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The application of the recently developed core doped ceramic Nd:YAG rods has the potential to provide better beam qualities compared to conventional rods since the hard aperture of the rod's boundary can be made wider while the width of the gain region remains the same. Thus, beam truncation and consequential diffraction can be reduced. We apply a finite elements model to calculate the resulting refractive index profiles in conventional and core doped rods. Propagating a Gaussian beam through both rod geometries the impact of aberrations and diffraction is compared for different side pumped scenarios. The potential advantage of the core doped geometry is discussed. (c) 2005 Optical Society of America
A pulsed, diode-laser-pumped Nd:YAG master oscillator power amplifier (MOPA) in rod geometry, frequency stabilized with a modified Pound-Drever-Hall scheme is presented. The apparatus delivers 33-ns pulses with a maximum pulse energy of 0.5 J at 1064 nm. The system was set up in two different configurations for repetition rates of 100 or 250 Hz. The beam quality was measured to be 1.5 times the diffraction limit at a pulse energy of 405 mJ and a repetition rate of 100 Hz. At 250 Hz with the same pulse energy, the M-2 was better than 2.1. The radiation is frequency converted with an efficiency of 50% to 532 nm. This MOPA system will be the pump laser of transmitters for a variety of high-end, scanning lidar systems. (C) 2005 Optical Society of America
We analyze the photometric data obtained by PLANET and OGLE on the caustic-crossing binary-lens microlensing event OGLE-2002-BLG-069. Thanks to the excellent photometric and spectroscopic coverage of the event, we are able to constrain the lens model up to the known ambiguity between close and wide binary lenses. The detection of annual parallax in combination with measurements of extended-source effects allows us to determine the mass, distance and velocity of the lens components for the competing models. While the model involving a close binary lens leads to a Bulge- Disc lens scenario with a lens mass of M = (0.51 ± 0.15) M-⊙ and distance of D-L = (2.9 ± 0.4) kpc, the wide binary lens solution requires a rather implausible binary black-hole lens ( M ≳ 126 M-⊙). Furthermore we compare current state-of-the-art numerical and empirical models for the surface brightness profile of the source, a G5III Bulge giant. We find that a linear limb-darkening model for the atmosphere of the source star is consistent with the data whereas a PHOENIX atmosphere model assuming LTE and with no free parameter does not match our observations
We present K band adaptive optics observations of three high-redshift ( z similar to 2.2) high-luminosity quasars, all of which were studied for the first time. We also observed several point spread function ( PSF) calibrators, non-simultaneously because of the small field of view. The significant temporal PSF variations on timescales of minutes inhibited a straightforward scaled PSF removal from the quasar images. Characterising the degree of PSF concentration by the radii encircling 20% and 80% of the total flux, respectively, we found that even under very different observing conditions the r(20) vs. r(80) relation varied coherently between individual short exposure images, delineating a well-defined relation for point sources. Placing the quasar images on this relation, we see indications that all three objects were resolved. We designed a procedure to estimate the significance of this result, and to estimate host galaxy parameters, by reproducing the statistical distribution of the individual short exposure images. We find in all three cases evidence for a luminous host galaxy, with a mean absolute magnitude of M-R = - 27.0 and scale lengths around similar to 4 - 12 kpc. Together with a rough estimate of the central black hole masses obtained from C.. line widths, the location of the objects on the bulge luminosity vs. black hole mass relation is not significantly different from the low-redshift regime, assuming only passive evolution of the host galaxy. Corresponding Eddington luminosities are L-nuc/L-Edd similar to 0.1 - 0.6
This paper is devoted to the digital processing of multicomponent seismograms using wavelet analysis. The goal of this processing is to identify Rayleigh surface elastic waves and determine their properties. A new method for calculating the ellipticity parameters of a wave in the form of a time-frequency spectrum is proposed, which offers wide possibilities for filtering seismic signals in order to suppress or extract the Rayleigh components. A model of dispersion and dissipation of elliptic waves written in terms of wavelet spectra of complex (two-component) signals is also proposed. The model is used to formulate a nonlinear minimization problem that allows for a high-accuracy calculation of the group and phase velocities and the attenuation factor for a propagating elliptic Rayleigh wave. All methods considered in the paper are illustrated with the use of test signals. (c) 2005 Pleiades Publishing, Inc
