1,721,043 research outputs found
Inducing oscillations of trapped particles in a near-critical Gaussian field
No full textWe study the nonequilibrium dynamics of two particles confined in two
spatially separated harmonic potentials and linearly coupled to the same
thermally fluctuating scalar field, a cartoon for optically trapped
colloids in contact with a medium close to a continuous phase
transition. When an external periodic driving is applied to one of these
particles, a nonequilibrium periodic state is eventually reached in
which their motion synchronizes thanks to the field-mediated effective
interaction, a phenomenon already observed in experiments. We fully
characterize the nonlinear response of the second particle as a function
of the driving frequency, in particular far from the adiabatic regime in
which the field can be assumed to relax instantaneously. We compare the
perturbative, analytic solution to its adiabatic approximation, thus
determining the limits of validity of the latter, and we qualitatively
test our predictions against numerical simulations
Slow dynamics in critical ferromagnetic vector models relaxing from a magnetized initial state
No full textWithin the universality class of ferromagnetic vector models with O(n) symmetry and purely dissipative dynamics, we study the non-equilibrium critical relaxation from a magnetized initial state. Transverse correlation and response functions are exactly computed for Gaussian fluctuations and in the limit of infinite number n of components of the order parameter. We find that the fluctuation-dissipation ratios (FDRs) for longitudinal and transverse modes differ already at the Gaussian level. In these two exactly solvable cases we completely describe the crossover from the short-time to the long-time behaviour, corresponding to a disordered and a magnetized initial condition, respectively. The effects of non-Gaussian fluctuations on longitudinal and transverse quantities are calculated in the first order in the epsilon-expansion (epsilon = 4-d) and reliable three-dimensional estimates of the two FDRs are obtained
On the definition of a unique effective temperature for non-equilibrium critical systems.
No full textWe consider the problem of the definition of an effective temperature via the long-time limit of the fluctuation-dissipation ratio X-infinity after a quench from the disordered state to the critical point of an O(N) model with dissipative dynamics. The scaling forms of the response and correlation functions of a generic observable O(t) are derived from the solutions of the corresponding renormalization group equations. We show that within the Gaussian approximation all the local observables have the same X-O(infinity), allowing for a definition of a unique effective temperature. This is no longer the case when fluctuations are taken into account beyond that approximation, as shown by a computation up to the first order in the epsilon-expansion for two quadratic observables. This implies that, contrarily to what is often conjectured, a unique effective temperature cannot be defined for this class of models
Aging and fluctuation-dissipation ratio for the dilute Ising model
No full textWe consider the out-of-equilibrium, purely relaxational dynamics of a weakly diluted Ising model in the aging regime at criticality. We derive at first order in a rootepsilon expansion the two-time response and correlation functions for vanishing momenta. The long-time limit of the critical fluctuation-dissipation ratio is computed at the same order in perturbation theory
Two-loop critical fluctuation-dissipation ratio for the relaxational dynamics of the O(N) Landau-Ginzburg Hamiltonian
No full textThe off-equilibrium purely dissipative dynamics (model A) of the O(N) vector model is considered at criticality in an epsilon=4-d>0 expansion up to O(epsilon(2)). The scaling behavior of two-time response and correlation functions at zero momentum, the associated universal scaling functions and the nontrivial limit of the fluctuation-dissipation ratio are determined in the aging regime
Ageing properties of critical systems
No full textIn the past few years, systems with slow dynamics have attracted considerable theoretical and experimental interest. Ageing phenomena are observed during this everlasting non-equilibrium evolution. A simple instance of such a behaviour is provided by the dynamics that takes place when a system is quenched from its high-temperature phase to the critical point. The aim of this review is to summarize the various numerical and analytical results that have been recently obtained for this case. Particular emphasis is put on the field-theoretical methods that can be used to provide analytical predictions for the relevant dynamical quantities. Fluctuation-dissipation relations are discussed and in particular the concept of fluctuation-dissipation ratio (FDR) is reviewed, emphasizing its connection with the definition of a possible effective temperature. The renormalization-group approach to critical dynamics is summarized and the scaling forms of the time-dependent non-equilibrium correlation and response functions of a generic observable are discussed. From them, the universality of the associated FDR follows as an amplitude ratio. It is then possible to provide predictions for ageing quantities in a variety of different models. In particular, the results for models A, B and C dynamics of the O(N) Ginzburg-Landau Hamiltonian, and model A dynamics of the weakly dilute Ising magnet and of the phi 3 theory are reviewed and compared with the available numerical results and exact solutions. The effect of a planar surface on the ageing behaviour of model A dynamics is also addressed within the mean-field approximation
Aging in ferromagnetic systems at criticality near four dimensions
No full textWe study the off-equilibrium response and correlation functions and the corresponding fluctuation-dissipation ratio for a purely dissipative relaxation of an O(N) symmetric vector model (model A) below its upper critical dimension. The scaling behavior of these quantities is analyzed and the associated universal functions are determined at first order in epsilon=4-d in the high-temperature phase and at criticality. A nontrivial limit of the fluctuation-dissipation ratio is found in the aging regime X(infinity)=1/2(1-(epsilon/4)(N+2)/(N+8))+O(epsilon(2))
Aging at criticality in model-C dynamics
No full textWe study the off-equilibrium two-point critical response and correlation functions for the relaxational dynamics with a coupling to a conserved density (model C) of the O(N) vector model. They are determined in an epsilon = 4-d expansion for vanishing momentum. We briefly discuss their scaling behaviors and the associated scaling forms are determined up to first order in epsilon. The corresponding fluctuation-dissipation ratio has a nontrivial large time limit in the aging regime and, up to one-loop order, it is the same as that of the model A for the physically relevant case N = 1. The comparison with predictions of local scale invariance is also discussed
Dynamics of fluctuations in the Gaussian model with dissipative Langevin Dynamics
No full textWe study the dynamics of the fluctuations of the variance s of the order parameter of the Gaussian model, following a temperature quench of the thermal bath. At each time t, there is a critical value s c(t) of s such that fluctuations with s > sc (t) are realized by condensed configurations of the systems, i.e., a single degree of freedom contributes macroscopically to s. This phenomenon, which is closely related to the usual condensation occurring on average quantities, is usually referred to as condensation of fluctuations. We show that the probability of fluctuations with s < inft[sc (t)], associated to configurations that never condense, after the quench converges rapidly and in an adiabatic way towards the new equilibrium value. The probability of fluctuations with s > inft[sc (t)], instead, displays a slow and more complex behavior, because the macroscopic population of the condensing degree of freedom is involved
Universal scaling functions of critical Casimir forces obtained by Monte Carlo simulations
No full textEffective Casimir forces induced by thermal fluctuations in the vicinity of bulk critical points are studied by means of Monte Carlo simulations in three-dimensional systems for film geometries and within the experimentally relevant Ising and XY universality classes. Several surface universality classes of the confining surfaces are considered, some of which are relevant for recent experiments. An approach introduced previously [O. Vasilyev , EPL 80, 60009 (2007)], based inter alia on an integration scheme of free-energy differences, is utilized to compute the universal scaling functions of the critical Casimir forces in the critical range of temperatures above and below the bulk critical temperature. The resulting predictions are compared with corresponding experimental data for wetting films of fluids and with available theoretical results
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