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When exposed to sufficiently high electric fields, polymer-foam electret materials with closed cells exhibit ferroelectric-like behavior and may therefore be called ferroelectrets. In cellular ferroelectrets, the influence of the cell size and shape distributions on the application-relevant properties is not yet understood. Therefore, controlled inflation experiments were carried out on cellular polypropylene films, and the resulting elastical and electromechanical parameters were determined. The elastic modulus in the thickness direction shows a minimum with a corresponding maximum in the electromechanical transducer coefficient. The resonance frequency shifts as a function of the elastic modulus and the relative density of the inflated cellular films. Therefore, the transducer properties of cellular ferroelectrets can be optimized by means of controlled inflation. (C) 2004 American Institute of Physics

We have undertaken a thorough dynamical investigation of five extrasolar planetary systems using extensive numerical experiments. The systems Gl 777 A, HD 72659, Gl 614, 47 Uma and HD 4208 were examined concerning the question of whether they could host terrestrial-like planets in their habitable zones (HZ). First we investigated the mean motion resonances between fictitious terrestrial planets and the existing gas giants in these five extrasolar systems. Then a fine grid of initial conditions for a potential terrestrial planet within the HZ was chosen for each system, from which the stability of orbits was then assessed by direct integrations over a time interval of 1 million years. For each of the five systems the 2-dimensional grid of initial conditions contained 80 eccentricity points for the Jovian planet and up to 160 semimajor axis points for the fictitious planet. The computations were carried out using a Lie-series integration method with an adaptive step size control. This integration method achieves machine precision accuracy in a highly efficient and robust way, requiring no special adjustments when the orbits have large eccentricities. The stability of orbits was examined with a determination of the Renyi entropy, estimated from recurrence plots, and with a more straightforward method based on the maximum eccentricity achieved by the planet over the 1 million year integration. Additionally, the eccentricity is an indication of the habitability of a terrestrial planet in the HZ; any value of e > 0.2 produces a significant temperature difference on a planet's surface between apoapse and periapse. The results for possible stable orbits for terrestrial planets in habitable zones for the five systems are: for Gl 777 A nearly the entire HZ is stable, for 47 Uma, HD 72659 and HD 4208 terrestrial planets can survive for a sufficiently long time, while for Gl 614 our results exclude terrestrial planets moving in stable orbits within the HZ. Studies such as this one are of primary interest to future space missions dedicated to finding habitable terrestrial planets in other stellar systems. Assessing the likelihood of other habitable planets, and more generally the possibility of other life, is the central question of astrobiology today. Our investigation indicates that, from the dynamical point of view, habitable terrestrial planets seem to be compatible with many of the currently discovered extrasolar systems

It has been found in recent measurements that the singlet-to-triplet exciton ratio in organic light-emitting diodes (OLEDs) is larger than expected from spin degeneracy, and that singlet excitons form at a larger rate than triplets. We employed the technique of optically detected magnetic resonance to measure the spin-dependent exciton formation rates in films of a polymer and corresponding monomer, and explore the relation between the formation rates and the actual singlet-to-triplet ratio measured previously in OLEDs. We found that the spin-dependent exciton formation rates can indeed quantitatively explain the observed exciton yields, and that singlet formation rates and yields are significantly enhanced only in polymer OLEDs, but not in OLEDs made from the corresponding monomer

Suppression of the keto-emission in polyfluorene light-emitting diodes : Experiments and models
(2004)

The spectral characteristics of polyfluorene (PF)-based light-emitting diodes (LEDs) containing a defined low concentration of either keto-defects or of the polymer poly(9.9-octylfuorene-co-benzothiadiazole) (F8BT) are preseneted. Both types of blend layers were tested in different device configurations with respect to the relative and absolute intensities of green blue emission components. It is shown that blending hole-transporting molecules into the emission layer at low concentration or incorporation of a suitable hole-transport layer reduces the green emission contribution in the electroluminescence (EL) spectrum of the PF:F8BT blend, which is similar to what is observed for the keto- containing PF layer. We conclude that the keto-defects in PF homopolymer layers mainly constitute weakly emissive electron traps, in agreement with the results of quantum-mechanical calculations

We study the behavior of time-periodic three-dimensional incompressible flows modelled by three-dimensional volume-preserving maps in the presence of a leakage. The distribution of residence times, and the chaotic saddle together with its stable and unstable invariant manifolds are described and characterized. They shed light. on typical filamentation of chaotic flows whose local stable and unstable manifolds are always of different, character (plane or line). We point out that leaking is a useful method which sheds light on typical filamentation of chaotic flows. In particular; the topology depends on the number of local expanding directions, and is the same in the leaked system as in the closed flow