We study metal abundances in the z = 0.9313 damped Ly alpha system observed along the two lines of sight, A and B, toward the gravitationally lensed double QSO HE 0512-3329. Spatially resolved Space Telescope Imaging Spectrograph spectra constrain the neutral-gas column density to be N(H I) = 1020.5 cm(-2) in both A and B. UV-visual Echelle Spectrograph spectra ( spectral resolution FWHM = 9.8 km s(-1)) show, in contrast, significant line-of-sight differences in the column densities of Mn II and Fe II; these are not due to observational systematics. We find that [Mn/H] = -1.44 and [Fe/H] = -1.52 in damped Ly alpha system A, while [Mn/H] = -0.98 and [Fe/H] > -1.32, and possibly as high as [Fe/H] approximate to -1, in damped Ly alpha system B. A careful assessment of possible systematic errors leads us to conclude that these transverse differences are significant at a 5 sigma level or greater. Although nucleosynthesis effects may also be at play, we favor differential dust depletion as the main mechanism producing the observed abundance gradient. The transverse separation is 5 h(70)(-1) kpc at the redshift of the absorber, which is also likely to be the lensing galaxy. The derived abundances therefore probe two opposite sides of a single galaxy hosting both damped Ly alpha systems. This is the first time firm abundance constraints have been obtained for a single damped system probed by two lines of sight. The significance of this finding for the cosmic evolution of metals is discussed
Interaction of particles of dust with vortex convective flows is under theoretical consideration. It is assumed that the volume fraction of solid phase is small, variations of density due to nonuniform distribution of particles and those caused by temperature nonisothermality of medium are comparable. Equations for the description of thermal buoyancy convection of a dusty medium are developed in the framework of the generalized Boussinesq approximation taking into account finite velocity of particle sedimentation. The capture of a cloud of dust particles by a vortex convective flow is considered, general criterion for the formation of such a cloud is obtained. The peculiarities of a steady state in the form of a dust cloud and backward influence of the solid phase on the carrier flow are studied in detail for a vertical layer heated from the sidewalls. It is shown that in the case, when this backward influence is essential, a hysteresis behavior is possible. The stability analysis of the steady state is performed. It turns out that there is a narrow range of governing parameters, in which such a steady state is stable. (c) 2005 American Institute of Physics
Many complex networks display strong heterogeneity in the degree (connectivity) distribution. Heterogeneity in the degree distribution often reduces the average distance between nodes but, paradoxically, may suppress synchronization in networks of oscillators coupled symmetrically with uniform coupling strength. Here we offer a solution to this apparent paradox. Our analysis is partially based on the identification of a diffusive process underlying the communication between oscillators and reveals a striking relation between this process and the condition for the linear stability of the synchronized states. We show that, for a given degree distribution, the maximum synchronizability is achieved when the network of couplings is weighted and directed and the overall cost involved in the couplings is minimum. This enhanced synchronizability is solely determined by the mean degree and does not depend on the degree distribution and system size. Numerical verification of the main results is provided for representative classes of small-world and scale-free networks
We present a modern method used in nonlinear time series analysis to investigate the relation of two oscillating systems with respect to their phases, independently of their amplitudes. We study the difference of the phase dynamics between El Nino/Southern Oscillation (ENSO) and the Indian Monsoon on inter-annual time scales. We identify distinct epochs, especially two intervals of phase coherence, 1886 - 1908 and 1964 - 1980, corroborating earlier findings from a new point of view. A significance test shows that the coherence is very unlikely to be the result of stochastic fluctuations. We also detect so far unknown periods of coupling which are invisible to linear methods. These findings suggest that the decreasing correlation during the last decades might be a typical epoch of the ENSO/ Monsoon system having occurred repeatedly. The high time resolution of the method enables us to present an interpretation of how volcanic radiative forcing could cause the coupling
