Kovacs-like memory effect in glassy systems and uniformly heated granular gases

by Antonio Prados (Universidad de Sevilla (Spain))

Tuesday 14 Oct 2014, 14:00 → 15:00 Europe/Rome

Aula 5 (Dipartimento di Fisica - Ed- E. Fermi)

The crossover or Kovacs effect is, roughly speaking, the non-monotonic relaxation of a physical quantity to equilibrium, from an initial non-equilibrium state in which the considered property had its equilibrium value. We investigate it in two cases (i) glassy systems, (ii) granular gases. In the former and with the usual cooling protocol, the time evolution of the energy always passes through a maximum (normal behavior). Within a very simple model, we discuss the relevance of recent linear response results for the understanding of this memory effect. The granular gas case is quite different, since we deal with an intrinsically out-of-equilibrium system due to the continuous loss of energy in collisions. By introducing a simple energy input mechanism (the so-called stochastic thermostat), the granular gas reaches an out-of-equilibrium steady state. When looking for memory effects, we have to substitute relaxation to equilibrium by relaxation to its non-equilibrium steady state. A simple à la Kovacs protocol for the driving is investigated, for which a crossover effect is displayed by the granular temperature (kinetic energy). Interestingly, it becomes anomalous for large enough inelasticity: the granular temperature shows a minimum instead of a maximum. The underlying physical mechanism for both the normal and anomalous behavior is also discussed, together with some perspectives for possible future research.