Off equilibrium fluctuations in a polymer glass

The fluctuation-dissipation relation (FDR) is measured on the dielectric properties a polymer glass (polycarbonate). It is observed that the fluctuation dissipation theorem is strongly violated for a quench from above to below the glass transition temperature. The amplitude and the persistence time of this violation are decreasing functions of frequency. Around $1Hz$ it may persist for several hours. The origin of this violation is a highly intermittent dynamics characterized by large fluctuations a strongly non-Gaussian statistics. The intermittent dynamics depends on the quenching rate and it disappears after slow quenches. The relevance of these results for recent models of aging are discussed.Comment: submitted to Physica

Off equilibrium dynamics in 2d-XY system

We study the non-equilibrium time evolution of the classical XY spin model in two dimensions. The two-time autocorrelation and linear response functions are considered for systems initially prepared in a high temperature state and in a completely ordered state. After a quench into the critical phase, we determine, via Monte Carlo simulations, the time-evolution of these quantities and extract the temperature dependence of the slope of the parametric plot susceptibility/correlation in the asymptotic regime. This slope is usually identified with the infinite fluctuation-dissipation ratio which measures the violation to the equilibrium fluctuation-dissipation theorem. However, a direct measure of this ratio leads to a vanishing value

Scaling properties in off equilibrium dynamical processes

In the present paper, we analyze the consequences of scaling hypotheses on dynamic functions, as two times correlations $C(t,t')$. We show, under general conditions, that $C(t,t')$ must obey the following scaling behavior $C(t,t') = \phi_1(t)^{f(\beta)}{\cal{S}}(\beta)$, where the scaling variable is $\beta=\beta(\phi_1(t')/\phi_1(t))$ and $\phi_1(t')$, $\phi_1(t)$ two undetermined functions. The presence of a non constant exponent $f(\beta)$ signals the appearance of multiscaling properties in the dynamics.Comment: 6 pages, no figure

Off equilibrium properties of vortex creep in superconductors

We study a model for the dynamics of vortices in type II superconductors. In particular, we discuss glassy ``off equilibrium'' properties and ``aging'' in magnetic creep. At low temperatures a crossover point is found, Tg, where relaxation times seem to diverge a' la Vogel-Tamman-Fulcher. Magnetic creep changes by crossing Tg: above Tg power law creep is found asymptotically followed by stretched exponential saturation; below Tg the creep is logarithmic and vortex motion strongly subdiffusive. In this region violation of time translation invariance is found along with important dynamical scaling properties. A thermodynamic glassy transition point can be found at a lower temperature Tc.Comment: published versio

Off-equilibrium relaxational dynamics with improved Ising Hamiltonian

We study the off-equilibrium relaxational dynamics at criticality in the three-dimensional Blume-Capel model whose static critical behaviour belongs to the 3d-Ising universality class. Using "improved" Hamiltonian (the leading corrections to scaling have vanishing amplitude) we perform Monte Carlo simulations of the relaxational dynamics after a quench from $T=\infty$ to $T_c$. Analysing the off-equilibrium dynamics at $T_c$ we obtain an estimate of the dynamical critical exponent $z=2.020(8)$ that is perfectly consistent with the Field Theory predictions.Comment: 14 pages, 7 figures, references added, to appear in J. Stat. Mec