We study several algorithms to simulate bone mass loss in two-dimensional and three-dimensional computed tomography bone images. The aim is to extrapolate and predict the bone loss, to provide test objects for newly developed structural measures, and to understand the physical mechanisms behind the bone alteration. Our bone model approach differs from those already reported in the literature by two features. First, we work with original bone images, obtained by computed tomography (CT); second, we use structural measures of complexity to evaluate bone resorption and to compare it with the data provided by CT. This gives us the possibility to test algorithms of bone resorption by comparing their results with experimentally found dependencies of structural measures of complexity, as well as to show efficiency of the complexity measures in the analysis of bone models. For two-dimensional images we suggest two algorithms, a threshold algorithm and a virtual slicing algorithm. The threshold algorithm simulates bone resorption on a boundary between bone and marrow, representing an activity of osteoclasts. The virtual slicing algorithm uses a distribution of the bone material between several virtually created slices to achieve statistically correct results, when the bone-marrow transition is not clearly defined. These algorithms have been tested for original CT 10 mm thick vertebral slices and for simulated 10 mm thick slices constructed from ten I mm thick slices. For three-dimensional data, we suggest a variation of the threshold algorithm and apply it to bone images. The results of modeling have been compared with CT images using structural measures of complexity in two- and three-dimensions. This comparison has confirmed credibility of a virtual slicing modeling algorithm for two-dimensional data and a threshold algorithm for three-dimensional data
Climate variability is triggered by several solar and orbital cycles as well as by the intern ocean dynamics. Consequently, paleoclimate proxy records are expected to vary on very different time scales ranging from subdecadal to millennial duration. We demonstrate, that Foster's (Foster, 1996) wavelet analysis technique is an appropriate tool for investigating temporarily changing spectral properties of records characterized by awkward sampling quality, which is a typical feature of climate proxy records. By applying it to the Holocene part of different glaciochemical records of Greenland ice cores we proof evidence for a significant contribution of the 1.47 kiloyears cycle over alomst the entire Holocene
We consider the additivity of the minimal output entropy and the classical information capacity of a class of quantum channels. For this class of channels, the norm of the output is maximized for the output being a normalized projection. We prove the additivity of the minimal output Renyi entropies with entropic parameters alpha is an element of [ 0, 2], generalizing an argument by Alicki and Fannes, and present a number of examples in detail. In order to relate these results to the classical information capacity, we introduce a weak form of covariance of a channel. We then identify various instances of weakly covariant channels for which we can infer the additivity of the classical information capacity. Both additivity results apply to the case of an arbitrary number of different channels. Finally, we relate the obtained results to instances of bi-partite quantum states for which the entanglement cost can be calculated
In this work, we report our investigations on the film-forming properties as well as the optical and electroluminescent characterisations of a series of lateral-substituted soluble oligo(phenylenevinylenes) of various conjugation length. Preliminary investigations show that these materials are potential candidates for use in organic light-emitting devices (OLEDs). Two types of OLEDs were fabricated: single layer (SL) and single heterostructure (SHS), with poly(p-phenylenevinylene) (PPV) as hole transporting layer. Our best results were obtained with single layer device emitting green light with a luminance of 0.18 cd m(-2) and 0.24 cd m(-2) at a driving voltage of 10 V. (c) 2004 Elsevier B.V. All rights reserved
Complex dynamical systems with many degrees of freedom may exhibit a wealth of collective phenomena related to high-dimensional chaos. This paper focuses on a lattice of coupled logistic maps to investigate the relationship between the loss of chaos synchronization and the onset of shadowing breakdown via unstable dimension variability in complex systems. In the neighborhood of the critical transition to strongly non-hyperbolic behavior, the system undergoes on-off intermittency with respect to the synchronization manifold. This has been confirmed by numerical diagnostics of synchronization and non-hyperbolic behavior, the latter using the statistical properties of finite-time Lyapunov exponents. (c) 2005 Elsevier B.V. All rights reserved
We quantify the long-term predictability of global mean daily temperature data by means of the Renyi entropy of second order K-2. We are interested in the yearly amplitude fluctuations of the temperature. Hence, the data are low- pass filtered. The obtained oscillatory signal has a more or less constant frequency, depending on the geographical coordinates, but its amplitude fluctuates irregularly. Our estimate of K-2 quantifies the complexity of these amplitude fluctuations. We compare the results obtained for the CRU data set (interpolated measured temperature in the years 1901- 2003 with 0.5 degrees resolution, Mitchell et al., 2005(1)) with the ones obtained for the temperature data from a coupled ocean-atmosphere global circulation model (AOGCM, calculated at DKRZ). Furthermore, we compare the results obtained by means of K-2 with the linear variance of the temperature data
