378 research outputs found
Dynamic facilitation explains democratic particle motion of metabasin transitions
Transitions between metabasins in supercooled liquids seem to occur through
rapid "democratic" collective particle rearrangements. Here we show that this
apparent homogeneous particle motion is a direct consequence of dynamic
facilitation. We do so by studying metabasin transitions in facilitated spin
models and constrained lattice gases. We find that metabasin transitions occur
through a sequence of locally facilitated events taking place over a relatively
short time frame. When observed on small enough spatial windows these events
appear sudden and homogeneous. Our results indicate that metabasin transitions
are essentially "non-democratic" in origin and yet another manifestation of
dynamical heterogeneity in glass formers.Comment: 6 pages, 6 figure
Phonons in Random Elastic Media and the Boson Peak
We show that the density of states of random wave equations, normalized by
the square of the frequency, has a peak - sometimes narrow and sometimes broad
- in the range of wave vectors between the disorder correlation length and the
interatomic spacing. The results of this letter may be relevant for
understanding vibrational spectra and light propagation in disordered solids
Boltzmann-type approach to transport in weakly interacting one-dimensional fermionic systems
We investigate transport properties of one-dimensional fermionic tight
binding models featuring nearest and next-nearest neighbor hopping, where the
fermions are additionally subject to a weak short range mutual interaction. To
this end we employ a pertinent approach which allows for a mapping of the
underlying Schr\"odinger dynamics onto an adequate linear quantum Boltzmann
equation. This approach is based on a suitable projection operator method. From
this Boltzmann equation we are able to numerically obtain diffusion
coefficients in the case of non-vanishing next-nearest neighbor hopping, i.e.,
the non-integrable case, whereas the diffusion coefficient diverges without
next-nearest neighbor hopping. For the latter case we analytically investigate
the decay behavior of the current with the result that arbitrarily small parts
of the current relax arbitrarily slowly which suggests anomalous diffusive
transport behavior within the scope of our approach.Comment: 6 pages, 4 figures, accepted for publication in Phys. Rev.
Bestimmung der Bioabbaubarkeit von nicht wasserlöslichen Flüssigkeiten (Schmieröle etc.) nach CEC L-33-A-94
The use of biodegradable lubricants as substitutes for lubricants based on mineral oils is necessary in areas where an impact on ecosystems is inevitable and cannot be excluded. These are applications in which a partial loss of lubricant into the environment occurs, e.g. chain
saw oils, two-stroke oils for outboard motors, and hydraulic oils, which are used in sensitive areas as dredging-machines near rivers or ground water sources. The use of commercially available biodegradable base liquids for lubricants is discussed and several current examples are given. These
water-insoluble products are frequently examined with the CEC test that can be used for a rough determination of the biodegradability. The development and application of this method has a long tradition at the EMPA DĂĽbendorf. Experimental details, limits, advantages, and disadvantages
will be discussed in this paper. Longterm measurements with the calibrating oils RL 130 and RL 110 together with results from a biodegradation study of several hydraulic fluids that are used in agricultural machinery are presented
Dynamical diversity and metastability in a hindered granular column near jamming
Granular media jam into a panoply of metastable states. The way in which
these states are achieved depends on the nature of local and global constraints
on grains; here we investigate this issue by means of a non-equilibrium
stochastic model of a hindered granular column near its jamming limit. Grains
feel the constraints of grains above and below them differently, depending on
their position. A rich phase diagram with four dynamical phases (ballistic,
activated, logarithmic and glassy) is revealed. The statistics of the jamming
time and of the metastable states reached as attractors of the zero-temperature
dynamics is investigated in each of these phases. Of particular interest is the
glassy phase, where intermittency and a strong deviation from Edwards' flatness
are manifest.Comment: 23 pages, 12 figure
Dynamic first-order phase transition in kinetically constrained models of glasses
We show that the dynamics of kinetically constrained models of glass formers
takes place at a first-order coexistence line between active and inactive
dynamical phases. We prove this by computing the large-deviation functions of
suitable space-time observables, such as the number of configuration changes in
a trajectory. We present analytic results for dynamic facilitated models in a
mean-field approximation, and numerical results for the Fredrickson-Andersen
model, the East model, and constrained lattice gases, in various dimensions.
This dynamical first-order transition is generic in kinetically constrained
models, and we expect it to be present in systems with fully jammed states.Comment: 4.1 pages, 3 figure
Thermodynamic picture of the glassy state
A picture for thermodynamics of the glassy state is introduced. It assumes
that one extra parameter, the effective temperature, is needed to describe the
glassy state. This explains the classical paradoxes concerning the Ehrenfest
relations and the Prigogine-Defay ratio. As a second part, the approach
connects the response of macroscopic observables to a field change with their
temporal fluctuations, and with the fluctuation-dissipation relation, in a
generalized non-equilibrium way.Comment: Proceedings of the Conference "Unifying Concepts in Glass Physics",
ICTP, Trieste, 15 - 18 September 199
Thermodynamic description of a dynamical glassy transition
For the dynamical glassy transition in the -spin mean field spin glass
model a thermodynamic description is given. The often considered marginal
states are not the relevant ones for this purpose. This leads to consider a
cooling experiment on exponential timescales, where lower states are accessed.
The very slow configurational modes are at quasi-equilibrium at an effective
temperature. A system independent law is derived that expresses their
contribution to the specific heat. -scaling in the aging regime of
two-time quantities is explained.Comment: 5 pages revte
Effect of physical aging on the low-frequency vibrational density of states of a glassy polymer
The effects of the physical aging on the vibrational density of states (VDOS)
of a polymeric glass is studied. The VDOS of a poly(methyl methacrylate) glass
at low-energy (<15 meV), was determined from inelastic neutron scattering at
low-temperature for two different physical thermodynamical states. One sample
was annealed during a long time at temperature lower than Tg, and another was
quenched from a temperature higher than Tg. It was found that the VDOS around
the boson peak, relatively to the one at higher energy, decreases with the
annealing at lower temperature than Tg, i.e., with the physical aging.Comment: To be published in Europhys. Let
Light scattering study of low-energy vibrational excitations in the metallic glass NiZr using electronic Raman scattering
The Raman response of the metallic glass NiZr is measured as a
function of polarization and temperature and analyzed theoretically.
Unexpectedly, the intensity in the range up to 300\wn increases upon cooling,
which is counterintuitive when the response originates from vibrations alone as
in insulators. The increase finds a natural explanation if the conduction
electrons are assumed to scatter on localized vibrations with a scattering
probability proportional to the Debye-Waller factor. None of our assumptions is
material specific, and the results are expected to be relevant for disordered
systems in general.Comment: 5 pages, 3 figure
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