arXiv subjects

Leticia F. Cugliandolo

Publications and source records attributed to Leticia F. Cugliandolo.

At least 163 records · Page 9Linked to original sources

arXiv2001

Glassy systems under time-dependent driving forces: application to slow granular rheology

We study the dynamics of a glassy model with infinite range interactions externally driven by an oscillatory force. We find a well-defined transition in the (Temperature-Amplitude-Frequency) phase diagram between (i) a `glassy' state characterized by the slow relaxation of one-time quantities, aging in two-time quantities and a modification of the equilibrium fluctuation-dissipation relation; and (ii) a `liquid' state with a finite relaxation time. In the glassy phase, the degrees of freedom governing the slow relaxation are thermalized to an effective temperature. Using Monte-Carlo simulations, we investigate the effect of trapping regions in phase space on the driven dynamics. We find that it alternates between periods of rapid motion and periods of trapping. These results confirm the strong analogies between the slow granular rheology and the dynamics of glasses. They also provide a theoretical underpinning to earlier attempts to present a thermodynamic description of moderately driven granular materials.

cond-mat.stat-mech↗

arXiv2000

Quantum TAP equations

We derive Thouless-Anderson-Palmer (TAP) equations for quantum disordered systems. We apply them to the study of the paramagnetic and glassy phases in the quantum version of the spherical p spin-glass model. We generalize several useful quantities (complexity, threshold level, etc.) and various ideas (configurational entropy crisis, etc), that have been developed within the classical TAP approach, to quantum systems. The analysis of the quantum TAP equations allows us to show that the phase diagram (temperature-quantum parameter) of the p spin-glass model should be generic. In particular, we argue that a crossover from a second order thermodynamic transition close to the classical critical point to a first order thermodynamic transition close to the quantum critical point is to be expected in a large class of systems.

cond-mat.dis-nn↗

arXiv2000

The entrance dynamics of polymers into a gel

We use computer simulations to study the (driven) dynamics of a charged polymer in a disordered medium, thus mimicking the setting used in gel electrophoresis. In agreement with experiments, we find that inside the gel the mobility of the polymer is only a weak function of its length N. In contrast to this, the mean entrance time into the gel, is a very strong function of N, \propto N^1.3, and does not show any sign of saturation with increasing N. We rationalize this effect by means of a simple model and propose an experimental setup that should allow to increase the separation ability of gel electrophoresis significantly.

cond-mat.stat-mech↗

arXiv2000

From second to first order transitions in a disordered quantum magnet

We study the spin-glass transition in a disordered quantum model. There is a region in the phase diagram where quantum effects are small and the phase transition is second order, as in the classical case. In another region, quantum fluctuations drive the transition first order. Across the first order line the susceptibility is discontinuous and shows hysteresis. Our findings reproduce qualitatively observations on LiHo$_x$Y$_{1-x}$F$_4$. We also discuss a marginally stable spin-glass state and derive some results previously obtained from the real-time dynamics of the model coupled to a bath.

cond-mat.stat-mech↗

arXiv2000

Hole-burning experiments within solvable glassy models

We reproduce the results of non-resonant spectral hole-burning experiments with fully-connected (equivalently infinite-dimensional) glassy models that are generalizations of the mode-coupling approach to nonequilibrium situations. We show that an ac-field modifies the integrated linear response and the correlation function in a way that depends on the amplitude and frequency of the pumping field. We study the effect of the waiting and recovery-times and the number of oscillations applied. This calculation will help descriminating which results can and which cannot be attributed to dynamic heterogeneities in real systems.

cond-mat.stat-mech↗

arXiv1999

A scenario for the dynamics in the small intropy production limit

We present a scenario for the nonequilibrium dynamics in the limit of small entropy production. We discuss (i) the appearence of different time-scales, (ii) the modification of the fluctuation-dissipation theorem and its relation to effective temperatures and partial equilibrations and (iii) the validity of Onsager reciprocity relations. We distinguish these properties by their reaction to infinitesimal perturbations. We recall that one can easily change the time dependence of observables by applying an infinitesimal force while time-reparametrization invariant features remain unchanged under the same perturbations. With the aim of better understanding these properties, we consider the effect of several baths with different temperatures and time-scales on the dynamics. This is done in two ways: numerically, by using a especially developed Monte Carlo algorithm that mimics the coupling to multiple baths; analytically, by computing the time-dependent probability density of simple systems in contact with multiple baths. We finally argue that these features are related to supersymmetry, the reparametrization invariance of the slow dynamics and its spontaneous breaking. This scenario is consistent within any perturbative scheme. A brief version of this article appeared in Physica A263, 242 (1999).