We present deep optical observations of the gravitational lens system CLASS B0218 + 357, from which we derive an estimate for the Hubble constant (H-0). Extensive radio observations using the VLA, MERLIN, the VLBA and VLBI have reduced the degeneracies between H-0 and the mass model parameters in this lens to one involving only the position of the radio-quiet lensing galaxy with respect to the lensed images. B0218 + 357 has an image separation of only 334 mas, so optical observations have, up until now, been unable to resolve the lens galaxy from the bright lensed images. Using the new Advanced Camera for Surveys (ACS), installed on the Hubble Space Telescope in 2002, we have obtained deep optical images of the lens system and surrounding field. These observations have allowed us to determine the separation between the lens galaxy centre and the brightest image, and so estimate H-0. We find an optical galaxy position, and hence an H0 value, that varies depending on our approach to the spiral arms in B0218 + 357. If the most prominent spiral arms are left unmasked, we find H-0 = 70 +/- 5 km s(-1) Mpc(-1) (95 per cent confidence). If the spiral arms are masked out, we find H-0 = 61 +/- 7 km s(-1) Mpc(-1) (95 per cent confidence)
Noise-sustained and controlled synchronization of stirred excitable media by external forcing
(2005)
Most of the previous studies on constructive effects of noise in spatially extended systems have focused on static media, e.g., of the reaction diffusion type. Because many active chemical or biological processes occur in a fluid environment with mixing, we investigate here the interplay among noise, excitability, mixing and external forcing in excitable media advected by a chaotic flow, in a two-dimensional FitzHugh-Nagumo model described by a set of reaction- advection-diffusion equations. In the absence of external forcing, noise may generate sustained coherent oscillations of the media in a range of noise intensities and stirring rates. We find that these noise-sustained oscillations can be synchronized by external periodic signals much smaller than the threshold. Analysis of the locking regions in the parameter space of the signal period, stirring rate and noise intensity reveals that the mechanism underlying the synchronization behaviour is a matching between the time scales of the forcing signal and the noise-sustained oscillations. The results demonstrate that, in the presence of a suitable level of noise, the stirred excitable media act as self-sustained oscillatory systems and become much easier to be entrained by weak external forcing. Our results may be verified in experiments and are useful to understand the synchronization of population dynamics of oceanic ecological systems by annual cycles
We show that realistic aftershock sequences with space-time characteristics compatible with observations are generated by a model consisting of brittle fault segments separated by creeping zones. The dynamics of the brittle regions is governed by static/kinetic friction, 3D elastic stress transfer and small creep deformation. The creeping parts are characterized by high ongoing creep velocities. These regions store stress during earthquake failures and then release it in the interseismic periods. The resulting postseismic deformation leads to aftershock sequences following the modified Omori law. The ratio of creep coefficients in the brittle and creeping sections determines the duration of the postseismic transients and the exponent p of the modified Omori law
We consider networks of chaotic maps with different network topologies. In each case, they are coupled in such a way as to generate synchronized chaotic solutions. By using the methods of control of chaos we are controlling a single map into a predetermined trajectory. We analyze the reaction of the network to such a control. Specifically we show that a line of one-dimensional logistic maps that are unidirectionally coupled can be controlled from the first oscillator whereas a ring of diffusively coupled maps cannot be controlled for more than 5 maps. We show that rings with more elements can be controlled if every third map is controlled. The dependence of unidirectionally coupled maps on noise is studied. The noise level leads to a finite synchronization lengths for which maps can be controlled by a single location. A two-dimensional lattice is also studied. (C) 2005 American Institute of Physics
Recently it has been shown that lateral carrier confinement in an InGaAs quantum well (QW) embedded in GaAs can be achieved by using a laterally patterned InGaP stressor layer on top of the heterostructure. To exploit this effect in a device the structure has to be planarized by a second epitaxial step. It has been shown that the lateral strain modulation almost vanishes after overgrowth with GaAs, whereas overgrowth with a single ternary layer of opposite strain compared to the stressor layer suffers from strain induced decomposition. Here we show that the lateral carrier confinement of the initially free standing nanostructure can almost be maintained using a two step process for overgrowth, where a strained thin ternary layer is grown first followed by GaAs up to complete planarization of the patterned structure. Thickness and composition of the ternary layer are adjusted on the basis of finite element calculations of the strain distribution (FEM). The strain field achieved after overgrowth is probed by X-ray grazing- incidence diffraction (GID). (c) 2005 Elsevier B.V. All rights reserved
