Institut für Physik
Refine
Has Fulltext
- no (1904)
Year of publication
Document Type
- Article (1552)
- Doctoral Thesis (236)
- Monograph/Edited Volume (110)
- Other (4)
- Review (2)
Is part of the Bibliography
- yes (1904)
Keywords
- CDA (1)
- Cassini (1)
- E-ring (1)
- anatomical connectivity (1)
- bifurcation analysis (1)
- biological robustness (1)
- computational biochemistry (1)
- cortical network (1)
- dust (1)
- dynamical cluster (1)
In July 2004 the Cassini–Huygens mission reached the Saturnian system and started its orbital tour. A total of 75 orbits will be carried out during the primary mission until August 2008. In these four years Cassini crosses the ring plane 150 times and spends approx. 400 h within Titan's orbit. The Cosmic Dust Analyser (CDA) onboard Cassini characterises the dust environment with its extended E ring and embedded moons. Here, we focus on the CDA results of the first year and we present the Dust Analyser (DA) data within Titan's orbit. This paper does investigate High Rate Detector data and dust composition measurements. The authors focus on the analysis of impact rates, which were strongly variable primarily due to changes of the spacecraft pointing. An overview is given about the ring plane crossings and the DA counter measurements. The DA dust impact rates are compared with the DA boresight configuration around all ring plane crossings between June 2004 and July 2005. Dust impacts were registered at altitudes as high as 100 000 km above the ring plane at distances from Saturn between 4 and 10 Saturn radii. In those regions the dust density of particles bigger than 0.5 can reach values of 0.001m-3.
To develop and investigate detailed mathematical models of metabolic processes is one of the primary challenges in systems biology. However, despite considerable advance in the topological analysis of metabolic networks, kinetic modeling is still often severely hampered by inadequate knowledge of the enzyme-kinetic rate laws and their associated parameter values. Here we propose a method that aims to give a quantitative account of the dynamical capabilities of a metabolic system, without requiring any explicit information about the functional form of the rate equations. Our approach is based on constructing a local linear model at each point in parameter space, such that each element of the model is either directly experimentally accessible or amenable to a straightforward biochemical interpretation. This ensemble of local linear models, encompassing all possible explicit kinetic models, then allows for a statistical exploration of the comprehensive parameter space. The method is exemplified on two paradigmatic metabolic systems: the glycolytic pathway of yeast and a realistic-scale representation of the photosynthetic Calvin cycle.
This paper discusses translocation features of the 20S proteasome in order to explain typical proteasome length distributions. We assume that the protein transport depends significantly on the fragment length with some optimal length which is transported most efficiently. By means of a simple one-channel model, we show that this hypothesis can explain both the one- and the three-peak length distributions found in experiments. A possible mechanism of such translocation is provided by so-called fluctuation-driven transport.
It is shown that several polymers can form insoluble interfacial layers on a poly (ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) layer after annealing of the double-layer structure. The thickness of the interlayer is dependent on the characteristics of the underlying PEDOT.PSS and the molecular weight of the polymers. It is further shown that the electronic structures of the interlayer polymers have a significant effect on the properties of red-light-emitting polymer-based electrophosphorescent devices. Upon increasing the highest occupied molecular orbital and lowest unoccupied molecular orbital positions, a significant increase in current density and device efficiency is observed. This is attributed to efficient blocking of electrons in combination with direct injection of holes from the interlayer to the phosphorescent dye. Upon proper choice of the interlayer polymer, efficient red, polymer-based electrophosphorescent devices with a peak luminance efficiency of 5.5 cd A(-1) (external quantum efficiency = 6 %) and a maximum power-conversion efficiency of 5 Im W-1 can be realized.
Recent research using the complex network approach has revealed a rich and complicated network topology in the cortical connectivity of mammalian brains. It is of importance to understand the implications of such complex network structures in the functional organization of the brain activities. Here we study this problem from the viewpoint of dynamical complex networks. We investigate synchronization dynamics on the corticocortical network of the cat by modeling each node (cortical area) of the network with a sub-network of interacting excitable neurons. We find that the network displays clustered synchronization behavior, and the dynamical clusters coincide with the topological community structures observed in the anatomical network. Our results provide insights into the relationship between the global organization and the functional specialization of the brain cortex.
