964 research outputs found
The influence of dust properties on the mass loss in pulsating AGB stars
We are currently studying carbon based dust types of relevance for
carbon-rich AGB stars, to obtain a better understanding of the influence of the
optical and chemical properties of the grains on the mass loss of the star. An
investigation of the complex interplay between hydrodynamics,radiative transfer
and chemistry has to be based on a better knowledge of the micro-physics of the
relevant dust species.Comment: 4 pages, 2 figures. Proceedings for IAU Colloquium 185 "Radial and
Nonradial Pulsations as Probes of Stellar Physics
Dust grain properties in atmospheres of AGB stars
We present self-consistent dynamical models for dust driven winds of
carbon-rich AGB stars. The models are based on the coupled system of
frequency-dependent radiation hydrodynamics and time-dependent dust formation.
We investigate in detail how the wind properties of the models are influenced
by the micro-physical properties of the dust grains that enter as parameters.
The models are now at a level where it is necessary to be quantitatively
consistent when choosing the dust properties that enters as input into the
models. At our current level of sophistication the choice of dust parameters is
significant for the derived outflow velocity, the degree of condensation and
the estimated mass loss rates of the models. In the transition between models
with and without mass-loss the choice ofmicro-physical parameters turns out to
be very significant for whether a particular set of stellar parameters will
give rise to a dust-driven mass loss or not.Comment: 10 pages, 3 figures. To appear in: Modelling of Stellar Atmospheres,
N.E. Piskunov, W.W. Weiss, D.F. Gray (eds.), IAU Symposium Vol. xxx.
Proceedings for the IAU Symposium 210, Uppsala, June 200
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.
The Excess Wing in the Dielectric Loss of Glass-Formers: A Johari-Goldstein beta-Relaxation?
Dielectric loss spectra of glass-forming propylene carbonate and glycerol at
temperatures above and below T_g are presented. By performing aging experiments
lasting up to five weeks, equilibrium spectra below T_g have been obtained.
During aging, the excess wing, showing up as a second power law at high
frequencies, develops into a shoulder. The results strongly suggest that the
excess wing, observed in a variety of glass formers, is the high-frequency
flank of a beta-relaxation.Comment: submitted to Phys. Rev. Lett., 4 figures, revised version after
referee report
Relaxation dynamics of a protein solution investigated by dielectric spectroscopy
In the present work, we provide a dielectric study on two differently
concentrated aqueous lysozyme solutions in the frequency range from 1 MHz to 40
GHz and for temperatures from 275 to 330 K. We analyze the three dispersion
regions, commonly found in protein solutions, usually termed beta-, gamma-, and
delta-relaxation. The beta-relaxation, occurring in the frequency range around
10 MHz and the gamma-relaxation around 20 GHz (at room temperature) can be
attributed to the rotation of the polar protein molecules in their aqueous
medium and the reorientational motion of the free water molecules,
respectively. The nature of the delta-relaxation, which often is ascribed to
the motion of bound water molecules, is not yet fully understood. Here we
provide data on the temperature dependence of the relaxation times and
relaxation strengths of all three detected processes and on the dc conductivity
arising from ionic charge transport. The temperature dependences of the beta-
and gamma-relaxations are closely correlated. We found a significant
temperature dependence of the dipole moment of the protein, indicating
conformational changes. Moreover we find a breakdown of the
Debye-Stokes-Einstein relation in this protein solution, i.e., the dc
conductivity is not completely governed by the mobility of the solvent
molecules. Instead it seems that the dc conductivity is closely connected to
the hydration shell dynamics.Comment: 11 pages, 7 figure
The excess wing in the dielectric loss of glass-forming ethanol: A relaxation process
A detailed dielectric investigation of liquid, supercooled liquid, and glassy
ethanol reveals a third relaxation process, in addition to the two processes
already known. The relaxation time of the newly detected process exhibits
strong deviations from thermally activated behavior. Most important, this
process is the cause of the apparent excess wing, which was claimed to be
present in the dielectric loss spectra of glass-forming ethanol. In addition,
marked deviations of the spectra of ethanol from the scaling proposed by Dixon
and Nagel have been detected.Comment: 8 pages including 4 figures submitted to Phys. Rev.
Non-intrinsic origin of the Colossal Dielectric Constants in CaCu3Ti4O12
The dielectric properties of CaCu3Ti4O12, a material showing colossal values
of the dielectric constant, were investigated in a broad temperature and
frequency range extending up to 1.3 GHz. A detailed equivalent circuit analysis
of the results and two crucial experiments, employing different types of
contacts and varying sample thickness, provide clear evidence that the
apparently high values of the dielectric constant in CaCu3Ti4O12 are
non-intrinsic and due to electrode polarization effects. The intrinsic
properties of CaCu3Ti4O12 are characterized by charge transport via hopping of
localized charge carriers and a relatively high dielectric constant of the
order of 100.Comment: 4 pages, 4 figure
Multiferroicity and colossal magneto-capacitance in Cr-thiospinels
The sulfur based Cr-spinels RCr2S4 with R = Cd and Hg exhibit the coexistence
of ferromagnetic and ferroelectric properties together with a pronounced
magnetocapacitive coupling. While in CdCr2S4 purely ferromagnetic order is
established, in HgCr2S4 a bond-frustrated magnetic ground state is realized,
which, however, easily can be driven towards a ferromagnetic configuration in
weak magnetic fields. This paper shall review our recent investigation for both
compounds. Besides the characterization of the magnetic properties, the complex
dielectric permittivity was studied by means of broadband dielectric
spectroscopy as well as measurements of polarization hysteresis and
pyro-currents. The observed colossal magneto-capacitive effect at the magnetic
transition seems to be driven by an enormous variation of the relaxation
dynamics.Comment: 10 pages, 11 figure
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