A series of novel arylene ladder polymers incorporating conjugated 1,5- and 2,6-naphthylene building blocks were synthesized. The polyketone ladder polymer precursors were prepared via a palladium-mediated Suzuki-type cross- coupling reaction using both conventional and microwave heating. While the 2,6-naphthylene polyketone precursor (2,6- NPK) was accessible from both heating protocols. the 1,5-naphthylene linked polyketones were only accessible via microwave-assisted (uW) procedures. and the polymer 1,5-NLP2 is the first example of a ladder polymer consisting exclusively of alternating six-membered rings prepared from this reaction sequence. The solution optical spectra of the final naphthylene ladder polymers exhibit the characteristic spectral shapes with a steep absorption edge and a vibronic fine structure common to fully rigidified ladder structures. The structural modification of the substitution pattern and the linkage positions at the naphthylene unit allows some tuning of the absorption and emission bands of the ladder polymers. The 2,6-naphthylene derivative exhibits an intense blue photoluminescence, while the two 1,5-naphthylene- linked ladder polymers exhibit a red shift of the 0-0 electronic transitions, leading to a blue-green photoluminescence
We combine sensitivity to atomic number, chemical shifts, probing depth, and magnetic order in a field- dependent magnetic circular X-ray dichroism study at the Mn L-edge of the diluted ferromagnetic semiconductor Ga1-xMnxAs and observe different Mn constituents: ferromagnetic Mn with an n(d) > 5 lineshape and paramagnetic Mn with distinct n(d) = 5 lineshape. The paramagnetic Mn is assigned to interstitials with surface segregation tendency. (c) 2005 Elsevier B.V. All rights reserved
Results of the combined investigation of atomic and electronic structure of the W(110)/C-R(15x3) surface carbide are reported. A variety of experimental techniques has been involved such as scanning tunneling microscopy (STM), low-energy electron diffraction, x-ray photoelectron spectroscopy, and angle-resolved photoemission (ARPES). Distance-dependent STM measurements show a nontrivial geometrical behavior in the topography data, demonstrating five different patterns representing the superstructure at different values of the tip-surface separation. Atomic resolution was achieved at lower tunneling gap resistance. An unexpected spatial asymmetry in the distribution of the local density of states across the surface unit cell has been observed as well. Photoelectron spectroscopy of C1s and W4f core levels clarifies the nature of the chemical bonding in the system. The band mapping with ARPES provides information on the wave- vector dependence of the electronic states. Notable quantum size and superlattice effects were discovered in the dispersion of the valence-band states. The experimental data suggests an apparent one-dimensional character of the electronic structure. Lateral quantization and umklapp scattering are proposed as explanation. Finally, based on photoemission and STM measurements, an improved crystallographic model of the tungsten surface carbide is introduced
The ground state electronic properties of the strongly correlated transition metal Ni are usually not accessible from the excitation spectra measured in photoelectron spectroscopy. We show that the bottom of the Ni d band along [111] can be probed through the energy dependence of the phase of quantum-well states in Ag/Ni(111). Our model description of the quantum-well energies measured by angle-resolved photoemission determines the bottom of the Lambda(1) d band of Ni as 2.6 eV, in full agreement with standard local density theory and at variance with the values of 1.7-1.8 eV from direct angle-resolved photoemission experiments of Ni
We present an excerpt of the document "Quantum Information Processing and Communication: Strategic report on current status, visions and goals for research in Europe", which has been recently published in electronic form at the website of FET (the Future and Emerging Technologies Unit of the Directorate General Information Society of the European Commission, http://www.cordis.lu/ist/fet/qipc-sr.htm). This document has been elaborated, following a former suggestion by FET, by a committee of QIPC scientists to provide input towards the European Commission for the preparation of the Seventh Framework Program. Besides being a document addressed to policy makers and funding agencies (both at the European and national level), the document contains a detailed scientific assessment of the state-of-the-art, main research goals, challenges, strengths, weaknesses, visions and perspectives of all the most relevant QIPC sub-fields, that we report here
