Fluctuation and Noise Letters

In a quantum system damping and noise enter through the coupling with an external large bath. The usual treatments computes the density matrix of the system by expanding in the coupling strength to the bath and averaging over the bath. The result, however, is known to be unacceptable as it leads to negative probabilities. It is here proposed to study correlation functions rather than the density matrix itself. Equations for them are derived and compared with the result obtained by the usual method.

An algorithm for the numerical integration of Langevin equations driven by green noise is proposed. The algorithm is local in time, hence it is particularly suitable for the integration when non equilibrium quantities (like, first exit times) are under investigation.

In this work, a new, physically based model for the low-frequency noise is investigated by statistical simulations. The proposed model is based only on superposition of generation-recombination centers, and can predict the frequency-, current- and area-dependence of the low-frequency noise, as well as the area-dependence of the variation in the noise level. Measurements on Bipolar Junction Transistors (BJTs) are found to be in excellent agreement with the simulated results. For devices with large emitter areas A_E, the model predicts a spectral density S_In ~ 1/f. For devices with submicron A_E, S_In strongly deviates from a 1/f behavior, and several generation-recombination centers dominate the spectrum. However, the average spectrum <S_In>, calculated from several BJTs with identical A_E, has a frequency dependence ~ 1/f. The extracted areal trap density within the frequency range 1-10⁴ Hz is n_T = 3 × 10⁹cm^-2. The simulations show that the condition for observing g-r noise in the spectrum, strongly depends on the number of traps N_T, as well as the distribution of the corresponding energy level for the traps. The relative noise level is found to vary in a non-symmetrical way around < S_In>, especially for small A_E. For A_E < 0.1 μm², the model predicts a relative variation in the noise level below <S_In>, and above <S_In>. For A_E > 0.3 μm², the variation is found to be .

In this letter, we will show that the current through the oxide in MOS structures is not always characterized by full shot noise, as it is generally assumed. We will report different noise behaviors associated to different conduction mechanisms through the oxide and we will discuss a case where the noise characterization has been used as a powerful probe of the charge transport mechanism, thus resulting in the discrimination between the several proposed models.

Terahertz pulse imaging (TPI) systems are used to obtain sub-millimeter spectroscopic measurements for a wide range of applications. This letter highlights the use of wavelet de-noising to markedly improve the SNR of the obtained data, increasing the SNR by up to 10 dB. A comparison of different wavelet families and properties is presented and the results demonstrated on THz image data of an oak leaf and an Australian $100 note.

Weak electrical noise applied in the water around small paddlefish, Polyodon spathula, increases the spatial range over which they can detect and capture planktonic prey (Daphnia), demonstrating stochastic resonance at the level of an animal's feeding behavior. Here we show that optimal-amplitude (~ 0.5 μ V·cm^-1) noise causes a fish to prefer more vertical angles of attack when striking at prey, as revealed in polar graphs. Increased spatial range is also seen in horizontal directions, as outlying shoulders in the probability distribution of horizontal strike distances. High levels of noise increased the distance that approaching prey travelled along the rostrum (an elongated appendage anterior to the head, functioning as an electrosensitive antenna), before the fish first showed a visible fin or body motion in response. There was no significant effect of optimal-amplitude noise on the rate of strikes, although high-amplitude noise reduced the strike rate. The behavioral data were confirmed in neurophysiological experiments demonstrating that stochastic resonance occurs in individual electroreceptors, and in fact occurs at a similar optimal noise level as in behavioral experiments. We conclude that stochastic resonance can be demonstrated in the behavior of animals, and that animals can make use of the increased sensory information available during near-threshold environmental noise.

Unlike the quasi-stationary escape flux, the flux on the time scales preceding the quasi-stationary stage may significantly depend on the initial state of the system. We analyze three characteristic initial stages: (i) the stable state of the noise-free system, i.e. the bottom of the potential well; (ii) the non-bottom state with given coordinate and velocity; (iii) a thermalized state. We prove rigorously and and demonstrate in simulations that, on the time scale of a period of eigenoscillation, the flux grows stepwise for cases (i) and (iii), and in an oscillatory manner for case (ii). Different steps/oscillations correspond to different topologies of the most probable escape path.

Asymmetric kinks bridging two adjacent potential valleys of equal depth but different curvature are predicted to drift with velocity proportional to their temperature, in close agreement with numerical simulation. Such a (white) noise rectification phenomenon, not to be mistaken for a ratchet-like effect, is an entropic mechanism with potential impact on noise-sustained signal propagation in linear bistable arrays, of both biological and microelectronic nature.

