151 research outputs found
Violation of the fluctuation-dissipation theorem in glassy systems: basic notions and the numerical evidence
No full textThis review reports on the research done during past years on violations of the fluctuation–dissipation theorem (FDT) in glassy systems. It is focused on the existence of a quasi-fluctuation–dissipation theorem (QFDT) in glassy systems and the current supporting knowledge gained from numerical simulation studies. It covers a broad range of non-stationary aging and stationary driven systems such as structural glasses, spin glasses, coarsening systems, ferromagnetic models at criticality, trap models, models with entropy barriers, kinetically constrained models, sheared systems and granular media. The review is divided into four main parts: (1) an introductory section explaining basic notions related to the existence of the FDT in equilibrium and its possible extension to the glassy regime (QFDT), (2) a description of the basic analytical tools and results derived in the framework of some exactly solvable models, (3) a detailed report of the current evidence in favour of the QFDT and (4) a brief digression on the experimental evidence in its favour. This review is intended for inexpert readers who want to learn about the basic notions and concepts related to the existence of the QFDT as well as for the more expert readers who may be interested in more specific results
Inherent structures, configurational entropy and slow glassy dynamics
No full textWe give a short introduction to the inherent structure approach to glassy systems, with particular emphasis on the Stillinger and Weber (SW) decomposition. We present some of the results obtained in the framework of spin-glass models and Lennard-Jones glasses. We discuss how to generalize the standard SW approach by including the entropy of inherent structures. Finally we discuss why this approach is probably insufficient to describe the behaviour of some kinetically constrained models
Equilibrium and ageing dynamics of simple models for glasses
No full textWe analyse the properties of the energy landscape of finite-size fully connected p-spin-like models. In the thermodynamic limit the high-temperature phase is described by the schematic mode-coupling theory of supercooled liquids. In this limit, the barriers between different basins are infinite below the critical dynamical temperature at which the ergodicity is broken in infinite time. We show that finite-size fully connected p-spin-like models, where activated processes are possible, exhibit properties typical of real supercooled liquid when both are near the critical glass transition. Our results support the conclusion that fully connected p-spin-like models are the natural statistical mechanical models for studying the glass transition in supercooled liquids
Are mean-field spin-glass models relevant for the structural glass transition?
No full textWe analyze the properties of the energy landscape of finite-size fully connected p-spin-like models whose high-temperature phase is described, in the thermodynamic limit, by the schematic Mode Coupling Theory of super-cooled liquids. We show that finite-size fully connected p-spin-like models, where activated processes are possible, do exhibit properties typical of real super-cooled liquid when both are near the critical glass transition. Our results support the conclusion that fully connected p-spin-like models are the natural statistical mechanical models for studying the glass transition in super-cooled liquids. (C) 2000 Elsevier Science B.V. All rights reserved
A glass transition scenario based on heterogeneities and entropy barriers
No full textWe propose a scenario for the glass transition based on the cooperative nature of nucleation processes and entropic effeets. The main point is the relation between the off-equilibrium energy dissipation and nucleation processes in off-equilibrium supercooled liquids which leads to a natural definition of the complexity. From the absence of coarsening growth we can derive an entropy-based fluctuation formula which relates the free-energy dissipation rate in the glass to the nucleation rate of the largest cooperative regions. As a by-product we obtain a new phenomenological relation between the longest relaxation time in the supercooled liquid phase and an effective temperature. This differs from the Adam-Gibbs relation in that it predicts no divergence of the primary relaxation time at the Kauzmann temperature and the existence of a crossover from fragile to strong behaviour
Frequency-domain study of α relaxation in the random orthogonal model
No full textThe time-dependent susceptibility for the finite-size mean-field random orthogonal model is studied numerically for temperatures above the mode-coupling temperature. The results show that the imaginary part χ „(ν ) of the susceptibility obeys the scaling form proposed for glass-forming liquids with the peak frequency decreasesing as the temperature is lowered consistently with the Vogel–Fulcher law with a critical temperature remarkably close to the known critical temperature T_c of the model where the configurational entropy vanishes
Absence of ageing in the remanent magnetization in Migdal-Kadanoff spin glasses
No full textWe study the non-equilibrium behaviour of three-dimensional spin glasses in the Migdal-Kadanoff approximation. This approximation is exact Tor disordered hierarchical lattices which have a unique ground state and equilibrium properties correctly described by the droplet model. Extensive numerical simulations show that this model lacks ageing in the remanent magnetization as well as a maximum in the magnetic viscosity in disagreement with experiments as well as with numerical studies of the Edwards-Anderson model. This result strongly limits the validity of the droplet model (at least in its simplest form) as a good model for real spin glasses
Intermittency of glassy relaxation and the emergence of a non-equilibrium spontaneous measure in the aging regime
No full textWe consider heat exchange processes between non-equilibrium aging systems (in their activated regime) and the thermal bath in contact. We discuss a scenario where two different heat exchange processes concur in the overall heat dissipation: a stimulated fast process determined by the temperature of the bath and a spontaneous intermittent process determined by the fact that the system has been prepared in a non-equilibrium state. The latter is described by a probability distribution function (PDF) that has an exponential tail of width given by a parameter A, and satisfies a fluctuation theorem (FT) governed by that parameter. The value of A is proportional to the so-called effective temperature, thereby providing a practical way to experimentally measure it by analyzing the PDF of intermittent events
Activated processes and inherent structure dynamics of finite-size mean-field models for glasses
No full textWe investigate the Inherent Structure (IS) dynamics of mean-field finite-size spin-glass models whose high-temperature dynamics is described in the thermodynamic limit by the schematic Mode Coupling Theory for supercooled liquids. Near the threshold energy the dynamics is ruled by activated processes which induce a logarithmic slow relaxation. We show the presence of aging in both the IS correlation and integrated response functions and check the validity of the one-step replica symmetry breaking scenario in the presence of activated processes. Our work shows: 1) the violation of the fluctuation-dissipation theorem can be computed from the configurational entropy obtained in the Stillinger and Weber approach, 2) the intermediate time regime (log(t) ∼ N) in mean-field theory automatically includes activated processes opening the way to analytically investigate activated processes by computing corrections beyond mean field
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