Aging is a log-Poisson process, not a renewal process

Real-space model for activated processes in rejuvenation and memory behavior of glassy systems

We offer an alternative real-space description, based purely on activated processes, for the understanding of relaxation dynamics in hierarchical landscapes. To this end, we use the cluster model, a coarse-grained lattice model of a jammed system, to analyze rejuvenation and memory effects during aging after a hard quench. In this model, neighboring particles on a lattice aggregate through local interactions into clusters that fragment with a probability based on their size. Despite the simplicity of the cluster model, it has been shown to reproduce salient observables of the aging dynamics in colloidal systems, such as those accounting for particle mobility and displacements. Here, we probe the model for more complex quench protocols and show that it exhibits rejuvenation and memory effects similar to those attributed to the complex hierarchical structure of a glassy energy landscape.

Crackling Noise during Slow Relaxations in Crumpled Sheets

The statistics of noise emitted by ultrathin crumpled sheets is measured while they exhibit logarithmic relaxations under load. We find that the logarithmic relaxation advanced via a series of discrete, audible, micromechanical events that are log-Poisson distributed (i.e., the process becomes a Poisson process when time stamps are replaced by their logarithms). The analysis places constraints on the possible mechanisms underlying the glasslike slow relaxation and memory retention in these systems.

Universal features of annealing and aging in compaction of granular piles

This paper links the nonequilibrium glassy relaxation behavior of otherwise athermal granular materials to those of thermally activated glasses. Thus, it demonstrates a much wider universality among complex glassy materials out of equilibrium. Our three-dimensional molecular dynamics simulations, fully incorporating friction and inelastic collisions, are designed to reproduce experimental behavior of tapped granular piles. A simple theory based on a dynamics of records explains why the typical phenomenology of annealing and aging after a quench should extend to such granular matter, activated by taps, beyond the more familiar realm of polymers, colloids, and magnetic materials that all exhibit thermal fluctuations.

Record-dynamics description of spin-glass thermoremanent magnetization: A numerical and analytical study

Thermoremanent magnetization data for the three-dimensional Edwards-Anderson (EA) spin glass are generated using the waiting time method as a simulational tool and interpreted using record dynamics. We verify that clusters of contiguous spins are overturned by quakes, nonequilibrium events linked to record-sized energy fluctuations, and we show that quaking is a log-Poisson process, i.e., a Poisson process whose average depends on the logarithm of the system age, counted from the initial quench. Our findings compare favorably with experimental thermoremanent magnetization findings and with the spontaneous fluctuation dynamics of the EA model. The logarithmic growth of the size of overturned clusters is related to similar experimental results and to the growing length scale of the spin-spin spatial correlation function. The analysis buttresses the applicability of the waiting time method as a simulational tool, and of record dynamics as a coarse-graining method for aging dynamics.

Barriers, trapping times, and overlaps between local minima in the dynamics of the disordered Ising p-spin model

We study the low-temperature out-of-equilibrium Monte Carlo dynamics of the disordered Ising p-spin Model with p=3 and a small number of spin variables. We focus on sequences of configurations that are stable against single spin flips obtained by instantaneous gradient descent from persistent ones. We analyze the statistics of energy gaps, energy barriers, and trapping times on subsequences such that the overlap between consecutive configurations does not overcome a threshold. We compare our results to the predictions of various trap models finding the best agreement with the step model when the p-spin configurations are constrained to be uncorrelated.

Maximum entropy approach to reliability

The aging process is a common phenomenon in engineering, biological, and physical systems. The hazard rate function, which characterizes the aging process, is a fundamental quantity in the disciplines of reliability, failure, and risk analysis. However, it is difficult to determine the entire hazard function accurately with limited observation data when the degradation mechanism is not fully understood. Inspired by the seminal work pioneered by Jaynes [Phys. Rev. 106, 620 (1956)PHRVAO0031-899X10.1103/PhysRev.106.620], this study develops an approach based on the principle of maximum entropy. In particular, the time-dependent hazard rate function can be established using limited observation data in a rational manner. It is shown that the developed approach is capable of constructing and interpreting many typical hazard rate curves observed in practice, such as the bathtub curve, the upside down bathtub, and so on. The developed approach is applied to model a classical single function system and a numerical example is used to demonstrate the method. In addition its extension to a more general multifunction system is presented. Depending on the interaction between different functions of the system, two cases, namely reducible and irreducible, are discussed in detail. A multifunction electrical system is used for demonstration.

Dynamics of dense hard sphere colloidal systems: A numerical analysis

The applicability to dense hard sphere colloidal suspensions of a general coarse-graining approach called Record Dynamics (RD) is tested by extensive molecular dynamics simulations. We reproduce known results as logarithmic diffusion and the logarithmic decay of the average potential energy per particle. We provide quantitative measures for the cage size and identify the displacements of single particles corresponding to intermittent cage breakings. We then partition the system into spatial domains and show that, within each domain, a subset of such intermittent events called quakes constitutes a log-Poisson process, as predicted by RD. Specifically, quakes are shown to be statistically independent and Poisson distributed with an average depending on the logarithm of time. Finally, we discuss the nature of the dynamical barriers surmounted by quakes and link RD to the phenomenology of aging hard sphere colloids.

Description of physical aging kinetics of glassy polymers by interpretation of parameters of the Kohlrausch-Williams-Watts relaxation function via simulation

The empirical Kohlrausch-Williams-Watts (KWW) relaxation function has been widely used to describe the nonexponential relaxation behavior of glassy polymers during their physical aging. The characteristic relaxation time (τ_{KWW}) and the stretching parameter (β_{KWW}) of the KWW equation are obtained from experimental relaxation data by a curve-fitting procedure. However, knowing the values of KWW parameters is not sufficient to completely characterize the physical aging kinetics of glassy materials in order to compare, for example, the aging rate of different systems. To be able to overcome such problems, at least, an exact interpretation of these parameters is absolutely vital and then it would be quite desirable to convert them into a single relaxation-time constant. This approach, indeed, provides the possibility to directly evaluate and compare the kinetics and aging rate of different systems as well as different methods used for monitoring the physical aging of the same materials. To do these, a method based on the notion of the relaxation spectrum is employed which is modeled via the well-known statistical distribution of log-normal. By detailed numerical simulations, some explicit expressions between KWW parameters with descriptor parameters of the distribution are established. From this point of view, it is tried to interpret τ_{KWW} and β_{KWW} and convert them into a single relaxation-time constant. Finally, the results of suggested simulations are compared with those obtained in the literature.

Two-time correlations for probing the aging dynamics of glassy colloids

We present results for the aging dynamics of a dense 2D colloidal system obtained with molecular dynamics simulations. To this end, systems are quenched to densities far above the glass transition with relaxation time scales that used to be prohibitive for such a comprehensive study. We performed extensive simulations to gather detailed statistics about rare rearrangement events. With a simple criterion for identifying irreversible events based on Voronoi tessellations, we find that the rate of those events decelerates hyperbolically. We track the probability density function for particle displacements, the van-Hove function, with sufficient statistics as to reveal its two-time dependence that is indicative of aging. Those displacements, measured from a waiting time tw after the quench up to times t, exhibit a data collapse as a function of t/tw. These findings can be explained comprehensively as manifestations of record dynamics, i.e., a relaxation dynamic driven by record-breaking fluctuations. We show that an on-lattice model of a colloid that was built on record dynamics indeed reproduces the experimental results in great detail.