Optical, structural and morphological properties of thin films of polyparaphenylenevinylene (PPV) formed by an alkyl sulfinyl precursor route have been studied. Thin films were fabricated on an optical glass and on quartz glass either by spin-coating of the precursor solution or by layer-by-layer deposition using Langmuir-Blodgett technique. PPV precursor films were also spin-coated on gold-coated glass in order to study thin-film optical parameters by surface plasmon spectroscopy. We have been successful in forming about 40 precursor mono layers on quartz glass by Langmuir- Blodgett technique using optimized surface pressure and dipping conditions. After thermal conversion of the precursor layers good quality fluorescent PPV films of yellow colour have been obtained. Optical characterization of the films was carried out by linear absorption and emission spectroscopy, ellipsometry, and surface plasmon spectroscopy. Structural and morphological studies on the thin films were carried out by using X-ray scattering and atomic force microscopy. Wave- guided travelling-wave laser action has been achieved in a PPV film on quartz glass. The sample was transversally pumped with picosecond laser pulses (wavelength 347.15 nm, duration 35 ps). Laser emission occurred at 550 nm for pump pulse energy densities above 1 x 10(-4) J cm(-2). (C) 2004 Elsevier B.V. All rights reserved

We suggest a method for suppression of synchrony in a globally coupled oscillator network, based on the time- delayed feedback via the mean field. Having in mind possible applications for suppression of pathological rhythms in neural ensembles, we present numerical results for different models of coupled bursting neurons. A theory is developed based on the consideration of the synchronization transition as a Hopf bifurcation

We investigate noise-controlled resonant response of active media to weak periodic forcing, both in excitable and oscillatory regimes. In the excitable regime, we find that noise-induced irregular wave structures can be reorganized into frequency-locked resonant patterns by weak signals with suitable frequencies. The resonance occurs due to a matching condition between the signal frequency and the noise-induced inherent time scale of the media. m:1 resonant regions similar to the Arnold tongues in frequency locking of self-sustained oscillatory media are observed. In the self-sustained oscillatory regime, noise also controls the oscillation frequency and reshapes significantly the Arnold tongues. The combination of noise and weak signal thus could provide an efficient tool to manipulate active extended systems in experiments

A statistical model describing the propensity for protein aggregation is presented. Only amino-acid hydrophobicity values and calculated net charge are used for the model. The combined effects of hydrophobic patterns as computed by the signal analysis technique, recurrence quantification, plus calculated net charge were included in a function emphasizing the effect of singular hydrophobic patches which were found to be statistically significant for predicting aggregation propensity as quantified by fluorescence studies obtained from the literature. These results suggest preliminary evidence for a mesoscopic principle for protein folding/aggregation. (C) 2004 Elsevier B.V. All rights reserved

We investigate the effects of rotation on the behavior of the helium-burning shell source in accreting carbon- oxygen white dwarfs, in the context of the single degenerate Chandrasekhar mass progenitor scenario for type la supernovae (SNe Ia). We model the evolution of helium-accreting white dwarfs of initially 1 M-circle dot, assuming four different constant accretion rates (2, 3, 5 and 10 x 10(-7) M-circle dot/yr). In a one-dimensional approximation, we compute the mass accretion and subsequent nuclear fusion of helium into carbon and oxygen, as well as angular momentum accretion, angular momentum transport inside the white dwarf, and rotationally induced chemical mixing. Our models show two major effects of rotation: a) The helium-burning nuclear shell source in the rotating models is much more stable than in corresponding non-rotating models - which increases the likelihood that accreting white dwarfs reach the stage of central carbon ignition. This effect is mainly due to rotationally induced mixing at the CO/He interface which widens the shell source, and due to the centrifugal force lowering the density and degeneracy at the shell source location. b) The C/O-ratio in the layers which experience helium shell burning - which may affect the energy of an SN Ia explosion - is strongly decreased by the rotationally induced mixing of a-particles into the carbon-rich layers. We discuss implications of our results for the evolution of SNe la progenitors

We demonstrate efficient single-layer polymer phosphorescent light-emitting devices based on a green-emitting iridium complex and a polymer host co-doped with electron-transporting and hole-transporting molecules. These devices can be operated at relatively low voltages, resulting in a power conversion efficiency of up to 24 lm/W at luminous efficiencies exceeding 30 cd/A. The overall performances of these devices suggest that efficient electrophosphorescent devices with acceptable operating voltages can be achieved in very simple device structures fabricated by spin coating. (C) 2004 American Institute of Physics

Various authors have investigated the problem of light deflection by radially moving gravitational lenses, but the results presented so far do not appear to agree on the expected deflection angles. Some publications claim a scaling of deflection angles with 1-v to first order in the radial lens velocity v, while others obtained a scaling with 1-2 v. In this paper we generalize the calculations for arbitrary lens velocities and show that the first result is the correct one. We discuss the seeming inconsistency of relativistic light deflection with the classical picture of moving test particles by generalizing the lens effect to test particles of arbitrary velocity, including light as a limiting case. We show that the effect of radial motion of the lens is very different for slowly moving test particles and light and that a critical test particle velocity exists for which the motion of the lens has no effect on the deflection angle to first order. An interesting and not immediately intuitive result is obtained in the limit of a highly relativistic motion of the lens towards the observer, where the deflection angle of light reduces to zero. This phenomenon is elucidated in terms of moving refractive media. Furthermore, we discuss the dragging of inertial frames in the field of a moving lens and the corresponding Lense-Thirring precession. in order to shed more light on the geometrical effects in the surroundings of a moving mass. In a second part we discuss the effect of transversal motion on the observed redshift of lensed sources. We demonstrate how a simple kinematic calculation explains the effects for arbitrary velocities of the lens and test particles. Additionally we include the transversal motion of the source and observer to show that all three velocities can be combined into an effective relative transversal velocity similar to the approach used in microlensing studies

