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