We discuss the influence of the material type in metal wires to the electromagnetic fluctuations in magnetic microtraps close to the surface of an atom chip. We show that significant reduction of the magnetic noise can be achieved by replacing the pure noble metal wires with their dilute alloys. The alloy composition provides an additional degree of freedom which enables a, controlled reduction of both magnetic noise and resistivity if the atom chip is cooled. In addition, we provide a careful re-analysis of the magnetically induced trap loss observed by Yu-Ju Lin et al. [Phys. Rev. Lett. 92 050404 (2004)] and find good agreement with an improved theory
We investigate the lifetime of magnetically trapped atoms above a planar, layered atom chip structure. Numerical calculations of the thermal magnetic noise spectrum are performed, based on the exact magnetic Green function and multi layer reflection coefficients. We have performed lifetime measurements where the center of a side guide trap is laterally shifted with respect to the current carrying wire using additional bias fields. Comparing the experiment to theory, we find a fair agreement and demonstrate that for a chip whose topmost layer is metallic, the magnetic noise depends essentially on the thickness of that layer, as long as the layers below have a, much smaller conductivity; essentially the same magnetic noise would be obtained with a metallic membrane suspended in vacuum. Based on our theory we give general scaling laws of how to reduce the effect of surface magnetic noise on the trapped atoms
The existing optical microscopes form an image by collecting photons emitted from an object. Here we report on the experimental realization of microscopy without the need for direct optical communication with the sample. To achieve this, we have scanned a single gold nanoparticle acting as a nanoantenna in the near field of a sample and have studied the modification of its intrinsic radiative properties by monitoring its plasmon spectrum
We develop an effective low-frequency theory of the electromagnetic field in equilibrium with thermal objects. The aim is to compute thermal magnetic noise spectra close to metallic microstructures. We focus on the limit where the material response is characterised by the electric conductivity. At the boundary between empty space and metallic microstructures, a large jump occurs in the dielectric function which leads to a partial screening of low-frequency magnetic fields generated by thermal current fluctuations. We resolve a, discrepancy between two approaches used in the past to compute magnetic field noise spectra close to microstructured materials
We establish strict upper limits for the Casimir interaction between multilayered structures of arbitrary dielectric or diamagnetic materials. We discuss the appearance of different power laws due to frequency-dependent material constants. Simple analytical expressions are in good agreement with numerical calculations based on Lifshitz theory. We discuss the improvements required for current ( meta) materials to achieve a repulsive Casimir force
We consider the effect of external noise on the dynamics of limit cycle oscillators. The Lyapunov exponent becomes negative under influence of small white noise, what means synchronization of two or more identical systems subject to common noise. We analytically study the effect of small nonidentities in the oscillators and in the noise, and derive statistical characteristics of deviations from the perfect synchrony. Large white noise can lead to desynchronization of oscillators, provided they are nonisochronous. This is demonstrated for the Van der Pol-Duffing system
We study the stability of self-sustained oscillations under the influence of external noise. For small-noise amplitude a phase approximation for the Langevin dynamics is valid. A stationary distribution of the phase is used for an analytic calculation of the maximal Lyapunov exponent. We demonstrate that for small noise the exponent is negative, which corresponds to synchronization of oscillators. (c) 2004 Elsevier B.V. All rights reserved
We study the random-field Ising chain in the limit of strong exchange coupling. In order to calculate the free energy we apply a continuous Langevin-type approach. This continuous model can be solved exactly, whereupon we are able to locate the crossover between an exponential and a power-law decay of the free energy with increasing coupling strength. In terms of magnetization, this crossover restricts the validity of the linear scaling. The known analytical results for the free energy are recovered in the corresponding limits. The outcomes of numerical computations for the free energy are presented, which confirm the results of the continuous approach. We also discuss the validity of the replica method which we then utilize to investigate the sample-to-sample fluctuations of the finite size free energy