We investigate the bifurcation structures in a two-dimensional parameter space (PS) of a parametrically excited system with two degrees of freedom both analytically and numerically. By means of the Renyi entropy of second order K-2, which is estimated from recurrence plots, we uncover that regions of chaotic behavior are intermingled with many complex periodic windows, such as shrimp structures in the PS. A detailed numerical analysis shows that, the stable solutions lose stability either via period doubling, or via intermittency when the parameters leave these shrimps in different directions, indicating different bifurcation properties of the boundaries. The shrimps of different sizes offer promising ways to control the dynamics of such a complex system.
How do diverse dynamical patterns arise from the topology of complex networks? We study synchronization dynamics in the cortical brain network of the cat, which displays a hierarchically clustered organization, by modeling each node (cortical area) with a subnetwork of interacting excitable neurons. We find that in the biologically plausible regime the dynamics exhibits a hierarchical modular organization, in particular, revealing functional clusters coinciding with the anatomical communities at different scales. Our results provide insights into the relationship between network topology and functional organization of complex brain networks.
Physiklehrer bestimmen durch die Gestaltung des Unterrichts und damit durch ihr professionelles Handeln maßgeblich mit, wie die individuellen Lernprozesse der Schüler zu Inhalten der Physik ablaufen. Für die Entwicklung ihrer professionellen Handlungskompetenz müssen zukünftige Physiklehrer einerseits physikalisches, physikdidaktisches und pädagogisches Wissen erwerben und andererseits motiviert sein, dieses Wissen auch anzuwenden. In ihrer Vorlesung geht Thorid Rabe der Frage nach, welche physikdidaktischen Kompetenzen Studierende im Rahmen der universitären Ausbildung erwerben sollten. Am Beispiel der Lehrveranstaltung "Physikalische Schulexperimente" zeigt sie, wie physikdidaktische Theorie und praktisches Lehrerhandeln aufeinander bezogen werden können. Zudem wird sie ein Forschungsprojekt vorstellen, das einem bisher vernachlässigten Aspekt professioneller Handlungskompetenz nachgeht, nämlich den domänenspezifischen Selbstwirksamkeitserwartungen - dem Zutrauen in sich selbst, als Physiklehrer angemessen und erfolgreich handeln zu können.
We have used polarized confocal Raman microspectroscopy and scanning near-field optical microscopy with a resolution of 60 nm to characterize photoinscribed grating structures of azobenzene doped polymer films on a glass support. Polarized Raman microscopy allowed determining the reorientation of the chromophores as a function of the grating phase and penetration depth of the inscribing laser in three dimensions. We found periodic patterns, which are not restricted to the surface alone, but appear also well below the surface in the bulk of the material. Near-field optical microscopy with nanoscale resolution revealed lateral two-dimensional optical contrast, which is not observable by atomic force and Raman microscopy.
Numerical simulation of the Antartic ice sheet and its dynamic response to external pertubations
(2012)
Electrically charged cellular polymer films can exhibit very high piezoelectric activity and are therefore more and more often employed in advanced electromechanical and electro-acoustical transducers. In this paper, we report an optimized sequence of steps for preparing such ferroelectrets from commercial nonvoided ploy(ethylene terephthalate) (PETP) films by means of foaming with CO2 biaxial mechanical stretching, controlled void inflation, and bipolar electric charging. The nonvoid PETP films foamed with supercritical CO2 at a suitably high pressure and subsequently annealed for stabilization. The cellular foam structure was further optimized by means of well controlled biaxial stretching in a commercial stretcher and sometimes subsequent inflamation in a pressure chamber. Bipolar electric charging of the internal voids was achieved through the application of high electric fields in an SF0 atmosphere. The new optimized PETP ferroelectric exhibit quite large piezoelectric coefficients up to almost 500 pCN(-1), for which unusually low elastic stiffness of only around 0.3 MPa are essential. The PETP foam ferroelectrics posses unclamped thickenss-extension resonance frequences between approximately 120 and 250 kHz, and are thus highly suitable for several established as well as novel ultrasonic-transductant applications.
