553 research outputs found

    Changes in the dynamical behavior of nonlinear systems induced by noise.

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    Weak noise acting upon a nonlinear dynamical system can have far-reaching consequences. The fundamental underlying problem - that of large deviations of a nonlinear system away from a stable or metastable state, sometimes resulting in a transition to a new stationary state, in response to weak additive or multiplicative noise - has long attracted the attention of physicists. This is partly because of its wide applicability, and partly because it bears on the origins of temporal irreversibility in physical processes. During the last few years it has become apparent that, in a system far from thermal equilibrium, even small noise can also result in qualitative change in the system's properties, e.g., the transformation of an unstable equilibrium state into a stable one, and vice versa, the occurrence of multistability and multimodality, the appearance of a mean field, the excitation of noise-induced oscillations, and noise-induced transport (stochastic ratchets). A representative selection of such phenomena is discussed and analyzed, and recent progress made towards their understanding is reviewed

    The origins of life on Earth

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    Essay review of "The Search for our Cosmic Ancestry" by Chandra Wickramasingh

    On the extraction of instantaneous frequencies from ridges in time-frequency representations of signals

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    The extraction of oscillatory components and their properties from different time-frequency representations, such as windowed Fourier transform and wavelet transform, is an important topic in signal processing. The first step in this procedure is to find an appropriate ridge curve: a sequence of amplitude peak positions (ridge points), corresponding to the component of interest. This is not a trivial issue, and the optimal method for extraction is still not settled or agreed. We discuss and develop procedures that can be used for this task and compare their performance on both simulated and real data. In particular, we propose a method which, in contrast to many other approaches, is highly adaptive so that it does not need any parameter adjustment for the signal to be analysed. Being based on dynamic path optimization and fixed point iteration, the method is very fast, and its superior accuracy is also demonstrated. In addition, we investigate the advantages and drawbacks that synchrosqueezing offers in relation to curve extraction. The codes used in this work are freely available for download.Comment: 13 pages, 7 figures, plus 4 supplementary figure

    Role of transdermal potential difference during intophoretic drug delivery.

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    Potential differences have been measured during transdermal iontophoresis in order to establish the effect of voltage, as opposed to current, on cutaneous blood flow. It is known that, even in the absence of drugs, the iontophoresis current can sometimes produce increased blood flow. The role of voltage in this process is studied through single-ended measurements (between electrode and body) of the potential difference during iontophoresis with 100-/spl mu/A, 20-s current pulses through deionized water, saturated 20.4% NaCl solution, 1 % acetylcholine, and 1 % sodium nitroprusside. It is found that the voltage needed to deliver the current varied by orders of magnitudes less than the differences in the conductance of these different electrolytes, and it is concluded that, at least for the present current protocol, the voltage as such is not an important factor in increasing the blood flow

    Liquid and solid helium.

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    Flow of superfluid 3He.

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    To push some of the newly discovered superfluid 3He through a fine tube, in order to see how it behaved, might seem a particularly obvious experiment, were it not for the daunting difficulties inherent in working at temperatures near 2 mK. In fact, it is a quite remarkable achievement in terms of experimental design and technique that has enabled R. M. Mueller, E. B. Flint and E. D. Adams to report (Phys. Rev. Lett., 36, 1460; 1976) the first studies of equilibrium flow phenomena, only four years after observation of the superfluid phases was first suspected. Their experiments were carried out in the Physics Department at the University of Florida

    Persistent currents in liquid 3He.

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    If you once start liquid lHe moving round a circle, it will continue to flow indefinitely without any further assistance and without slowing down at all, according to a recent experiment at Cornell University. The work in question, which was carried out by P.L.Gammeland J.D. Reppy of Cornell in collaboration with H.E. Hall of the University of Manchester, is reported in Physical Review Letters (52, 121; 1984)

    Do cosmic rays account for superfluid 3He transition?

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    In theory, the transition between the two principal superfluid forms of liquid 3He ought not to be able to occur. So why is it routinely observed to take place? A possible answer to this intriguing question has recently been put forward by A.J. Leggett. Writing in Physical Review Letters 53, 1096; 1984, he suggests that the transition, although prohibited from occuring spontaneously, readily takes place because of the occasional high energy cosmic ray that passes through any experimental chamber on the Earth's surface

    Spin waves in superfluid 3He.

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    One of the most intriguing features of the recently discovered superfluid phases of liquid 3He is that in sharp contrast to 4He, the only other superfluid known in nature, they have magnetic properties. It is hardly surprising, therefore, that efforts to understand superfluid 3He have been influenced by making comparisons with more conventional types of magnetic material, attempting to explore both the similarities and the fundamental differences in their properties. One characteristic feature of ordered magnetic solids is that they may support so-called spin waves, a form of collective excitation among the magnetic elements. The possible existence of spin waves in 3He has consequently been a subject of much discussion during the four years which have elapsed since the superfluid phases were discovered by D.D. Osheroff, then working at Cornell University. It is of particular interest, therefore, that Osheroff and his co-workers at Bell Laboratories have now reported (Phys. Rev. Left. 38, 134; 1977) the first experimental observation of spin waves in the B phase of superfluid 3He
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