regime between
alpha-peak and boson-peak. Similar to our previous results in other glass
forming materials we find evidence for additional processes in the crossover
regime. However, significant differences concerning the spectral form at high
frequencies are found. We compare our results to the susceptibilities obtained
from light scattering and to the predictions of various models of the glass
transition.Comment: 13 pages, 9 figures, submitted to Phys. Rev.
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
- âŠ