B0218 + 357 is one of the most promising systems to determine the Hubble constant from time-delays in gravitational lenses. Consisting of two bright images, which are well resolved in very long baseline interferometry (VLBI) observations, plus one of the most richly structured Einstein rings, it potentially provides better constraints for the mass model than most other systems. The main problem left until now was the very poorly determined position of the lensing galaxy. After presenting detailed results from classical lens modelling, we apply our improved version of the LENSCLEAN algorithm which for the first time utilizes the beautiful Einstein ring for lens modelling purposes. The primary result using isothermal lens models is a now very well defined lens position of (255 +/- 6, 119 +/- 4) mas relative to the A image, which allows the first reliable measurement of the Hubble constant from the time-delay of this system. The result of H-0 = (78 +/- 6) km s(-1) Mpc(-1) (2sigma) is very high compared with other lenses. It is, however, compatible with local estimates from the Hubble Space Telescope (HST) key project and with WMAP results, but less prone to systematic errors. We furthermore discuss possible changes of these results for different radial mass profiles and find that the final values cannot be very different from the isothermal expectations. The power-law exponent of the potential is constrained by VLBI data of the compact images and the inner jet to be beta = 1.04 +/- 0.02, which confirms that the mass distribution is approximately isothermal (corresponding to beta = 1), but slightly shallower. The effect on H-0 is reduced from the expected 4 per cent decrease by an estimated shift of the best galaxy position of circa 4 mas to at most 2 per cent. Maps of the unlensed source plane produced from the best LENSCLEAN brightness model show a typical jet structure and allow us to identify the parts which are distorted by the lens to produce the radio ring. We also present a composite map which for the first time shows the rich structure of B0218 + 357 on scales ranging from mas to arcsec, both in the image plane and in the reconstructed source plane. Finally, we use a comparison of observations at different frequencies to investigate the question of possible weakening of one of the images by propagation effects and/or source shifts with frequency. The data clearly favour the model of significant 'extinction' without noticeable source position shifts. The technical details of our variant of the LENSCLEAN method are presented in the accompanying Paper I.

LensClean revisited
(2004)

We discuss the LENSCLEAN algorithm which for a given gravitational lens model fits a source brightness distribution to interferometric radio data in a similar way as standard CLEAN does in the unlensed case. The lens model parameters can then be varied in order to minimize the residuals and determine the best model for the lens mass distribution. Our variant of this method is improved in order to be useful and stable even for high dynamic range systems with nearly degenerated lens model parameters. Our test case B0218 + 357 is dominated by two bright images but the information needed to constrain the unknown parameters is provided only by the relatively smooth and weak Einstein ring. The new variant of LENSCLEAN is able to fit lens models even in this difficult case. In order to allow the use of general mass models with LENSCLEAN, we develop the new method LENTIL which inverts the lens equation much more reliably than any other method. This high reliability is essential for the use as part of LENSCLEAN. Finally a new method is developed to produce source plane maps of the unlensed source from the best LENSCLEAN brightness models. This method is based on the new concept of 'dirty beams' in the source plane. The application to the lens B0218 + 357 leads to the first useful constraints for the lens position and thus to a result for the Hubble constant. These results are presented in the accompanying Paper II, together with a discussion of classical lens modelling for this system

In cellular, electromechanically active polymer films, the so-called ferroelectrets, the cell size and shape distributions can be varied through a controlled inflation process. Up to now, high-pressure treatments were usually performed at elevated temperatures. There are, however, significant experimental limitations and complications if the pressure and temperature treatments are performed at the same time. Here, we demonstrate the controlled inflation of cellular polypropylene films by means of sepal-ate pressure and temperature treatments. Separate procedures are Much easier to implement. Excellent electromechanical properties were achieved with Such a two-step inflation process. The technique has significant potential for inflating large-area transducer films for electromechanical and electroacoustical applications

Piezo-, pyro- and ferroelectricity in poly (vinylidene fluoride-hexafluoropropylene) copolymer films
(2004)

Thin films of poly(vinylidene fluoride-hexafluoropropylene) P(VDF-HFP) show significant electroactive properties, such as piezoelectricity, pyroelectricity and electrostriction. Suitable polar P(VDF-HFP) copolymer films can be prepared by melt-pressing or solution-casting. Dipolar orientation causes the macroscopic polarization and thus also the symmetry breaking necessary for electroactive properties. We discuss the polarization build-up in thin, stretched and non-stretched, films of P(VDF-HFP) copolymer with a HFP content of 15%. Poling currents measured in-situ during electric poling are analyzed and the polarization is calculated. Suitable electric poling leads to hysteresis phenomena of the polarization as a function of the electric field as well as to significant polarization during switching experiments. Our results indicate dipolar orientation also in non-stretched P(VDF-HFP) films

We review the problem of estimating parameters and unobserved trajectory components from noisy time series measurements of continuous nonlinear dynamical systems. It is first shown that in parameter estimation techniques that do not take the measurement errors explicitly into account, like regression approaches, noisy measurements can produce inaccurate parameter estimates. Another problem is that for chaotic systems the cost functions that have to be minimized to estimate states and parameters are so complex that common optimization routines may fail. We show that the inclusion of information about the time-continuous nature of the underlying trajectories can improve parameter estimation considerably. Two approaches, which take into account both the errors-in-variables problem and the problem of complex cost functions, are described in detail: shooting approaches and recursive estimation techniques. Both are demonstrated on numerical examples