cond-mat.stat-mech↗

arXiv1999

A Search for Fluctuation-Dissipation Theorem Violations in Spin-Glasses from Susceptibility Data

We propose an indirect way of studying the fluctuation-dissipation relation in spin-glasses that only uses available susceptibility data. It is based on a dynamic extension of the Parisi-Toulouse approximation and a Curie-Weiss treatment of the average magnetic couplings. We present the results of the analysis of several sets of experimental data obtained from various samples.

cond-mat.stat-mech↗

arXiv1999

Mean-field theory of temperature cycling experiments in spin-glasses

We study analytically the effect of temperature cyclings in mean-field spin-glasses. In accordance with real experiments, we obtain a strong reinitialization of the dynamics on decreasing the temperature combined with memory effects when the original high temperature is restored. The same calculation applied to mean-field models of structural glasses shows no such reinitialization, again in accordance with experiments. In this context, we derive some relations between experimentally accessible quantities and propose new experimental protocols. Finally, we briefly discuss the effect of field cyclings during isothermal aging.

cond-mat↗

arXiv1999

Effective temperatures out of equilibrium

We describe some interesting effects observed during the evolution of nonequilibrium systems, using domain growth and glassy systems as examples. We breafly discuss the analytical tools that have been recently used to study the dynamics of these systems. We mainly concentrate on one of the results obtained from this study, the violation of the fluctuation-dissipation theorem and we discuss, in particular, its relation to the definition and measurement of effective temperatures out of equilibrium.

cond-mat↗

arXiv1998

Thermal properties of slow dynamics

The limit of small entropy production is reached in relaxing systems long after preparation, and in stationary driven systems in the limit of small driving power. Surprisingly, for extended systems this limit is not in general the Gibbs-Boltzmann distribution, or a small departure from it. Interesting cases in which it is not are glasses, phase-separation, and certain driven complex fluids. We describe a scenario with several coexisting temperatures acting on different timescales, and partial equilibrations at each time scale. This scenario entails strong modifications of the fluctuation-dissipation equalities and the existence of some unexpected reciprocity relations. Both predictions are open to experimental verification, particularly the latter. The construction is consistent in general, since it can be viewed as the breaking of a symmetry down to a residual group. It does not assume the presence of quenched disorder. It can be -- and to a certain extent has been -- tested numerically, while some experiments are on their way. There is furthermore the perspective that analytic arguments may be constructed to prove or disprove its generality.

cond-mat↗

arXiv1998

Real-time non-equilibrium dynamics of quantum glassy systems

We develop a systematic analytic approach to aging effects in quantum disordered systems in contact with an environment. Within the closed-time path-integral formalism we include dissipation by coupling the system to a set of independent harmonic oscillators that mimic a quantum thermal bath. After integrating over the bath variables and averaging over disorder we obtain an effective action that determines the real-time dynamics of the system. The classical limit yields the Martin-Siggia-Rose generating functional associated to a colored noise. We apply this general formalism to a prototype model related to the $p$ spin-glass. We show that the model has a dynamic phase transition separating the paramagnetic from the spin-glass phase and that quantum fluctuations depress the transition temperature until a quantum critical point is reached. We show that the dynamics in the paramagnetic phase is stationary but presents an interesting crossover from a region controlled by the classical critical point to another one controlled by the quantum critical point. The most characteristic property of the dynamics in a glassy phase, namely aging, survives the quantum fluctuations. In the sub-critical region the quantum fluctuation-dissipation theorem is modified in a way that is consistent with the notion of effective temperatures introduced for the classical case. We discuss these results in connection with recent experiments in dipolar quantum spin-glasses and the relevance of the effective temperatures with respect to the understanding of the low temperature dynamics.

