1,774 research outputs found
Non-Hamiltonian dynamics in optical microcavities resulting from wave-inspired corrections to geometric optics
We introduce and investigate billiard systems with an adjusted ray dynamics
that accounts for modifications of the conventional reflection of rays due to
universal wave effects. We show that even small modifications of the specular
reflection law have dramatic consequences on the phase space of classical
billiards. These include the creation of regions of non-Hamiltonian dynamics,
the breakdown of symmetries, and changes in the stability and morphology of
periodic orbits. Focusing on optical microcavities, we show that our adjusted
dynamics provides the missing ray counterpart to previously observed wave
phenomena and we describe how to observe its signatures in experiments. Our
findings also apply to acoustic and ultrasound waves and are important in all
situations where wavelengths are comparable to system sizes, an increasingly
likely situation considering the systematic reduction of the size of electronic
and photonic devices.Comment: 6 pages, 4 figures, final published versio
Modeling Barkhausen Noise in Magnetic Glasses with Dipole-Dipole Interactions
Long-ranged dipole-dipole interactions in magnetic glasses give rise to
magnetic domains having labyrinthine patterns. Barkhausen Noise is then
expected to result from the movement of domain boundaries which is supposed to
be modeled by the motion of elastic membranes with random pinning. We propose
an atomistic model of such magnetic glasses in which we measure the Barkhausen
Noise which indeed results from the movement of domain boundaries. Nevertheless
the statistics of the Barkhausen Noise is found in striking disagreement with
the expectations in the literature. In fact we find exponential statistics
without any power law, stressing the fact that Barkhausen Noise can belong to
very different universality classes. In this glassy system the essence of the
phenomenon is the ability of spin-carrying particles to move and minimize the
energy without any spin flip. A theory is offered in excellent agreement with
the measured data without any free parameter.Comment: 5 Pages, 5 Figures, Submitted to EP
Near-field diffraction of fs and sub-fs pulses: super-resolutions of NSOM in space and time
The near-field diffraction of fs and sub-fs light pulses by nm-size slit-type
apertures and its implication for near-field scanning optical microscopy (NSOM)
is analyzed. The amplitude distributions of the diffracted wave-packets having
the central wavelengths in the visible spectral region are found by using the
Neerhoff and Mur coupled integral equations, which are solved numerically for
each Fourier's component of the wave-packet. In the case of fs pulses, the
duration and transverse dimensions of the diffracted pulse remain practically
the same as that of the input pulse. This demonstrates feasibility of the NSOM
in which a fs pulse is used to provide the fs temporal resolution together with
nm-scale spatial resolution. In the sub-fs domain, the Fourier spectrum of the
transmitted pulse experiences a considerable narrowing that leads to the
increase of the pulse duration in a few times. This imposes a limit on the
simultaneous resolutions in time and space.Comment: 5 figure
Detailing the relation between renal T(2)* and renal tissue pO2 using an integrated approach of parametric magnetic resonance imaging and invasive physiological measurements
OBJECTIVES: This study was designed to detail the relation between renal T2* and renal tissue pO2 using an integrated approach that combines parametric magnetic resonance imaging (MRI) and quantitative physiological measurements (MR-PHYSIOL. MATERIALS AND METHODS: Experiments were performed in 21 male Wistar rats. In vivo modulation of renal hemodynamics and oxygenation was achieved by brief periods of aortic occlusion, hypoxia, and hyperoxia. Renal perfusion pressure (RPP), renal blood flow (RBF), local cortical and medullary tissue pO2, and blood flux were simultaneously recorded together with T2*, T2 mapping, and magnetic resonance-based kidney size measurements (MR-PHYSIOL). Magnetic resonance imaging was carried out on a 9.4-T small-animal magnetic resonance system. Relative changes in the invasive quantitative parameters were correlated with relative changes in the parameters derived from MRI using Spearman analysis and Pearson analysis. RESULTS: Changes in T2* qualitatively reflected tissue pO2 changes induced by the interventions. T2* versus pO2 Spearman rank correlations were significant for all interventions, yet quantitative translation of T2*/pO2 correlations obtained for one intervention to another intervention proved not appropriate. The closest T2*/pO2 correlation was found for hypoxia and recovery. The interlayer comparison revealed closest T2*/pO2 correlations for the outer medulla and showed that extrapolation of results obtained for one renal layer to other renal layers must be made with due caution. For T2* to RBF relation, significant Spearman correlations were deduced for all renal layers and for all interventions. T2*/RBF correlations for the cortex and outer medulla were even superior to those between T2* and tissue pO2. The closest T2*/RBF correlation occurred during hypoxia and recovery. Close correlations were observed between T2* and kidney size during hypoxia and recovery and for occlusion and recovery. In both cases, kidney size correlated well with renal vascular conductance, as did renal vascular conductance with T2*. Our findings indicate that changes in T2* qualitatively mirror changes in renal tissue pO2 but are also associated with confounding factors including vascular volume fraction and tubular volume fraction. CONCLUSIONS: Our results demonstrate that MR-PHYSIOL is instrumental to detail the link between renal tissue pO2 and T2* in vivo. Unravelling the link between regional renal T2* and tissue pO2, including the role of the T2* confounding parameters vascular and tubular volume fraction and oxy-hemoglobin dissociation curve, requires further research. These explorations are essential before the quantitative capabilities of parametric MRI can be translated from experimental research to improved clinical understanding of hemodynamics/oxygenation in kidney disorders
Multifractal nature of stock exchange prices
The multifractal structure of the temporal dependence of the Deutsche
Aktienindex (DAX) is analyzed. The -th order moments of the structure
functions and the singular measures are calculated. The generalized Hurst
exponent and the curve indicate a hierarchy of power law
exponents. This approach leads to characterizing the nonstationarity and
intermittency pertinent to such financial signals, indicating differences with
turbulence data. A list of results on turbulence and financial markets is
presented for asserting the analogy.Comment: 6 pages, 3 figures; an invited talk at: Conference on Computational
Physics 2001, Sept 5-8, Aachen, German
Nonlinear anomalous diffusion equation and fractal dimension: Exact generalized gaussian solution
In this work we incorporate, in a unified way, two anomalous behaviors, the
power law and stretched exponential ones, by considering the radial dependence
of the -dimensional nonlinear diffusion equation where , ,
, and are real parameters and is a time-dependent
source. This equation unifies the O'Shaugnessy-Procaccia anomalous diffusion
equation on fractals () and the spherical anomalous diffusion for
porous media (). An exact spherical symmetric solution of this
nonlinear Fokker-Planck equation is obtained, leading to a large class of
anomalous behaviors. Stationary solutions for this Fokker-Planck-like equation
are also discussed by introducing an effective potential.Comment: Latex, 6 pages. To appear in Phys. Rev.
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