The quasar HE 0047-1756, at z = 1.67, is found to be split into two images 1."144 apart by an intervening galaxy acting as a gravitational lens. The flux ratio for the two components is roughly 3.5:1, depending slightly upon wavelength. The lensing galaxy is seen on images obtained in the i (800 nm) and K-s bands (2.1 mum); there is also a nearby faint object which may be responsible for some shear. The spectra of the two quasar images are nearly identical, but the emission line ratio between the two components scale differently from the continuum. Moreover, the fainter component has a bluer continuum slope than the brighter one. We argue that these small differences are probably due to microlensing. There is evidence for a partial Einstein ring emanating from the brighter image toward the fainter one

Cross-sections for ground and excited state absorptions of the charge transfer system 3-(3-tert- butoxycarbonylamino-propyl)-4'-pyren-1-yl-biphenyl-2,5-dicarbo xylicacid dimethyl ester (Py-C) are determined from nonlinear absorption and fluorescence measurements as a function of solvent. While in non-polar solvents no stable charge transfer (CT) state occurs after optical excitation, in polar solvents the CT state is stabilized. (C) 2004 Elsevier B.V. All rights reserved

Unstable dimension variability is a mechanism whereby an invariant set of a dynamical system, like a chaotic attractor or a strange saddle, loses hyperbolicity in a severe way, with serious consequences on the shadowability properties of numerically generated trajectories. In dynamical systems possessing a variable parameter, this phenomenon can be triggered by the bifurcation of an unstable periodic orbit. This Letter aims at discussing the possible types of codimension-one bifurcations leading to unstable dimension variability in a two-dimensional map, presenting illustrative examples and displaying numerical evidences of this fact by computing finite-time Lyapunov exponents. (C) 2004 Elsevier B.V. All rights reserved

We present spatially resolved spectrophotometric observations of multiply imaged QSOs, using the Potsdam Multi- Aperture Spectrophotometer (PMAS), with the intention to search for spectral differences between components indicative of either microlensing or dust extinction. For the quadruple QSO HE 0435-1223 we find that the continuum shapes are indistinguishable, therefore differential extinction is negligible. The equivalent widths of the broad emission lines are however significantly different, and we argue that this is most likely due to microlensing. Contrariwise, the two components of the well-known object UM 673 have virtually identical emission line properties, but the continuum slopes differ significantly and indicate different dust extinction along both lines of sight

In this letter, elastic properties of highly anisotropic cellular poly(propylene) films are reported. The material shows peculiar elastic properties compared to other foams in the literature. The data is displayed as the relative Young's modulus E*/E-s versus relative density rho*/rho(s). Almost all the data from the literature are located on the region E*/E-s = (rho*/rho(s))(n) with 1 less than or equal to n less than or equal to 6. The introduced material on the other hand has lower relative Young's modulus at high relative densities, n greater than or equal to 6. (C) 2004 Elsevier B.V. All rights reserved

Dielectric relaxation in disordered dielectric mixtures is presented by emphasizing the interfacial polarization. The obtained results coincide with and cause confusion with those of the low frequency dispersion behaviour. The considered systems are composed of two phases on two-dimensional square and triangular topological networks. We use the finite element method to calculate the effective dielectric permittivities of randomly generated structures. The dielectric relaxation phenomena together with the dielectric permittivity values at constant frequencies are investigated, and significant differences of the square and triangular topologies are observed. The frequency dependent properties of some of the generated structures are examined. We conclude that the topological disorder may lead to the normal or anomalous low frequency dispersion if the electrical properties of the phases are chosen properly, such that for 'slightly' reciprocal mixture-when sigma(1) >> sigma(2), and epsilon(1) < epsilon(2)-normal, and while for 'extreme' reciprocal mixture-when sigma(1) >> sigma(2), and epsilon(1) << epsilon(2)-anomalous low frequency dispersions are obtained. Finally, comparison with experimental data indicates that one can obtain valuable information from simulations when the material properties of the constituents are not available and of importance

A small fraction of all quasars are strongly lensed and multiply imaged, with usually a galaxy acting as the main lens. Some, or maybe all of these quasars are also affected by microlensing, the effect of stellar mass objects in the lensing galaxy. Usually only the photometric aspects of microlensing are considered: the apparent magnitudes of the quasar images vary independently because the relative motion between source, lens and observer leads to uncorrelated magnification changes as a function of time. However, stellar microlensing on quasars has yet another effect, which was first explored by Lewis & lbata (1998): the position of the quasar - i.e. the center-of-light of the many microimages - can shift by tens of microarcseconds due to the relatively sudden (dis-)appearance of a pair of microimages when a caustic is being crossed. Here we explore quantitatively the astrometric effects of microlensing on quasars for different values of the lensing parameters kappa and gamma (surface mass density and external shear) covering most of the known multiple quasar systems. We show examples of microlens-induced quasar motion (i.e. astrometric changes) and the corresponding light curves for different quasar sizes. We evaluate statistically the occurrence of large "jumps" in angular position and their correlation with apparent brightness fluctuations. We also show statistical relations between positional offsets and time from random starting points. As the amplitude of the astrometric offset depends on the source size, astrometric microlensing signatures of quasars - combined with the photometric variations - will provide. very good constraints on the sizes of quasars as a function of wavelength. We predict that such signatures will be detectable for realistic microlensing scenarios with near future technology in the infrared/optical (Keck- Interferometry, VLTI, SIM, GAIA). Such detections will show that not even high redshift quasars define a "fixed" coordinate system

