1/f1/f noise and avalanche scaling in plastic deformation

We study the intermittency and noise of dislocation systems undergoing shear deformation. Simulations of a simple two-dimensional discrete dislocation dynamics model indicate that the deformation rate exhibits a power spectrum scaling of the type $1/f^{\alpha}$. The noise exponent is far away from a Lorentzian, with $\alpha \approx 1.5$. This result is directly related to the way the durations of avalanches of plastic deformation activity scale with their size.Comment: 6 pages, 5 figures, submitted to Phys. Rev.

Helium condensation in aerogel: avalanches and disorder-induced phase transition

We present a detailed numerical study of the elementary condensation events (avalanches) associated to the adsorption of $^4$He in silica aerogels. We use a coarse-grained lattice-gas description and determine the nonequilibrium behavior of the adsorbed gas within a local mean-field analysis, neglecting thermal fluctuations and activated processes. We investigate the statistical properties of the avalanches, such as their number, size and shape along the adsorption isotherms as a function of gel porosity, temperature, and chemical potential. Our calculations predict the existence of a line of critical points in the temperature-porosity diagram where the avalanche size distribution displays a power-law behavior and the adsorption isotherms have a universal scaling form. The estimated critical exponents seem compatible with those of the field-driven Random Field Ising Model at zero temperature.Comment: 16 pages, 14 figure

Hysteresis and Avalanches in the Random Anisotropy Ising Model

The behaviour of the Random Anisotropy Ising model at T=0 under local relaxation dynamics is studied. The model includes a dominant ferromagnetic interaction and assumes an infinite anisotropy at each site along local anisotropy axes which are randomly aligned. Two different random distributions of anisotropy axes have been studied. Both are characterized by a parameter that allows control of the degree of disorder in the system. By using numerical simulations we analyze the hysteresis loop properties and characterize the statistical distribution of avalanches occuring during the metastable evolution of the system driven by an external field. A disorder-induced critical point is found in which the hysteresis loop changes from displaying a typical ferromagnetic magnetization jump to a rather smooth loop exhibiting only tiny avalanches. The critical point is characterized by a set of critical exponents, which are consistent with the universal values proposed from the study of other simpler models.Comment: 40 pages, 21 figures, Accepted for publication in Phys. Rev.

Dynamics of a ferromagnetic domain wall and the Barkhausen effect

We derive an equation of motion for the the dynamics of a ferromagnetic domain wall driven by an external magnetic field through a disordered medium and we study the associated depinning transition. The long-range dipolar interactions set the upper critical dimension to be $d_c=3$, so we suggest that mean-field exponents describe the Barkhausen effect for three-dimensional soft ferromagnetic materials. We analyze the scaling of the Barkhausen jumps as a function of the field driving rate and the intensity of the demagnetizing field, and find results in quantitative agreement with experiments on crystalline and amorphous soft ferromagnetic alloys.Comment: 4 RevTex pages, 3 ps figures embedde

Dynamics of a ferromagnetic domain wall: avalanches, depinning transition and the Barkhausen effect

We study the dynamics of a ferromagnetic domain wall driven by an external magnetic field through a disordered medium. The avalanche-like motion of the domain walls between pinned configurations produces a noise known as the Barkhausen effect. We discuss experimental results on soft ferromagnetic materials, with reference to the domain structure and the sample geometry, and report Barkhausen noise measurements on Fe$_{21}$Co$_{64}$B$_{15}$ amorphous alloy. We construct an equation of motion for a flexible domain wall, which displays a depinning transition as the field is increased. The long-range dipolar interactions are shown to set the upper critical dimension to $d_c=3$, which implies that mean-field exponents (with possible logarithmic correction) are expected to describe the Barkhausen effect. We introduce a mean-field infinite-range model and show that it is equivalent to a previously introduced single-degree-of-freedom model, known to reproduce several experimental results. We numerically simulate the equation in $d=3$, confirming the theoretical predictions. We compute the avalanche distributions as a function of the field driving rate and the intensity of the demagnetizing field. The scaling exponents change linearly with the driving rate, while the cutoff of the distribution is determined by the demagnetizing field, in remarkable agreement with experiments.Comment: 17 RevTeX pages, 19 embedded ps figures + 1 extra figure, submitted to Phys. Rev.

Universal Pulse Shape Scaling Function and Exponents: A Critical Test for Avalanche Models applied to Barkhausen Noise

In order to test if the universal aspects of Barkhausen noise in magnetic materials can be predicted from recent variants of the non-equilibrium zero temperature Random Field Ising Model (RFIM), we perform a quantitative study of the universal scaling function derived from the Barkhausen pulse shape in simulations and experiment. Through data collapses and scaling relations we determine the critical exponents $\tau$ and $1/\sigma\nu z$ in both simulation and experiment. Although we find agreement in the critical exponents, we find differences between theoretical and experimental pulse shape scaling functions as well as between different experiments.Comment: 19 pages (in preprint format), 5 figures, 1 tabl

Non-invasive assessment of risk for severe tachyarrhythmias by means of non-linear analysis techniques

Sudden death remains a phenomenon of disturbing proportions, displaying a mean incidence of 300,000-350,000 persons/year in the USA (0.1-0.2% of the general population). In Europe, the figures are very similar. In 90% of cases, sudden death has an arrhythmic cause. Prevention of Sudden Cardiac Death (SCD) constitutes one of the most important challenges of modern cardiology. In order to make a real progress in this field it is crucial to precisely identify increased risk for serious ventricular tachyarrhythmias. In this study the effectiveness of different methods of the non-linear analysis (NLA) of ECG in the risk stratification of patients with ventricular arrhythmias is evaluated, and these non-invasive parameters are correlated with the results of invasive electrophysiological study (EPS). We evaluated 25 patients with history of cardiac arrest, syncope, sustained or nonsustained ventricular tachycardia (VT). The study group was compared with a control group of 25 healthy subjects. All patients underwent both electrophysiologic study (EPS) and non-linear analysis (NLA) of ECG. Patients were classified through the application of a clustering procedure to the whole set of functions, and a comparison between the results of non-linear analysis of ECG and EPS was performed. Results are presented and discussed

Non-invasive assessment of risk for severe tachyarrhythmias by means of non-linear analysis techniques

Sudden death remains a phenomenon of disturbing proportions, displaying a mean incidence of 300,000-350,000 persons/year in the USA (0.1-0.2% of the general population). In Europe, the figures are very similar. In 90% of cases, sudden death has an arrhythmic cause. Prevention of Sudden Cardiac Death (SCD) constitutes one of the most important challenges of modern cardiology. In order to make a real progress in this field it is crucial to precisely identify increased risk for serious ventricular tachyarrhythmias. In this study the effectiveness of different methods of the non-linear analysis (NLA) of ECG in the risk stratification of patients with ventricular arrhythmias is evaluated, and these non-invasive parameters are correlated with the results of invasive electrophysiological study (EPS). We evaluated 25 patients with history of cardiac arrest, syncope, sustained or nonsustained ventricular tachycardia (VT). The study group was compared with a control group of 25 healthy subjects. All patients underwent both electrophysiologic study (EPS) and non-linear analysis (NLA) of ECG. Patients were classified through the application of a clustering procedure to the whole set of functions, and a comparison between the results of non-linear analysis of ECG and EPS was performed. Results are presented and discussed