21/06/2018 Colóquio Explaining slow relaxation in jammed materials and aging in terms of the dynamics of record fluctuations - Stefan Boettcher (Physics Department, Emory University)
Explaining slow relaxation in jammed materials and aging in terms of the dynamics of record fluctuations
Stefan Boettcher (Physics Department, Emory University)
Leaving the familiar regularity of crystalline ordering behind, understanding amorphous, disordered or otherwise glassy materials poses entirely new challenges. Determining their most relaxed, optimal configuration often rivals the complexity of the hardest problems in computer sciences and combinatorial optimization. Dynamically, they readily fall out of equilibrium under quenching (to a state of low temperature, high density, etc), even in the absence of a corresponding equilibrium transition to an ordered phase. Their relaxation soon reaches astronomical time-scales, during which tantalizing memory effects arise due to breaking of time-translational invariance. The simplest of those effects is called "aging", the increasingly sluggish dynamics widely observed in the jammed state of glassy materials, where all future observations depend markedly on the age t_w of the system after the quench. In this talk, I will introduce "record dynamics" as a unified description of aging. We find that structural evolution in aging materials requires ever larger, record-sized rearrangements in an uncorrelated sequence of intermittent events (avalanches or quakes), manifestations of spatial *and* temporal heterogeneity. Based on record statistics, these (irreversible!) rearrangements occur at a rate ˜ 1/t, indicative of a log-Poisson process, where the number of events between age t_w and any later time t integrates to ˜ ln(t/t_w). Thus, any observable inherits the t/t_w dependence that is the hallmark of pure aging. Thereby, we can explain the relaxation dynamics observed in a broad range of materials, such as in simulations of low-temperature spin glasses and in experiments on high-density colloids and granular piles . We have proposed a phenomenological model of record dynamics that reproduces salient aspects of the experiments, for example, persistence, intermittency and dynamic heterogeneity, and in fact, the entire van-Hove function . Our analysis also rules out some other explanations of aging based on trap models and continuous-time walks .