Random telegraph fluctuations of the resistivity exceeding 10% have been observed in current induced low resistive metastable state of low-dopped La_1-xCa_xMnO₃ single crystal at temperatures below the Curie point. Telegraph amplitudes and average life-times depend on dc bias. Near the characteristic minimum in the resistance vs current curve the bias dependent duty cycle of the telegraph signal crosses 0.5. The pronounced telegraph fluctuations are tentatively associated with fluctuating conductivity of intrinsic tunnel barriers between ferromagnetic domains joined by antiferromagnetic insulating matrix in percolating paths of the phase separated manganite.

Classical particle transport, such as the diffusion and slowing down of neutrons in a multiplying system, is a random process. As a result, the number of neutrons in the system, or the number of detector counts, is a random number. Because of the branching property of the cascade, i.e. producing two or more neutrons in the fission process, correlations will exist between the neutrons in the system. This will lead to non-trivial statistics in the cascade, which, among others, carries information on the dynamical properties of the system. Neutron fluctuations have thus been used for a long time to measure either nuclear physics parameters such as the effective delayed neutron fraction, or the so-called multiplication constant, which describes the criticality properties of the system.

In addition to the above described inherent random property of the particle transport in steady-state systems, another possibility of neutron fluctuations arises in systems whose properties fluctuate in time. This type of noise is observed in systems with high power. The reason for neutron fluctuations, that is neutron noise, is the fluctuations of the reactor material. There is boiling of the coolant, vibration of various components such as control rods, the core barrel etc., which all will lead to parametric excitation of the neutron noise. In this paper we shall however mostly deal with the neutron noise in low power systems, i.e. with inherent fluctuations of the neutron transport process, and the case of power reactor noise will only be touched upon.

The neutron noise in low power systems has received a renewed strong interest recently, due to the appearance of a new concept, the so-called accelerator driven systems (ADS). An ADS is a subcritical reactor, driven by a strong external source, which utilizes spallation. The advantages of such a system is that it has excellent operational safety properties (a criticality accident is practically impossible), can utilize fertile nuclides as fuel such as thorium or U-238 easier than traditional reactors (hereby having access to more abundant fuel reserves as the current reactors), and finally, on the long run and with proper design, it would produce less high-level nuclear waste. As a matter of fact, an ADS can incinerate more waste than it produces, so it can also be used primarily for transmutation of nuclear waste with energy production as a by-product.

In an ADS, however, the statistical properties of the neutron source are different from those of the classical radioactive sources that were used in the development of the theory in the past. A radioactive source emits one neutron at a time, and thus all source neutrons are independent (they obey Poisson statistics). In a spallation source, on the other hand, a large number of neutrons are produced in each spallation event, and the number of neutrons generated is a random number. Thus the source neutrons are correlated. In addition to time correlations, energy correlations may exist between spallation neutrons even if the neutron energies in a single spallation even are independent. This fact will be illustrated i the paper. All of the above mentioned (i.e. time and energy) correlations will influence the statistics of the neutrons in an ADS and thus they need to be accounted for.

This paper presents an extended mathematical base for the interpretation of cluster processes in introducing an intermittent stochastic process; this is characterized by clusters of events being separated by distinct breaks so-called intermissions. The spectral features of such an intermittent process are investigated for the case that either intermission δ or duration of cluster τ_c is power-law distributed like t^z. Correspondingly, a spectral shape like 1/f^b(z) is found; a pure 1/f shape (b=1) is obtained for z = -2. The most striking result is the dependence of exponent b(z) on quotient δ/τ_c: we obtain b≤2 for δ > τ_c and b≤ 1 for δ < τ_c. This behavior is explained by mutual time relations between clusters which cannot be neglected in case intermissions are small in comparison to duration of clusters (δ < τ_c). The conditions under which superimposed intermittent processes converge to the clustering Poisson process exhibiting 1/f noise are investigated. Possible applications are discussed for 1/f noise in neuronal spike trains, ionic channels, on-off intermittency and semiconductors.

The major goal of the present paper is to find out the manifestation of the boundedness of fluctuations. Two different subjects are considered: (i) an ergodic Markovian process associated with a new type of large scale fluctuations at spatially homogeneous reaction systems; (ii) simulated dynamical systems that possess strange attractors. Their common property is that the fluctuations are bounded. It is found out that the mathematical description of the stochasticity at both types of systems is identical. Then, it is to be expected that it exhibits certain common features whose onset is the stochasticity, namely: (i) The power spectrum of a time series of length T comprises a striclty decreasing band that uniformly fits the shape 1/f^α(f) where and α(f) strictly increases to the value α(∞) = p(p > 2) as f. approaches infinity. Practically, at low frequencies this shape is 1/f-like with high accuracy because the deviations of the non-constant exponent α(f) from 1 are very small and become even smaller as the frequency tends to 1/T. The greatest advantage of the shape 1/f^α(f) is that it ensures a finite variance of fluctuations. (ii) It is found out that the structure of a physical and a strange attractor is identical and they are non-homogeneous. (iii) The Kolmogorov entropy is finite.