Thin azobenzene polymer films show a very unusual property, namely optically induced material transport. The underlying physics for this phenomenon has not yet been thoroughly explained. Nevertheless, this effect enables one to inscribe different patterns onto film surfaces, including one- and two-dimensional periodic structures. Typical sizes of such structures are of the order of micrometers, i.e. related to the interference pattern made by the laser used for optical excitation. In this study we have measured the mechanical properties of one- and two-dimensional gratings, with a high lateral resolution, using force-distance curves and pulse force mode of the atomic force microscope. We also report on the generation of considerably finer structures, with a typical size of 100 nm, which were inscribed onto the polymer surface by the tip of a scanning near-field optical microscope used as an optical pen. Such inscription not only opens new application possibilities but also gives deeper insight into the fundamentals physics underlying optically induced material transport
We present a new method to detect phase as well as generalized synchronization in a wide class of complex systems. It is based on the recurrences of the system's trajectory to the neighborhood of a former state in phase space. We illustrate the applicability of the algorithm for the paradigmatic chaotic Rossler system in the funnel regime and for noisy data, where other methods to detect phase synchronization fail. Furthermore, we demonstrate for electrochemical experiments that the method can easily detect phase and generalized synchronization in non-phase- coherent and even non-stationary time series
The evolution of X-ray emission from young massive star clusters is modelled, taking into account the emission from the stars as well as from the cluster wind. It is shown that the level and character of the soft (0.2-10 keV) X-ray emission change drastically with cluster age and are tightly linked with stellar evolution. Using the modem X-ray observations of massive stars, we show that the correlation between bolometric and X-ray luminosity known for single O stars also holds for O + O and (Wolf-Rayet) WR + O binaries. The diffuse emission originates from the cluster wind heated by the kinetic energy of stellar winds and supernova explosions. To model the evolution of the cluster wind, the mass and energy yields from a population synthesis are used as input to a hydrodynamic model. It is shown that in a very young cluster the emission from the cluster wind is low. When the cluster evolves, WR stars are formed. Their strong stellar winds power an increasing X-ray emission of the cluster wind. Subsequent supernova explosions pump the level of diffuse emission even higher. Clusters at this evolutionary stage may have no X-ray-bright stellar point sources, but a relatively high level of diffuse emission. A supernova remnant may become a dominant X-ray source, but only for a short time interval of a few thousand years. We retrieve and analyse Chandra and XMM-Newton observations of six massive star clusters located in the Large Magellanic Cloud (LMC). Our model reproduces the observed diffuse and point-source emission from these LMC clusters, as well as from the Galactic clusters Arches, Quintuplet and NGC 3603
Recent experiments using time- and angle-resolved two-photon photoemission (2PPE) spectroscopy at metal/polar adsorbate interfaces succeeded in time-dependent analysis of the process of electron solvation. A fully quantum mechanical, two-dimensional simulation of this process, which explicitly includes laser excitation, is presented here, confirming the origin of characteristic features, such as the experimental observation of an apparently negative dispersion. The inference of the spatial extent of the localized electron states from the angular dependence of the 2PPE spectra has been found to be non-trivial and system-dependent. (C) 2005 American Institute of Physics
In the light of recent intensity-voltage low energy electron diffraction (LEED-IV) experiments [Surf. Sci. 316, 92 (1994); Surf. Rev. Lett. 10, 487 (2003)], the electronic and geometric structure of a water bilayer adsorbed at the Ru(0001) surface are investigated through first-principles total energy calculations, using periodic slab geometries and gradient-corrected density functional theory (DFT). We consider five possible bilayer structures, all roughly consistent with the LEED-IV analysis (three intact structures and two half-dissociated), and a water single layer at Ru(0001). Adsorption energies and substrate-adsorbate geometry parameters are given and discussed in the light of the experiments. We also give a comparative analysis of the electron density redistribution (Delta rho) and of the dipole moment change (Delta mu) induced by water adsorption on the Ru(0001) surface. In agreement with Feibelman [Science 295, 99 (2002)], the half-dissociated structures are found to be more stable than the intact ones, and their adsorption geometries in better agreement with the LEED-IV data. However, the Delta rho analysis shows that a half-dissociated structure induces a Delta mu>0, which would be incompatible with the experimentally measured decrease of the work function following bilayer adsorption; the latter would be consistent, instead, with the Delta mu < 0 induced by the intact structures. It is the aim of this paper to compare various possible adsorption structures, most of them already considered previously, with one and the same method. For this purpose, thick slabs and restrictive computational parameters are chosen to generally address the accuracy and the limits of DFT in reproducing adsorption energies and bond lengths of water-metal interacting systems