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 study synchronization behavior in networks of coupled chaotic oscillators with heterogeneous connection degrees. Our focus is on regimes away from the complete synchronization state, when the coupling is not strong enough, when the oscillators are under the influence of noise or when the oscillators are nonidentical. We have found a hierarchical organization of the synchronization behavior with respect to the collective dynamics of the network. Oscillators with more connections (hubs) are synchronized more closely by the collective dynamics and constitute the dynamical core of the network. The numerical observation of this hierarchical synchronization is supported with an analysis based on a mean field approximation and the master stability function. (C) 2006 American Institute of Physics
Realistic networks display not only a complex topological structure, but also a heterogeneous distribution of weights in the connection strengths. Here we study synchronization in weighted complex networks and show that the synchronizability of random networks with a large minimum degree is determined by two leading parameters: the mean degree and the heterogeneity of the distribution of node's intensity, where the intensity of a node, defined as the total strength of input connections, is a natural combination of topology and weights. Our results provide a possibility for the control of synchronization in complex networks by the manipulation of a few parameters
Dynamical organization of connection weights is studied in scale-free networks of chaotic oscillators, where the coupling strength of a node from its neighbors develops adaptively according to the local synchronization property between the node and its neighbors. We find that when complete synchronization is achieved, the coupling strength becomes weighted and correlated with the topology due to a hierarchical transition to synchronization in heterogeneous networks. Importantly, such an adaptive process enhances significantly the synchronizability of the networks, which could have meaningful implications in the manipulation of dynamical networks
We present an approach to generate (multivariate) twin surrogates (TS) based on recurrence properties. This technique generates surrogates which correspond to an independent copy of the underlying system, i.e. they induce a trajectory of the underlying system starting at different initial conditions. We show that these surrogates are well suited to test for complex synchronisation and exemplify this for the paradigmatic system of Rossler oscillators. The proposed test enables to assess the statistical relevance of a synchronisation analysis from passive experiments which are typical in natural systems
The third-generation X-ray source BESSYII (Berlin, Germany) provides coherent X-ray radiation which can be used for static and dynamic speckle analysis. Recently we have demonstrated that one can perform experiments with coherent white radiation provided by a bending magnet (5 < E < 20 keV). In this paper we show that the diffraction figure of the initial pinhole must be considered for the interpretation of coherent experiments. The reflectivity spectrum of a sample results from the Fresnel diffraction of the incident pinhole deformed by the static speckle features of the sample surface. For dynamical experiments all speckle like features alter with time whereas the pure Fresnel fringes remain constant. (c) 2007 Published by Elsevier B.V.
Graphical models applying partial coherence to multivariate time series are a powerful tool to distinguish direct and indirect interdependencies in multivariate linear systems. We carry over the concept of graphical models and partialization analysis to phase signals of nonlinear synchronizing systems. This procedure leads to the partial phase synchronization index which generalizes a bivariate phase synchronization index to the multivariate case and reveals the coupling structure in multivariate synchronizing systems by differentiating direct and indirect interactions. This ensures that no false positive conclusions are drawn concerning the interaction structure in multivariate synchronizing systems. By application to the paradigmatic model of a coupled chaotic Roessler system, the power of the partial phase synchronization index is demonstrated
New ferroelectrets were developed on the basis of foams from cyclo-olefin polymers and copolymers. The results obtained on the cyclo-olefin polymer foam demonstrate a significant improvement of the service temperature for ferroelectret transducer materials. Suitable compounding and preparation led to cyclo-olefin ferroelectrets with an electromechanical activity of around 15 pC/N, which is thermally stable at least up to 110 degrees C. The properties in sensor and actuator applications are strongly dependent on the processing parameters related to film-making, sensor and actuator preparation, gas content and electric charging. The processing window for the film stretching was very narrow compared to the earlier developed polypropylene ferroelectrets. The film porosity, softness and thus the electromechanical activity are adjusted by gas-diffusion expansion. The activity of the electromechanically operating sensors and actuators was increased by stacking several layers of cellular cyclo-olefin film. For applications such as flat loudspeakers, the foamed films are tuned by tensioning them on a support frame. Correct tensioning was essential also for reducing the distortion levels.
The first application of a pnCCD detector for X-ray scattering experiments using white synchrotron radiation at BESSY II is presented. A Cd arachidate multilayer was investigated in reflection geometry within the energy range 7 keV < E < 35 keV. At fixed angle of incidence the two-dimensional diffraction pattern containing several multilayer Bragg peaks and respective diffuse-resonant Bragg sheets were observed. Since every pixel of the detector is able to determine the energy of every incoming photon with a resolution Delta E/E similar or equal to 10(-2). a three-dimensional dataset is finally obtained. In order to achieve this energy resolution the detector was operated in the so-called single-photon- counting mode. A full dataset was evaluated taking into account all photons recorded within 10(5) detector frames at a readout rate of 200 Hz. By representing the data in reciprocal-space coordinates, it becomes obvious that this experiment with the pnCCD detector provides the same information as that obtained by combining a large number of monochromatic scattering experiments using conventional area detectors.