Ocean convection is a highly non-linear and local process. Typically, a small-scale phenomenon of this kind entails numerical problems in the modelling of ocean circulation. One of the tasks to solve is the improvement of convection parameterization schemes, but the question of grid geometry also plays a considerable role. Here, this question is studied in the context of global ocean models coupled to an atmosphere model. Such ocean climate models have mostly structured, coarsely resolved grids. Using a simple conceptual two-layer model, we compare the discretization effects of a rectangular grid with those of a grid with hexagonal grid cells, focussing on average properties of the ocean. It turns out that systematic errors tend to be clearly smaller with the hexagonal grid. In a hysteresis experiment with the atmospheric boundary condition as a hysteresis parameter, the spatially averaged behaviour shows nonnegligible artificial steps for quadratic grid cells. This bias is reduced with the hexagonal grid. The same holds for the directional sensitivity (or horizontal anisotropy) which is found for different angles of the advection velocity. The grid with hexagonal grid cells shows much more isotropic results. From the limited viewpoint of these test experiments, it seems that the hexagonal grid (i.e. icosahedral-hexagonal grids on the sphere) is recommendable for ocean climate models. (C) 2003 Elsevier Ltd. All rights reserved

Time-resolved photoluminescence analysis of distribution and migration of terbium ions in zeolites X
(2004)

The photoluminescence (PL) dynamics of terbium-exchanged zeolites X was investigated upon laser excitation at 355 nm. The results evidenced the presence of at least two terbium main environments with PL lifetimes varying between 391-411 and 753-770 mus. The two-site nature of terbium distribution in zeolites X permitted a quantitative analysis of the migration process of terbium ions inside the pores and cavities upon dehydration in air at 200 degreesC. Besides the increase of the PL lifetimes with about 30% and 80% compared to those of the hydrated zeolite, a fraction of almost 30% of terbium ions was estimated to migrate from the supercages to the neighboring sodalites or hexagonal prisms. Our results evidenced for the first time the capability of time-resolved luminescence spectroscopy in quantitatively tracking for the intrazeolitic migration of lanthanides. (C) 2004 Elsevier B.V. All rights reserved

Fourier surrogate data are artificially generated time series, that - based on a resampling scheme - share the linear properties with an observed time series. In this paper we study a statistical surrogate hypothesis test to detect deviations from a linear Gaussian process with respect to asymmetry in time (Q-statistic). We apply this test to a Fourier representable function and obtain a representation of the asymmetry in time of the sample data, a characteristic for nonlinear processes, and the significance in terms of the Fourier coefficients. The main outcome is that we calculate the expected value of the mean and the standard deviation of the asymmetries of the surrogate data analytically and hence, no surrogates have to be generated. To illustrate the results we apply our method to the saw tooth function, the Lorenz system and to measured X-ray data of Cygnus X-1

In this paper we show that two dynamical invariants, the second order Renyi entropy and the correlation dimension, can be estimated from recurrence plots (RPs) with arbitrary embedding dimension and delay. This fact is interesting as these quantities are even invariant if no embedding is used. This is an important advantage of RPs compared to other techniques of nonlinear data analysis. These estimates for the correlation dimension and entropy are robust and, moreover, can be obtained at a low numerical cost. We exemplify our results for the Rossler system, the funnel attractor and the Mackey-Glass system. In the last part of the paper we estimate dynamical invariants for data from some fluid dynamical experiments and confirm previous evidence for low dimensional chaos in this experimental system. (C) 2004 American Institute of Physics

Recurrence plots have recently been recognized as a powerful tool for the analysis of data. Not only the visualization of structures of the time series but also the possibility to estimate invariants from them and the possibility to analyze non-stationary data sets are remarkable. However, the question of how much information is encoded in such a two-dimensional and binary representation has not been discussed so far. In this Letter we show that-under some conditions-it is possible to reconstruct an attractor from the recurrence plot, at least topologically. This means that all relevant dynamical information is contained in the plot. (C) 2004 Elsevier B.V. All rights reserved

We show a Lefschetz fixed point formula for holomorphic functions in a bounded domain D with smooth boundary in the complex plane. To introduce the Lefschetz number for a holomorphic map of D, we make use of the Bergman kernel of this domain. The Lefschetz number is proved to be the sum of the usual contributions of fixed points of the map in D and contributions of boundary fixed points, these latter being different for attracting and repulsing fixed points

An approach is presented for the reconstruction of phase synchronization phenomena in a chaotic CO2 laser from experimental data. We analyze this laser system in a regime able to phase synchronize with a weak sinusoidal forcing. Our technique recovers the synchronization diagram of the experimental system from only few measurement data sets, thus allowing the prediction of the regime of phase synchronization as well as nonsynchronization in a broad parameter space of forcing frequency and amplitude without further experiments

The science case of the PEPSI high-resolution echelle spectrograph and polarimeter for the LBT
(2004)