We present calculations on the deformation of two- and three-layer electret systems. The electrical field is coupled with the stress-strain equations by means of the Maxwell stress tensor. In the simulations, two-phase systems are considered, and intrinsic relative dielectric permittivity and Young's modulus of the phases are altered. The numerically calculated electro-mechanical activity is compared to an analytical expression. Simulations are performed on two- and three-layer systems. Various parameters in the model are systematically varied and their influence on the resulting piezoelectricity is estimated. In three-layer systems with bipolar charge, the piezoelectric coefficients exhibit a strong dependence on the elastic moduli of the phases. However, with mono-polar charge, there is no significant piezoelectric effect. A two-dimensional simulation illustrated that higher piezoelectric coefficients can be obtained for non-uniform surface charges and low Poisson's ratio of phases. Irregular structures considered exhibit low piezoelectric activity compared to two-layer structures. (C) 2004 Elsevier B.V. All rights reserved
Cellular polypropylene (PP) films were treated with sulfur hexafluoride (SF6) gas in order to study the SF6 penetration behaviour and optimize the electric charging conditions. There were differences in the penetration of SF6 for different cellular PP materials, depending on the microscopic properties, which manifest themselves in the voided structure as well as in the mechanical stiffnesses of the cellular films. The penetration of SF6 after long-term pressure treatment is confirmed in strongly inflated cellular PP films with a low mechanical stiffness of about 1 MPa. No SF6 penetration occurs for slightly inflated cellular PP films with smaller void sizes and higher mechanical stiffnesses of around 5.8 MPa. The observed thickness variations, the higher charging fields during corona charging because of SF6 penetration and the SF6 environment, as well as the resulting electromechanical properties are discussed
Space-charge depth profiles in various electret polymers have been measured in both the time and the frequency domain using thermal pulses and waves, respectively. A comparison of the two techniques on corona-charged polytetrafluoroethylene showed that the thermal-pulse method yielded similar results as the thermal-wave technique, but approximately 20-50 times faster. The article discusses sensitivity limitations as well as possible applications, including the real-time monitoring of space-charge decay under UV irradiation. (C) 2005 American Institute of Physics
High-resolution, large-area three-dimensional mapping of polarization profiles in electret polymers was carried out by means of a fast thermal pulse technique with a focused laser beam. A lateral resolution of 38 mu m and a near- surface depth resolution of less than 0.5 mu m was achieved. At larger depths, fast thermal diffusion in the metal electrode rather than the laser spot size becomes the limiting factor for the lateral resolution. (c) 2005 American Institute of Physics
Piezoelectric cellular polypropylene films, so-called ferroelectrets, are assembled in a stack with two active transducer layers. The stack is characterized with respect to its linear and quadratic response in a frequency range from 1 kHz to 80 kHz. A relatively smooth frequency response in the sound-pressure level is found for the individual layers as well as for both layers driven in phase. The piezoelectric response of the two-layer stack is twice the response of an individual layer over a rather broad frequency range. Furthermore, the influence of the preparation conditions on the resonance frequency and the effect of the quadratic distortion on the radiated sound are investigated both for the individual transducer films in the stack and for the stack system as a whole
In this paper, a recently developed numerical method to analyze dielectric-spectroscopy data is applied to alpha-phase polyvinylidene fluoride (PVDF). The numerical procedure is non-parametric and does not contain any of the extensively used empirical formulas mentioned in the literature. The method basically recovers the unknown distribution of relaxation times of the generalized dielectric function representation by simultaneous application of the Monte Carlo integration method and of the constrained least-squares optimization. The relaxation map constructed after the numerical analysis is compared to a-phase PVDF data presented in the literature and results of the parametric analysis with a well- known empirical formula. (c) 2005 Elsevier B.V. All rights reserved
The conductivity of alpha-polyvinylidene fluoride is obtained from dielectric measurements performed in the frequency domain at several temperatures. At temperatures above the glass-transition, the conductivity can be interpreted as an ionic conductivity, which confirms earlier results reported in the literature. Our investigation shows that the observed ionic conductivity is closely related to the amorphous phase of the polymer. (C) 2005 American Institute of Physics