3,197 research outputs found
Contributions to the mixed-alkali effect in molecular dynamics simulations of alkali silicate glasses
The mixed-alkali effect on the cation dynamics in silicate glasses is
analyzed via molecular dynamics simulations. Observations suggest a description
of the dynamics in terms of stable sites mostly specific to one ionic species.
As main contributions to the mixed--alkali slowdown longer residence times and
an increased probability of correlated backjumps are identified. The slowdown
is related to the limited accessibility of foreign sites. The mismatch
experienced in a foreign site is stronger and more retarding for the larger
ions, the smaller ions can be temporarily accommodated. Also correlations
between unlike as well as like cations are demonstrated that support
cooperative behavior.Comment: 10 pages, 12 figures, 1 table, revtex4, submitted to Phys. Rev.
Non Markovian persistence in the diluted Ising model at criticality
We investigate global persistence properties for the non-equilibrium critical
dynamics of the randomly diluted Ising model. The disorder averaged persistence
probability of the global magnetization is found to decay
algebraically with an exponent that we compute analytically in a
dimensional expansion in . Corrections to Markov process are
found to occur already at one loop order and is thus a novel
exponent characterizing this disordered critical point. Our result is
thoroughly compared with Monte Carlo simulations in , which also include a
measurement of the initial slip exponent. Taking carefully into account
corrections to scaling, is found to be a universal exponent,
independent of the dilution factor along the critical line at , and
in good agreement with our one loop calculation.Comment: 7 pages, 4 figure
Backward correlations and dynamic heterogeneities: a computer study of ion dynamics
We analyse the correlated back and forth dynamics and dynamic
heterogeneities, i.e. the presence of fast and slow ions, for a lithium
metasilicate system via computer simulations. For this purpose we define, in
analogy to previous work in the field of glass transition, appropriate
three-time correlation functions. They contain information about the dynamics
during two successive time intervals. First we apply them to simple model
systems in order to clarify their information content. Afterwards we use this
formalism to analyse the lithium trajectories. A strong back-dragging effect is
observed, which also fulfills the time-temperature superposition principle.
Furthermore, it turns out that the back-dragging effect is long-ranged and
exceeds the nearest neighbor position. In contrast, the strength of the dynamic
heterogeneities does not fulfill the time-temperature superposition principle.
The lower the temperature, the stronger the mobility difference between fast
and slow ions. The results are then compared with the simple model systems
considered here as well as with some lattice models of ion dynamics.Comment: 12 pages, 10 figure
Energy landscape, two-level systems and entropy barriers in Lennard-Jones clusters
We develop an efficient numerical algorithm for the identification of a large
number of saddle points of the potential energy function of Lennard- Jones
clusters. Knowledge of the saddle points allows us to find many thousand
adjacent minima of clusters containing up to 80 argon atoms and to locate many
pairs of minima with the right characteristics to form two-level systems (TLS).
The true TLS are singled out by calculating the ground-state tunneling
splitting. The entropic contribution to all barriers is evaluated and
discussed.Comment: 4 pages, RevTex, 2 PostScript figure
Origin of non-exponential relaxation in a crystalline ionic conductor: a multi-dimensional 109Ag NMR study
The origin of the non-exponential relaxation of silver ions in the
crystalline ion conductor Ag7P3S11 is analyzed by comparing appropriate
two-time and three-time 109Ag NMR correlation functions. The non-exponentiality
is due to a rate distribution, i.e., dynamic heterogeneities, rather than to an
intrinsic non-exponentiality. Thus, the data give no evidence for the relevance
of correlated back-and-forth jumps on the timescale of the silver relaxation.Comment: 4 pages, 3 figure
Finite-Size Effects in a Supercooled Liquid
We study the influence of the system size on various static and dynamic
properties of a supercooled binary Lennard-Jones liquid via computer
simulations. In this way, we demonstrate that the treatment of systems as small
as N=65 particles yields relevant results for the understanding of bulk
properties. Especially, we find that a system of N=130 particles behaves
basically as two non-interacting systems of half the size.Comment: Proceedings of the III Workshop on Non Equilibrium Phenomena in
Supercooled Fluids, Glasses and Amorphous Materials, Sep 2002, Pis
Complex lithium ion dynamics in simulated LiPO3 glass studied by means of multi-time correlation functions
Molecular dynamics simulations are performed to study the lithium jumps in
LiPO3 glass. In particular, we calculate higher-order correlation functions
that probe the positions of single lithium ions at several times. Three-time
correlation functions show that the non-exponential relaxation of the lithium
ions results from both correlated back-and-forth jumps and the existence of
dynamical heterogeneities, i.e., the presence of a broad distribution of jump
rates. A quantitative analysis yields that the contribution of the dynamical
heterogeneities to the non-exponential depopulation of the lithium sites
increases upon cooling. Further, correlated back-and-forth jumps between
neighboring sites are observed for the fast ions of the distribution, but not
for the slow ions and, hence, the back-jump probability depends on the
dynamical state. Four-time correlation functions indicate that an exchange
between fast and slow ions takes place on the timescale of the jumps
themselves, i.e., the dynamical heterogeneities are short-lived. Hence, sites
featuring fast and slow lithium dynamics, respectively, are intimately mixed.
In addition, a backward correlation beyond the first neighbor shell for highly
mobile ions and the presence of long-range dynamical heterogeneities suggest
that fast ion migration occurs along preferential pathways in the glassy
matrix. In the melt, we find no evidence for correlated back-and-forth motions
and dynamical heterogeneities on the length scale of the next-neighbor
distance.Comment: 12 pages, 13 figure
What does the potential energy landscape tell us about the dynamics of supercooled liquids and glasses?
For a model glass-former we demonstrate via computer simulations how
macroscopic dynamic quantities can be inferred from a PEL analysis. The
essential step is to consider whole superstructures of many PEL minima, called
metabasins, rather than single minima. We show that two types of metabasins
exist: some allowing for quasi-free motion on the PEL (liquid-like), the others
acting as traps (solid-like). The activated, multi-step escapes from the latter
metabasins are found to dictate the slowing down of dynamics upon cooling over
a much broader temperature range than is currently assumed
An anomalous Wtb coupling at a linear collider
Differential cross sections of secondary particles in a process of top quark
pair production and decay into six fermions at a linear collider with an
unpolarized and a longitudinally polarized electron beam are computed to the
lowest order in the standard model and in the presence of an anomalous Wtb
coupling. It is illustrated that the latter has a little impact on the
differential cross sections. In particular, it is shown that the angular
distribution of a secondary lepton receives practically no contribution from
the anomalous Wtb coupling, even if the top quark is produced off shell and the
non-double resonance background contributions are taken into account. This
finding is in accordance with the decoupling theorem that has been proven in
literature in the narrow top quark width approximation.Comment: 11 pages, 9 figures, axodraw.st
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