We lay out the scientific rationale for and present the instrumental requirements of a high-resolution adaptive- optics Echelle spectrograph with two full-Stokes polarimeters for the Large Binocular Telescope (LBT) in Arizona. Magnetic processes just like those seen on the Sun and in the space environment of the Earth are now well recognized in many astrophysical areas. The application to other stars opened up a new field of research that became widely known as the solar-stellar connection. Late-type stars with convective envelopes are all affected by magnetic processes which give rise to a rich variety of phenomena on their surface and are largely responsible for the heating of their outer atmospheres. Magnetic fields are likely to play a crucial role in the accretion process of T-Tauri stars as well as in the acceleration and collimation of jet-like flows in young stellar objects (YSOs). Another area is the physics of active galactic nucleii (AGNs), where the magnetic activity of the accreting black hole is now believed to be responsible for most of the behavior of these objects, including their X-ray spectrum, their notoriously dramatic variability, and the powerful relativistic jets they produce. Another is the physics of the central engines of cosmic gamma-ray bursts, the most powerful explosions in the universe, for which the extreme apparent energy release are explained through the collimation of the released energy by magnetic fields. Virtually all the physics of magnetic fields exploited in astrophysics is somehow linked to our understanding of the Sun's and the star's magnetic fields

Optically induced mass transport studied by scanning near-field optical- and atomic force microscopy
(2004)

Some functionalised thin organic films show a very unusual property, namely the light induced material transport. This effect enables to generate three-dimensional structures on surfaces of azobenzene containing films only caused by special optical excitation. The physical mechanisms underlying this phenomenon have not yet been fully understood, and in addition, the dimensions of structures created in that way are macroscopic because of the optical techniques and the wavelength of the used light. In order to gain deeper insight into the physical fundamentals of this phenomenon and to open possibilities for applications it is necessary to create and study structures not only in a macroscopic but also in nanometer range. We first report about experiments to generate optically induced nano structures even down to 100 nm size. The optical stimulation was therefore made by a Scanning Near-field Optical Microscope (SNOM). Secondly, physical conditions inside optically generated surface relief gratings were studied by measuring mechanical properties with high lateral resolution via pulse force mode and force distance curves of an AFM

We present an entropy and complexity analysis of intracranially recorded EEG from patients suffering from a left frontal lobe epilepsy. Our approach is based on symbolic dynamics and Shannon entropy. In particular, we will discuss the possibility to monitor long-term dynamical changes in brain electrical activity. This might offer an alternative approach for the analysis and more fundamental understanding of human epilepsies

We derive kinetic equations covering coagulation and fragmentation of granular gases including a combined dynamics of the mass spectrum and the velocity distribution. We will focus on coagulation; that can only occur at low impact velocities where attractive forces and dissipation prevent a post-collisional separation. We calculate an impact speed-dependent threshold velocity g(c) for coagulation to occur based on binary collision dynamics of viscoelastic Iranular particles including adhesive forces and determined by the masses, and the material of the colliding particles. Growth processes are immensely slowed down due to g(c) and the resulting restriction in phase space, and do furthermore depend on the ratio of threshold and thermal velocity of a considered particle ensemble. The Smoluchowski equation emerges from the general kinetic approach as a special case

Films of emeraldine base of polyaniline (PAni) doped by various transition metal salts have been prepared, and current-voltage characteristics of the indium-tin oxide (ITO)/PAni film/metal electrode heterostructures were investigated. It was found that the electrical characteristics of the heterostructures are greatly affected by the dopant used and the metal electrode used. Different dopants resulted in different current anomalies with asymmetric current-voltage characteristics. Depending on the dopant used, the exponential and power law of the current behavior can be distinguished. Depending on the metal electrode used, two different regimes of current passing have been found at low applied voltages, namely, a nearly ohmic regime for the indium electrode, and a diode regime for the aluminum electrode. The diode regime was found to accompany by a positive charge accumulation in the film near the film/metal interface, which creates a built-in potential in the film. The amount of positive charges accumulated at the interface and therefore the value of the built-in potential can be reversibly increased or reduced by successive runs of the applied voltage in the forward or reverse direction, respectively. (C) 2004 Elsevier B.V. All rights reserved

Noise and fluctuations are ubiquitous in living systems. Still, the interaction between complex biochemical regulatory systems and the inherent fluctuations ('noise') is only poorly understood. As a paradigmatic example, we study the implications of noise on a recently proposed model of the eukaryotic cell cycle, representing a complex network of interactions between several genes and proteins. The purpose of this work is twofold: First, we show that the inclusion of noise into the description of the system accounts for several recent experimental findings, as e.g. the existence of quantized cycle times in wee1(-) cdc25Delta double-mutant cells of fission yeast. In the main part, we then focus on more general aspects of the interplay between noise and the dynamics of the system. In particular, we demonstrate that a stochastic description leads to qualitative changes in the dynamics, such as the emergence of noise-induced oscillations. These findings will be discussed in the light of an ongoing debate on models of cell division as limit-cycle oscillators versus checkpoint mechanisms. (C) 2004 Elsevier Ltd. All rights reserved