The results of the theoretical consideration of stochastic resonance in overdamped bistable oscillators are given. These results are founded not on the model of two states as in [McNamara B, Wiesenfeld K. Theory of stochastic resonance. Phys Rev A 1989;39:4854-69], but on splitting of motion into regular and random and the rigorous solution of the Fokker-Planck equation for the random component. We show that this resonance is caused by a change, under the influence of noise, of the system's effective stiffness and damping factor contained in the equation for the regular component. For a certain value of the noise intensity the effective stiffness is minimal, and this fact causes non-monotonic change of the output signal amplitude as the noise intensity changes. It is important that the location of the minimum and its value depend essentially on the signal frequency.
We show a scenario of a two-frequeney torus breakdown, in which a global bifurcation occurs due to the collision of a quasi-periodic torus T-2 with saddle points, creating a heteroclinic saddle connection. We analyze the geometry of this torus-saddle collision by showing the local dynamics and the invariant manifolds (global dynamics) of the saddle points. Moreover, we present detailed evidences of a heteroclinic saddle-focus orbit responsible for the type- if intermittency induced by this global bifurcation. We also characterize this transition to chaos by measuring the Lyapunov exponents and the scaling laws.
Detuning-dependent dominance of oscillation death in globally coupled synthetic genetic oscillators
(2009)
We study dynamical regimes of globally coupled genetic relaxation oscillators in the presence of small detuning. Using bifurcation analysis, we find that under strong coupling via the slow variable, the detuning can eliminate standard oscillatory solutions in a large region of the parameter space, providing the dominance of oscillation death. This result is substantially different from previous results on oscillation quenching, where for homogeneous populations, the coexistence of oscillation death and limit cycle oscillations is always present. We propose further that this effect of detuning-dependent dominance could be a powerful regulator of genetic network's dynamics.
Agglomeration in a fluid flow, when collisions of aggregates with channel walls are important is analyzed. We assume the diffusion-limited mechanism for clusters growth and the Stokes' force exerted on the agglomerates from the flow. Collisions of the particles with the channel walls are modeled by a random Poisson process. We develop an analytical theory for the size distribution of the aggregates and check the theoretical predictions by Monte Carlo simulations. The numerical data agree well with the analytical results.
We investigate the sensitivity of a coarse resolution coupled climate model to the representation of the overflows over the Greenland-Scotland ridge. This class of models suffers from a poor representation of the water mass exchange between the Nordic Seas and the North Atlantic, a crucial part of the large-scale oceanic circulation. We revisit the explicit representation of the overflows using a parameterisation by hydraulic constraints and compare it with the enhancement of the overflow transport by artificially deepened passages over the Greenland-Scotland ridge, a common practice in coarse resolution models. Both configurations increase deep water formation in the Nordic Seas and represent the large-scale dynamics of the Atlantic realistically in contrast to a third model version with realistic sill depths but without the explicit overflow transport. The comparison of the hydrography suggests that for the unperturbed equilibrium the Nordic Seas are better represented with the parameterised overflows. As in previous studies, we do not find a stabilising effect of the overflow parameterisation on the Atlantic meridional overturning circulation but merely on the overflow transport. As a consequence the surface air temperature in the Nordic Seas is less sensitive to anomalous surface fresh water forcing. Special attention is paid to changes in the subpolar gyre circulation. We find it sensitive to the overflow transport and the density of these water masses through baroclinic adjustments. The analysis of the governing equations confirms the presence of positive feedbacks inherent to the subpolar gyre and allows us to isolate the influence of the overflows on its dynamics.
We perform a quantitative analysis of extensive chess databases and show that the frequencies of opening moves are distributed according to a power law with an exponent that increases linearly with the game depth, whereas the pooled distribution of all opening weights follows Zipf's law with universal exponent. We propose a simple stochastic process that is able to capture the observed playing statistics and show that the Zipf law arises from the self-similar nature of the game tree of chess. Thus, in the case of hierarchical fragmentation the scaling is truly universal and independent of a particular generating mechanism. Our findings are of relevance in general processes with composite decisions.