cond-mat↗

arXiv1997

Quantum aging in mean-field models

We study the real-time dynamics of quantum models with long-range interactions coupled to a heat-bath within the closed-time path-integral formalism. We show that quantum fluctuations depress the transition temperature. In the subcritical region there are two asymptotic time-regimes with (i) stationary, and (ii) slow aging dynamics. We extend the quantum fluctuation-dissipation theorem to the nonequilibrium case in a consistent way with the notion of an effective temperature that drives the system in the aging regime. The classical results are recovered for $\hbar\to 0$.

cond-mat↗

arXiv1997

Fluctuation-Dissipation theorems and entropy production in relaxational systems

We show that for stochastic dynamical systems out of equilibrium the violation of the fluctuation-dissipation equality is bounded by a function of the entropy production. The result applies to a much wider situation than `near equilibrium', comprising diffusion as well as glasses and other macroscopic systems far from equilibrium. For aging systems this bounds the age-frequency regimes in which the susceptibilities satisfy FDT in terms of the rate of decay of the H-function, a question intimately related to the reading of a thermometer placed in contact with the system.

cond-mat.stat-mech↗

arXiv1997

Out of equilibrium dynamics in spin-glasses and other glassy systems

We review recent theoretical progress on glassy dynamics, with special emphasis on the importance and universality of the ``aging regime'', which is relevant to many experimental situations. The three main subjects which we address are: (i) Phenomenological models of aging (coarsening, trap models), (ii) Analytical results for the low-temperature dynamics of mean-field models (corresponding to the mode-coupling equations); and (iii) Simple non-disordered models with glassy dynamics. We discuss the interrelation between these approaches, and also with previous work in the field. Several open problems are underlined -- in particular the precise relation between mean-field like (or mode-coupling) descriptions and finite dimensional problems.

cond-mat.dis-nn↗

arXiv1996

Dynamics of particles and manifolds in a quenched random force field

We study the dynamics of a directed manifold of internal dimension D in a d-dimensional random force field. We obtain an exact solution for $d \to \infty$ and a Hartree approximation for finite d. They yield a Flory-like roughness exponent $ζ$ and a non trivial anomalous diffusion exponent $ν$ continuously dependent on the ratio $g_{T}/g_{L}$ of divergence-free ($g_{T}$) to potential ($g_{L}$) disorder strength. For the particle (D=0) our results agree with previous order $ε^2$ RG calculations. The time-translational invariant dynamics for $g_{T} >0$ smoothly crosses over to the previously studied ultrametric aging solution in the potential case.

cond-mat.dis-nn↗

arXiv1996

Aging and effective temperatures in the low temperature mode-coupling equations

The low-temperature generalization of the mode-coupling equations corresponds to the dynamics of mean-field disordered models in the glassy phase. The system never achieves equilibrium, preserving the memory of the time elapsed after the quench throughout its evolution. A concept of effective temperature can be made quite rigorous in this context by considering readings of thermometers in different time-scales and the thermalization of weakly coupled subsystems.

cond-mat.dis-nn↗

arXiv1996

Energy flow, partial equilibration and effective temperatures in systems with slow dynamics

We show that, in nonequilibrium systems with small heat flows, there is a time-scale dependent effective temperature which plays the same role as the thermodynamical temperature, in that it controls the direction of heat flows and acts as a criterion for thermalization. We simultaneously treat the case of stationary systems with weak stirring and of glassy systems that age after cooling and show that they exhibit very similar behavior provided that time dependences are expressed in terms of the correlations of the system. We substantiate our claims with examples taken from solvable models with non-trivial low-temperature dynamics, but argue that they have a much wider range of validity. We suggest experimental checks of these ideas.

cond-mat.dis-nn↗

arXiv1996

Slow dynamics and aging in spin-glasses

Contribution presented by Eric Vincent in the Conference `Complex Behaviour of Glassy Systems', Sitges, Barcelona, Spain, June, 1996. It contains a review of the experimental results on Slow dynamics and aging in spin-glasses. It also presents their comparison with recent theoretical developments in the description of the out of equilibrium dynamics of disordered systems; namely, the trap model and the mean-field theory.

cond-mat↗