We study the effects of mutual and external chaotic phase synchronization in ensembles of bursting oscillators. These oscillators (used for modeling neuronal dynamics) are essentially multiple time scale systems. We show that a transition to mutual phase synchronization takes place on the bursting time scale of globally coupled oscillators, while on the spiking time scale, they behave asynchronously. We also demonstrate the effect of the onset of external chaotic phase synchronization of the bursting behavior in the studied ensemble by a periodic driving applied to one arbitrarily taken neuron. We also propose an explanation of the mechanism behind this effect. We infer that the demonstrated phenomenon can be used efficiently for controlling bursting activity in neural ensembles

The photoalignment ability of poly[methyl(phenyl)silylene] (PMPSi) films makes it possible to use them as hole- transporting substrates for the preparation of organic oriented films. A PMPSi layer prepared by spin coating was irradiated, after drying, with linearly polarized UV light. Then, water-soluble hydroxyaluminium phthalocyaninesulfonate [Al(OH)Pc(SO3Na)(1-2)] was deposited by casting. The cell ITO/PMPSi/AI(OH)Pc(SO3Na)(1-2)/Al showed non-linear current- voltage characteristics. For applied voltages higher than 10 V, polarized electroluminescence was observed. Its spectral characteristic consisted of two peaks with maxima at about 320 and 700 nm; their polarized anisotropies R-EL = Phi(parallel to) / Phi(perpendicular to) were ca. 15 and 0.5, respectively

We study the dynamics of Bose-Einstein condensates in time-dependent microtraps for the purpose of understanding the influence of the mean field interaction on the performance of interferometers. We identify conditions where the nonlinearity due to atom interactions increases the sensitivity of interferometers to a phase shift. This feature is connected with the adiabatic generation of a dark soliton. We analyse the robustness of this phenomenon with respect to thermal fluctuations, due to excited near fields in an electromagnetic surface trap

Results of energy-dispersive x-ray reflectivity and grazing incidence diffraction studies of Langmuir-Blodgett films exhibited evolution of conventional three-dimensional melting from continuous melting, characteristic of two- dimensional systems, as a function of deposited monolayers. Continuous expansion followed by a sharp phase transition of the in-plane lattice was observed before the melting point and found to be independent of number of deposited layers. Evolution of conventional melting with an increase in the number of monolayers could be quantified by measuring stiffness against tilting of the vertical stack of molecules, which are kept together by an internal field. The internal field as defined in this model reduces as the in-plane lattice expands and the sample temperature approaches melting point. The sharpness of the melting transition, which has been approximated by a Langevin function, increases with the number of deposited monolayers

We study the inference of long-range correlations by means of Detrended Fluctuation Analysis (DFA) and argue that power-law scaling of the fluctuation function and thus long-memory may not be assumed a priori but have to be established. This requires the investigation of the local slopes. We account for the variability characteristic for stochastic processes by calculating empirical confidence regions. Comparing a long-memory with a short-memory model shows that the inference of long-range correlations from a finite amount of data by means of DFA is not specific. We remark that scaling cannot be concluded from a straight line fit to the fluctuation function in a log-log representation. Furthermore, we show that a local slope larger than alpha=0.5 for large scales does not necessarily imply long memory. We also demonstrate, that it is not valid to conclude from a finite scaling region of the fluctuation function to an equivalent scaling region of the autocoffelation function. Finally, we review DFA results for the Prague temperature data set and show that long-range correlations cannot not be concluded unambiguously

In this paper, we present a detailed evaluation of cross wavelet analysis of bivariate time series. We develop a statistical test for zero wavelet coherency based on Monte Carlo simulations. If at least one of the two processes considered is Gaussian white noise, an approximative formula for the critical value can be utilized. In a second part, typical pitfalls of wavelet cross spectra and wavelet coherency are discussed. The wavelet cross spectrum appears to be not suitable for significance testing the interrelation between two processes. Instead, one should rather apply wavelet coherency. Furthermore we investigate problems due to multiple testing. Based on these results, we show that coherency between ENSO and NAO is an artefact for most of the time from 1900 to 1995. However, during a distinct period from around 1920 to 1940, significant coherency between the two phenomena occurs

We present a comprehensive study of the Magellanic Cloud planetary nebula SMP 61 and of its nucleus, a Wolf- Rayet type star classified [WC 5-6]. The observational material consists of HST STIS spectroscopy and imaging, together with optical and UV spectroscopic data collected from the literature and infrared fluxes measured by IRAS. We have performed a detailed spectral analysis of the central star, using the Potsdam code for expanding atmospheres in non-LTE. For the central star we determine the following parameters: L-star = 10(3.96) L-., R-star = 0.42 R-., T-star = 87.5 kK, (M) over dot = 10(-6.12) M-. yr(-1), v(infinity) = 1400 km s(-1), and a clumping factor of D = 4. The elemental abundances by mass are X-He = 0.45, X-C = 0.52, X-N < 5 x 10(-5), X-O = 0.03, and X-Fe < 1 x 10(-4). The fluxes from the model stellar atmosphere were used to compute photoionization models of the nebula. All the available observations, within their error bars, were used to constrain these models. We find that the ionizing fluxes predicted by the stellar model are consistent with the fluxes needed by the photoionization model to reproduce the nebular emission, within the error margins. However, there are indications that the stellar model overestimates the number and hardness of Lyman continuum photons. The photoionization models imply a clumped density structure of the nebular material. The observed C III] lambda1909/C II lambda4267 line ratio implies the existence of carbon-rich clumps in the nebula. Such clumps are likely produced by stellar wind ejecta, possibly mixed with the nebular material. We discuss our results with regard to the stellar and nebular post-AGB evolution. The observed Fe-deficiency for the central star indicates that the material which is now visible on the stellar surface has been exposed to s-process nucleosynthesis during previous thermal pulses. The absence of nitrogen allows us to set an upper limit to the remaining H-envelope mass after a possible AGB final thermal pulse. Finally, we infer from the total amount of carbon detected in the nebula that the strong [WC] mass- loss may have been active only for a limited period during the post-AGB evolution

