Biomedical subjects

F K Diakonos

Publications and source records attributed to F K Diakonos.

9 recordsLinked to original sources

PubMed2006

Hyperacceleration in a stochastic Fermi-Ulam model.

Fermi acceleration in a Fermi-Ulam model, consisting of an ensemble of particles bouncing between two, infinitely heavy, stochastically oscillating hard walls, is investigated. It is shown that the widely used approximation, neglecting the displacement of the walls (static wall approximation), leads to a systematic underestimation of particle acceleration. An improved approximative map is introduced, which takes into account the effect of the wall displacement, and in addition allows the analytical estimation of the long term behavior of the particle mean velocity as well as the corresponding probability distribution, in complete agreement with the numerical results of the exact dynamics. This effect accounting for the increased particle acceleration--Fermi hyperacceleration--is also present in higher-dimensional systems, such as the driven Lorentz gas.

Journal Article↗

PubMed2006

Critical opalescence in baryonic QCD matter.

We show that critical opalescence, a clear signature of second-order phase transition in conventional matter, manifests itself as critical intermittency in QCD matter produced in experiments with nuclei. This behavior is revealed in transverse momentum spectra as a pattern of power laws in factorial moments, to all orders, associated with baryon production. This phenomenon together with a similar effect in the isoscalar sector of pions (sigma mode) provide us with a set of observables associated with the search for the QCD critical point in experiments with nuclei at high energies.

Journal Article↗

PubMed2006

Abrupt transition in a sandpile model.

We present a fixed energy sandpile model which, by increasing the initial energy, undergoes, at the level of individual configurations, a discontinuous transition. The model is obtained by modifying the toppling procedure in the Bak-Tang-Wiesenfeld (BTW) [Phys. Rev. Lett. 59, 381 (1987); Phys. Rev. A 38, 364 (1988)] rules: the energy transfer from a toppling site takes place only to neighboring sites with less energy (negative gradient constraint) and with a time ordering (asynchronous). The model is minimal in the sense that removing either of the two above-mentioned constraints (negative gradient or time ordering) the abrupt transition goes over to a continuous transition as in the usual BTW case. Therefore, the proposed model offers a unique possibility to explore at the microscopic level the basic mechanisms underlying discontinuous transitions.

Journal Article↗

PubMed2006

Lyapunov instability versus relaxation time in two coupled oscillators.

We consider the relation between relaxation time and the largest Lyapunov exponent in a system of two coupled oscillators, one of them being harmonic. It has been found that in a rather broad region of parameter space, contrary to the common expectation, both Lyapunov exponent and relaxation time increase as a function of the total energy. This effect is attributed to the fact that above a critical value of the total energy, although the Lyapunov exponent increases, Kolmogorov-Arnold-Moser tori appear and the chaotic fraction of phase space decreases. We examine the required conditions and demonstrate the key role of the dispersion relation for this behavior to occur. This study is useful, among other things, in the understanding of the damping of nuclear giant resonances.

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PubMed2004

Scattering off two oscillating disks: dilute chaos.

We investigate the role of the unstable periodic orbits and their manifolds in the dynamics of a time-dependent two-dimensional scattering system. As a prototype we use two oscillating disks on the plane with the oscillation axes forming an angle theta. The phase space of the system is five dimensional and it possesses a variety of families of unstable periodic orbits (UPOs) with intersecting manifolds. We perform numerical experiments to probe the structure of distinct scattering functions, in one and two dimensions, near the location of the UPOs. We find that the corresponding manifolds occur only in a very particular and localized way in the high-dimensional phase space. As a consequence the underlying fractal structure is ubiquitous only in higher-dimensional, e.g., two-dimensional, scattering functions. Both two-dimensional and one-dimensional scattering functions are dominated by seemingly infinite sequences of discontinuities characterized by small values of the magnitude of the projectile's outgoing velocity. These peaks accumulate toward the phase-space locations of the UPOs, with a rate which monotonically depends on the corresponding instability exponent. They represent the intersections of the set of the initial conditions with invariant sets of larger dimensionality embedded in the phase space of the system, which are not directly related with the UPOs. We adopt the term "dilute chaos" to characterize these phenomenological aspects of the scattering dynamics.

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PubMed2004

Criticality in the relaxation phase of a spontaneously contracting atria isolated from a frog's heart.

We investigate the spontaneous contraction generated by the atria of a frog's heart isolated in a physiological solution. In the relaxation phase, the recorded time series for two different sampling rates possesses an intermittent component similar to the dynamics of the order parameter's fluctuations of a thermal critical system belonging to the mean field universality class. This behavior is not visible through conventional analysis in the frequency space due to the presence of Brownian noise dominating the corresponding power spectrum.

Animals↗

PubMed2002

Intermittent dynamics of critical fluctuations.

We argue that the fluctuations of the order parameter in a complex system at the critical point can be described in terms of intermittent dynamics of type I. Based on this observation we develop an algorithm to calculate the isothermal critical exponent delta for a "thermal" critical system. We apply successfully our approach to the 3D Ising model. The intermittent character of these "critical" dynamics guides to the introduction of a new exponent which extends the notion of the exponent delta to nonthermal systems.

Algorithms↗

PubMed2001

Detecting unstable periodic orbits in chaotic continuous-time dynamical systems.

We extend the recently developed method for detecting unstable periodic points of chaotic time-discrete dynamical systems to find unstable periodic orbits in time-continuous systems, given by a set of ordinary differential equations. This is achieved by the reduction of the continuous flow to a Poincaré map which is then searched for periodic points. The algorithm has global convergence properties and needs no a priori knowledge of the system. It works well for both dissipative and Hamiltonian dynamical systems which is demonstrated by exploring the Lorenz system and the hydrogen atom in a strong magnetic field. The advantages and general features of the approach are discussed in detail.

Journal Article↗

PubMed2001

Nonperiodic delay mechanism and fractallike behavior in classical time-dependent scattering.

We study the occurrence of delay mechanisms other than periodic orbits in scattering systems with time-dependent potentials. By using as model system two harmonically oscillating disks on a plane, we have found the existence of a mechanism not related to the periodic orbits of the system, that delays trajectories in the scattering region. This mechanism creates a fractallike structure in the scattering functions and can possibly occur in several time-dependent scattering systems.

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