628 research outputs found
Johari-Goldstein relaxation far below Tg: Experimental evidence for the Gardner transition in structural glasses?
Experimental evidence for the Gardner transition, theoretically predicted to
arise deep in the glassy state of matter, is scarce. At this transition, the
energy landscape sensed by the particles forming the glass is expected to
become more complex. In the present work, we report the dielectric response of
two typical glass formers with well-pronounced Johari-Goldstein beta relaxation
following this response down to unprecedented low temperatures, far below the
glass transition. As the Johari-Goldstein process is believed to arise from the
local structure of the energy landscape, its investigation seems an ideal tool
to seek evidence for the Gardner transition. Indeed, we find an unusual
broadening of the beta relaxation below TG ~ 110 K for sorbitol and TG ~ 100 K
for xylitol, in excess of the expected broadening arising from a distribution
of energy barriers. Thus, these results provide hints at the presence of the
Gardner transition in canonical structural glass formers.Comment: 6 pages, 3 figures + 2 pages, 3 figures in Supplemental Materia
- and - Relaxation Dynamics of a fragile plastic crystal
We present a thorough dielectric investigation of the relaxation dynamics of
plastic crystalline Freon112, which exhibits freezing of the orientational
degrees of freedom into a glassy crystal below 90 K. Among other plastic
crystals, Freon112 stands out by being relatively fragile within Angell's
classification scheme and by showing an unusually strong -relaxation.
Comparing the results to those on Freon112a, having only a single molecular
conformation, points to the importance of the presence of two molecular
conformations in Freon112 for the explanation of its unusual properties.Comment: 17 pages, 6 figure
Cooperativity and Heterogeneity in Plastic Crystals Studied by Nonlinear Dielectric Spectroscopy
The glassy dynamics of plastic-crystalline cyclo-octanol and ortho-carborane,
where only the molecular reorientational degrees of freedom freeze without
long-range order, is investigated by nonlinear dielectric spectroscopy. Marked
differences to canonical glass formers show up: While molecular cooperativity
governs the glassy freezing, it leads to a much weaker slowing down of
molecular dynamics than in supercooled liquids. Moreover, the observed
nonlinear effects cannot be explained with the same heterogeneity scenario
recently applied to canonical glass formers. This supports ideas that molecular
relaxation in plastic crystals may be intrinsically non-exponential. Finally,
no nonlinear effects were detected for the secondary processes in
cyclo-octanol.Comment: Final version as accepted for publication in Phys. Rev. Lett. 6
pages, 5 figures (including 1 page and figure in Supplemental Material
Apparent giant dielectric constants, dielectric relaxation, and ac-conductivity of hexagonal perovskites La1.2Sr2.7BO7.33 (B = Ru, Ir)
We present a thorough dielectric investigation of the hexagonal perovskites
La1.2Sr2.7IrO7.33 and La1.2Sr2.7RuO7.33 in a broad frequency and temperature
range, supplemented by additional infrared measurements. The occurrence of
giant dielectric constants up to 10^5 is revealed to be due to electrode
polarization. Aside of dc and ac conductivity contributions, we detect two
intrinsic relaxation processes that can be ascribed to ionic hopping between
different off-center positions. In both materials we find evidence for charge
transport via hopping of localized charge carriers. In the infrared region,
three phonon bands are detected, followed by several electronic excitations. In
addition, these materials provide further examples for the occurrence of a
superlinear power law in the broadband ac conductivity, which recently was
proposed to be a universal feature of all disordered matter.Comment: 8 pages, 7 figure
Scaling of broadband dielectric data of glass-forming liquids and plastic crystals
The Nagel-scaling and the modified scaling procedure proposed recently by
Dendzik et al. have been applied to broadband dielectric data on two glass-
forming liquids (glycerol and propylene carbonate) and three plastic crystals
(ortho-carborane, meta-carborane, and 1-cyano-adamantane). Our data extend the
upper limit of the abscissa range to considerably higher values than in
previously published analyses. At the highest frequencies investigated,
deviations from a single master curve show up which are most pronounced in the
Dendzik-scaling plot. The loss curves of the plastic crystals do not scale in
the Nagel-plot, but they fall onto a separate master curve in the Dendzik-plot.
In addition, we address the question of a possible divergence of the static
susceptibility near the Vogel-Fulcher temperature. For this purpose, the
low-temperature evolution of the high-frequency wing of the dielectric loss
peaks is investigated in detail. No convincing proof for such a divergence can
be deduced from the present broadband data.Comment: 7 pages including 6 figures submitted to Eur. Phys. J.
Nonlinear dielectric response of Debye, alpha, and beta relaxation in 1-propanol
We present nonlinear dielectric measurements of glass-forming 1-propanol, a
prototypical example for the monohydroxy alcohols that are known to exhibit
unusual relaxation dynamics, namely an additional Debye relaxation, slower than
the structural alpha relaxation. Applying high ac fields of 468 kV/cm allows
for a detailed investigation of the nonlinear properties of all three
relaxation processes occurring in 1-propanol, namely the Debye, alpha, and beta
relaxation. Both the field-induced variations of dielectric constant and loss
are reported. Polarization saturation and the absorption of field energy govern
the findings in the Debye-relaxation regime, well consistent with the suggested
cluster-like nature of the relaxing entities. The behavior of the alpha
relaxation is in good accord with the expectations for a heterogeneous
relaxation scenario. Finally, the Johari-Goldstein beta-relaxation in
1-propanol seems to exhibit no or only weak field dependence, in agreement with
recent findings for the excess wing of canonical glass formers.Comment: 8 pages, 4 figure
Low temperature dielectric relaxation study of aqueous solutions of diethylsulfoxide
In the present work, dielectric spectra of mixtures of diethylsulfoxide
(DESO) and water are presented, covering a concentration range of 0.2 - 0.3
molar fraction of DESO. The measurements were performed at frequencies between
1 Hz and 10 MHz and for temperatures between 150 and 300 K. It is shown that
DESO/water mixtures have strong glass-forming abilities. The permittivity
spectra in these mixtures reveal a single relaxation process. It can be
described by the Havriliak-Negami relaxation function and its relaxation times
follow the Vogel-Fulcher-Tammann law, thus showing the typical signatures of
glassy dynamics. The concentration dependence of the relaxation parameters,
like fragility, broadening, and glass temperature, are discussed in detail.Comment: 20 pages, 5 figure
Ions in glass forming glycerol: Close correlation of alpha and fast beta relaxation
We provide broadband dielectric loss spectra of glass-forming glycerol with
varying additions of LiCl. The measurements covering frequencies up to 10 THz
extend well into the region of the fast beta process, commonly ascribed to
caged molecule dynamics. Aside of the known variation of the structural alpha
relaxation time and a modification of the excess wing with ion content, we also
find a clear influence on the shallow loss minimum arising from the fast beta
relaxation. Within the framework of mode-coupling theory, the detected
significant broadening of this minimum is in reasonable accord with the found
variation of the alpha-relaxation dynamics. A correlation between
alpha-relaxation rate and minimum position holds for all ion concentrations and
temperatures, even below the critical temperature defined by mode-coupling
theory.Comment: 5 pages, 5 figure
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