352 research outputs found
Comment on "First order amorphous-amorphous transformation in silica"
In a recent letter (Phys. Rev. Lett. 84, 4629 (2000)), Lacks presents
evidence of a first order amorphous-amorphous transition in silica at T=0. He
calculates the free energy along a path of compression and successive
decompression of a sample of 108 SiO2 units. The free energy of the two
branches cross each other, and this is interpreted as evidence of a first order
transition. We show that this conclusion does not follow from the shown data,
since qualitatively the same phenomenology is obtained in a model where a first
order transition does not exist.Comment: 1 page, 1 figure, to appear in Phys. Rev. Let
Minimum energy configurations of repelling particles in two dimensions
Geometrical arrangements of minimum energy of a system of identical repelling
particles in two dimensions are studied for different forms of the interaction
potential. Stability conditions for the triangular structure are derived, and
some potentials not satisfying them are discussed. It is shown that in addition
to the triangular lattice, other structures may appear (some of them with
non-trivial unit cells, and non-equivalent positions of the particles) even for
simple choices of the interaction. The same qualitative behavior is expected in
three dimensions.Comment: 6 pages, 6 figures include
Delayed dynamic triggering of earthquakes: Evidences from a statistical model of seismicity
I study a recently proposed statistical model of earthquake dynamics that
incorporates aging as a fundamental ingredient. The model is known to generate
earthquake sequences that quantitatively reproduce the spatial and temporal
clustering of events observed in actual seismic patterns. The aim of the
present work is to investigate if this model can give support to the empirical
evidence that earthquakes can be triggered by transient small perturbations,
particularly by the passing of seismic waves originated in events occurring in
far geographical locations. The effect of seismic waves is incorporated into
the model by assuming that they produce instantaneous small modifications in
the dynamical state of the system at the time they are applied. This change in
the dynamical state has two main effects. On one side, it induces earthquakes
that occur right at the application of the perturbation. These are called
immediate events. On the other side, after the application of the perturbation
there is a delayed effect: the seismic activity increases abruptly after the
perturbation, then falls down below the level of background activity, and
eventually recovers to the background value. The time scale of these variations
depends on the internal dynamics of the system, and is totally independent of
the duration of the perturbation. The number of delayed events in excess of the
background activity is typically observed to be around a factor of twenty
larger than the number of immediate events. The origin of the enhanced activity
period following the perturbation is associated to the existence of aging
relaxation, and it does not occur if relaxation is absent. These findings give
support to the experimental evidence that earthquake can be remotely triggered
by small transient perturbations as those produced by seismic waves.Comment: 6 pages, 3 figures, comments welcom
Fragile-strong transitions and polyamorphism in glass former fluids
A simple model of a glass former fluid, consisting of a bidisperse mixture of
penetrable spheres is studied. The model shows a transition from fragile to
strong behavior as temperature is reduced. This transition is driven by the
competition between the two mechanisms that contribute to diffusivity in the
model: collective rearrangement of particles (responsible for the fragile
behavior), and individual particle motion (which gives rise to the strong
behavior at low temperature). We also observe a maximum of diffusivity as a
function of pressure that can be interpreted within the same framework. The
connection between this behavior and polyamorphism is addressed.Comment: 5 pages, 6 figure
Aftershock production rate of driven viscoelastic interfaces
We study analytically and by numerical simulations the statistics of the
aftershocks generated after large avalanches in models of interface depinning
that include viscoelastic relaxation effects. We find in all the analyzed cases
that the decay law of aftershocks with time can be understood by considering
the typical roughness of the interface and its evolution due to relaxation. In
models where there is a single viscoelastic relaxation time there is an
exponential decay of the number of aftershocks with time. In models in which
viscoelastic relaxation is wave-vector dependent we typically find a power law
dependence of the decay rate, compatible with the Omori law. The factors that
determine the value of the decay exponent are analyzed
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