The frequency of giant arcs - highly distorted and strongly gravitationally lensed background galaxies - is a powerful test for cosmological models. Previous comparisons of arc statistics for the currently favored concordance cosmological model ( lambda cold dark matter [LCDM]) with observations have shown an apparently large discrepancy in underpredicting cluster arcs. We present new ray-shooting results, based on a high-resolution (1024(3) particles in a 320 h(-1) Mpc box) large-scale structure simulation normalized to the Wilkinson Microwave Anisotropy Probe (WMAP) observations. We follow light rays through a pseudo - three-dimensional matter distribution approximated by up to 38 lens planes and evaluate the occurrence of arcs for various source redshifts. We find that the frequency of strongly lensed background galaxies is a steep function of source redshift: the optical depth for giant arcs increases by a factor of 5 when background sources are moved from redshift z(s) = 1.0 to 1.5. This is a consequence of a small decrease of the critical surface mass density for lensing, combined with the very steep cluster mass function at the high-mass end plus a modest contribution from secondary lens planes. Our results are consistent with those of Bartelmann et al. if we - as they did - restrict all sources to be at z(s) = 1. If we allow sources extending to or beyond z(s) greater than or equal to 1.5, the apparent discrepancy vanishes: the frequency of arcs increases by about a factor of 10 as compared to previous estimates, and results in roughly one arc per 20 deg(2) over the sky, in good agreement with the observed frequency of arcs

Ab initio calculations have been carried out using the FP-APW+lo method in order to understand the atomic origin of the inverse piezoelectric effect in x-quartz. The external electric field was modelled by a saw-like potential V-ext in order to achieve translational symmetry within a supercell (SC) containing 72 atoms. The original trigonal quartz structure was repeated along the [110] direction, which corresponds to the direction of the external field. An electric field with 550 kV/mm was applied and the atomic positions of the SC were relaxed until the forces acting on the atoms vanished. In parts of the SC, V-ext changes almost linearly and thus the relaxed atomic positions can be used to determine the structural response due to the external electric field. The calculations provide the piezoelectric modulus of the correct order of magnitude. In contrast to previous models and in agreement with recent experimental results, the atomic origin of the piezoelectric effect can be described by a rotation of slightly deformed SiO4 tetrahedra against each other. The change of the Si-O bond lengths and the tetrahedral O-Si-O angles is one order of magnitude smaller than that of the Si-O-Si angles between neighbouring tetrahedra. The calculated changes of X-ray structure factors are in agreement with experiment when the theoretical data are extrapolated down to the much smaller field strength that is applied in the experiment (E < 10 kV/mm). (C) 2004 Elsevier Ltd. All rights reserved

Microlensing is the only known direct method to measure the masses of stars that lack visible companions. In terms of microlensing observables, the mass is given by M (c(2)/4G)(r) over tilde (E)theta(E) and so requires the measurement of both the angular Einstein radius theta(E) and the projected Einstein radius (r) over tilde (E). Simultaneous measurement of these two parameters is extremely rare. Here we analyze OGLE-2003-BLG-238, a spectacularly bright (I-min 10.3), high-magnification (A(max) 170) microlensing event. Pronounced finite-source effects permit a measurement of theta(E) = 650 muas. Although the timescale of the event is only t(E) 38 days, one can still obtain weak constraints on the microlens parallax: 4.4 AU < <(r)over tilde>(E) < 18 AU at the 1 σ level. Together these two parameter measurements yield a range for the lens mass of 0.36 M-&ODOT; < M < 1.48 M-&ODOT;. As was the case for MACHO- LMC-5, the only other single star (apart from the Sun) whose mass has been determined from its gravitational effects, this estimate is rather crude. It does, however, demonstrate the viability of the technique. We also discuss future prospects for single-lens mass measurements

Surface relief gratings on azobenzene containing polymer films were prepared under irradiation by actinic light. Finite element modeling of the inscription process was carried out using linear viscoelastic analysis. It was assumed that under illumination the polymer film undergoes considerable plastification, which reduces its original Young's modulus by at least three orders of magnitude. Force densities of about 10(11) N/m(3) were necessary to reproduce the growth of the surface relief grating. It was shown that at large deformations the force of surface tension becomes comparable to the inscription force and therefore plays an essential role in the retardation of the inscription process. In addition to surface profiling the gradual development of an accompanying density grating was predicted for the regime of continuous exposure. Surface grating development under pulselike exposure cannot be explained in the frame of an incompressible fluid model. However, it was easily reproduced using the viscoelastic model with finite compressibility. (C) 2004 American Institute of Physics