L.B. Kiss

Non-Dynamical Stochastic Resonance: Theory and Experiments with White and Arbitrarily Coloured Noise

We describe the simplest system which shows stochastic resonance. Theoretical results for white and (almost) arbitrarily coloured noise are presented. The new system has new, unique properties which originate from its non-dynamical character; for example, the strength and phase shift of periodic response of the system is independent of the frequency. Experiments have been carried out with the following noise processes: (physical) white noise, (physical) Lorentzian noise and (physical) 1/f noise. With a small extension of the system, its linear-response regime can be significantly increased. As the system is similar to some simple models of neurons, the new results might have not only physical but also biological importance.

A STOCHASTIC RESONATOR IS ABLE TO GREATLY IMPROVE SIGNAL-TO-NOISE RATIO

Abstract After a decade of doubts, for the first time in the history of stochastic resonance (SR), we demonstrate that a simple stochastic resonator does greatly improve the signal-to-noise ratio (SNR) of a periodic signal with additive Gaussian noise. The particular stochastic resonator is a level-crossing detector (LCD) driven by the sum of a periodic spike train signal and a band-limited Gaussian white noise. To reach the improvement of the SNR, the stochastic resonator has to work in the strongly nonlinear response limit and the noise has to have a high cut-off frequency compared to the reciprocal duration of the spikes. We demonstrate by analog and computer simulations that the SNR gain goes beyond four orders of magnitude at practical conditions. These findings get a particular importance due the fact that simplest neurone models behave very similarly to our arrangement, so the results might have direct applications in neural systems.

Signal-to-noise ratio gain by stochastic resonance in a bistable system

It was shown recently that the signal-to-noise ratio (SNR) could be improved by stochastic resonance (SR) in certain monostable systems and certain systems with monotonous nonlinearity working in the nonlinear response (NLR) regime. Here we demonstrate th

Biased percolation and abrupt failure of electronic devices

We propose a new percolation model as an aid to understand abrupt failure of electronic devices. It is called biased percolation because we assume that local Joule heating determines the probability of generating defects causing percolative breakdown of the device. We take as a simple geometry a homogeneous thin film, modelled as a two-dimensional resistor network. By carrying out Monte Carlo simulations we investigate the evolution of the system including: the damage pattern, current distribution, resistance degradation, resistance relative fluctuations and its power spectrum associated with 1/f noise. Our results show that biased percolation efficiently simulates degradation of thin films in good agreement with available experiments and predicts several features that should take place close to the abrupt failure of most devices.

Conductance noise and percolation in YBa2Cu3O7 thin films

Abstract Experimental results of the conductivity noise in the superconducting transition region of YBCO thin films prepared by co-evaporation are presented. In the case of ex situ fabricated samples, Cooper-pair number fluctuations (induced by electron trapping) have been identified in the high-temperature part of the transition. Classical percolation noise was found in the low-temperature part of the transition. In the case of samples made by the in situ method, the noise is smaller by several orders of magnitude in the upper part of the transition. This indicates a much more ordered microstructure in these samples. Electron mobility fluctuations, shunted by the conductance of Cooper-pairs, were identified in this temperature range. In the low-temperature part of the transition, a new type of fluctuation has been discovered; the fluctuation of the volume fraction of the superconducting phase. This implies new scaling exponents very different from the exponents of classical percolation models. This effect is a consequence of intergrain critical current fluctuations and can be caused by, for instance, magnetic flux motion, defect motion or trapping of electrons in the barriers between grains. Moreover, in the in situ fabricated samples, a dimensional crossover, 3D→2D, of the percolating network has been observed. From this effect, the length scale of the microscopic disorder can be estimated.

Thermal effects on the electrical degradation of thin film resistors

Recently we introduced a biased percolation model to study the electrical failure of thin-film resistors. Here we extend this model by allowing thermal interactions among first neighbour elemental resistances and accounting for the dependence of each elemental resistance on the local temperature. Monte Carlo simulations are performed to investigate the main properties of the film degradation such as: damage pattern, film lifetime, evolution of the resistance and of the 1/f resistance–noise spectrum.

Biased percolation and electrical breakdown

To analyse the degradation of a thin-film conductor we have extended the biased percolation model to the case of electrical breakdown associated with a systematic decrease of the resistance. As relevant indicators of the degradation process we have chosen the damage pattern, the current and temperature distributions, the change of resistance, the lifetime, the relative resistance fluctuations and its power spectrum associated with 1/f noise. Our results are in a satisfactory agreement with available experiments, exhibiting several features which take place close to the abrupt failure of a thin-film device, and confirm the usefulness of the biased percolation model as a tool to investigate degradation processes. Analogies and differences between the two opposite situations when degradation occurs with a systematic increase or decrease of the resistance are discussed.

Non-dynamical stochastic resonance: Theory and experiments with white and various coloured noises

SummaryWe describe the simplest system which shows stochastic resonance. A linear(ized) theory for white and (almost) arbitrarily coloured noise is presented. The presented new system has new, unique properties which originate from itsnon-dynamical character; for example, the strength and phase shift of periodic response of the system is independent of the frequency. Experiments have been carried out with the following noise processes: (physical) white noise, (physical) Lorentzian noise and (physical) 1/f noise. With a small extension of the system, its linear-response regime can be significantly increased. As the system is similar to some simple models of neurones, the new results might have not only physical but also biological importance.

Methods for determining the distribution of Josephson coupling energies in high-Tc superconductors

Abstract On the basis of recent results from measurements of spontaneous conductivity fluctuations in YBa 2 Cu 3 O 7 thin films, it is proposed that a number of properties of high- T c superconductors can be understood by means of percolation theory. Using percolation theory, a quantitative analysis of the effects of disorder in these materials is obtained. Using this analysis, suggestions are made on how to achieve higher critical temperatures and stronger critical current densities.

Scaling spontaneous fluctuations in high-Tc superconductor films☆

Abstract Spontaneous conductivity fluctuations in high-Tc superconducting films on sapphire and SrTiO3 substrates have been measured through the superconducting transition. It is found that the normalized spectrum for the sapphire film is two orders of magnitude larger than for the SrTiO3 film, due to a stronger disorder. At sufficiently low temperatures, the normalized spectrum scales with the conductivity, which proves the existence of a percolation superconducting network.