Institut für Physik und Astronomie
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Understanding the functional dynamics of the mammalian brain is one of the central aims of modern neuroscience. Mathematical modeling and computational simulations of neural networks can help in this quest. In recent publications, a multilevel model has been presented to simulate the resting-state dynamics of the cortico-cortical connectivity of the mammalian brain. In the present work we investigate how much of the dynamical behavior of the multilevel model can be reproduced by a strongly simplified model. We find that replacing each cortical area by a single Rulkov map recreates the patterns of dynamical correlation of the multilevel model, while the outcome of other models and setups mainly depends on the local network properties, e. g. the input degree of each vertex. In general, we find that a simple simulation whose dynamics depends on the global topology of the whole network is far from trivial. A systematic analysis of different dynamical models and coupling setups is required.
We show that Scattering Infrared Near-field Microscopy (SNIM) allows chemical mapping of polymer monolayers that can serve as designed nanostructured surfaces with specific surface chemistry properties on a nm scale. Using s- SNIM a minimum volume of 100 nm x 100 nm x 15 nm is sufficient for a recording of a "chemical'' IR signature which corresponds to an enhancement of at least four orders of magnitudes compared to conventional FT-IR microscopy. We could prove that even in cases where it is essentially difficult to distinguish between distinct polymer compositions based solely on topography, nanophase separated polymers can be clearly distinguished according to their characteristic near-field IR response.
We identify a large class of quantum many-body systems that can be solved exactly: natural frustration-free spin-1/2 nearest-neighbor Hamiltonians on arbitrary lattices. We show that the entire ground-state manifold of such models can be found exactly by a tensor network of isometries acting on a space locally isomorphic to the symmetric subspace. Thus, for this wide class of models, real-space renormalization can be made exact. Our findings also imply that every such frustration-free spin model satisfies an area law for the entanglement entropy of the ground state, establishing a novel large class of models for which an area law is known. Finally, we show that our approach gives rise to an ansatz class useful for the simulation of almost frustration-free models in a simple fashion, outperforming mean- field theory.
Stellar winds appear as a persistent feature of hot stars, irrespective of their wide range of different luminosities, masses, and chemical composition. Among the massive stars, the Wolf-Rayet types show considerably stronger mass loss than the O stars. Among hot low-mass stars, stellar winds are seen at central stars of planetary nebulae, where again the hydrogen-deficient stars show much stronger winds than those central stars with "normal" composition. We also studied mass-loss from a few extreme helium stars and sdOs. Their mass-loss rate roughly follows the same proportionality with luminosity to the power 1.5 as the massive O stars. This relation roughly marks a lower limit for the mass loss from hot stars of all kinds, and provides evidence that radiation pressure on spectral lines is the basic mechanism at work. For certain classes of stars the mass-loss rates lie significantly above this relation, for reasons that are not yet fully understood. Mass loss from low-mass stars may affect their evolution, by reducing the envelope mass, and can easily prevent diffusion from establishing atmospheric abundance patterns. In close binary systems, their winds can feed the accretion onto a companion.
Stochastic bifurcations and coherencelike resonance in a self-sustained bistable noisy oscillator
(2010)
We investigate the influence of additive Gaussian white noise on two different bistable self-sustained oscillators: Duffing-Van der Pol oscillator with hard excitation and a model of a synthetic genetic oscillator. In the deterministic case, both oscillators are characterized with a coexistence of a stable limit cycle and a stable equilibrium state. We find that under the influence of noise, their dynamics can be well characterized through the concept of stochastic bifurcation, consisting in a qualitative change of the stationary amplitude distribution. For the Duffing-Van der Pol oscillator analytical results, obtained for a quasiharmonic approach, are compared with the result of direct computer simulations. In particular, we show that the dynamics is different for isochronous and anisochronous systems. Moreover, we find that the increase of noise intensity in the isochronous regime leads to a narrowing of the spectral line. This effect is similar to coherence resonance. However, in the case of anisochronous systems, this effect breaks down and a new phenomenon, anisochronous-based stochastic bifurcation occurs.
We have developed and characterized a hard x-ray accumulating streak camera that achieves subpicosecond time resolution by using single-photon counting. A high repetition rate of 2 kHz was achieved by use of a readout camera with built-in image processing capabilities. The effects of sweep jitter were removed by using a UV timing reference. The use of single-photon counting allows the camera to reach a high quantum efficiency by not limiting the divergence of the photoelectrons.
Superexponential droplet fractalization as a hierarchical formation of dissipative compactons
(2010)
We study the dynamics of a thin film over a substrate heated from below in a framework of a strongly nonlinear one-dimensional Cahn-Hilliard equation. The evolution leads to a fractalization into smaller and smaller scales. We demonstrate that a primitive element in the appearing hierarchical structure is a dissipative compacton. Both direct simulations and the analysis of a self-similar solution show that the compactons appear at superexponentially decreasing scales, which means vanishing dimension of the fractal.
The surface morphology of icy moons is affected by several processes implicating exchanges between their subsurfaces and atmospheres (if any). The possible exchange of material between the subsurface and the surface is mainly determined by the mechanical properties of the lithosphere, which isolates the deep, warm and ductile ice material from the cold surface conditions. Exchanges through this layer occur only if it is sufficiently thin and/or if it is fractured owing to tectonic stresses, melt intrusion or impact cratering. If such conditions are met, cryomagma can be released, erupting fresh volatile-rich materials onto the surface. For a very few icy moons (Titan, Triton, Enceladus), the emission of gas associated with cryovolcanic activity is sufficiently large to generate an atmosphere, either long- lived or transient. For those moons, atmosphere-driven processes such as cryovolcanic plume deposition, phase transitions of condensable materials and wind interactions continuously re-shape their surfaces, and are able to transport cryovolcanically generated materials on a global scale. In this chapter, we discuss the physics of these different exchange processes and how they affect the evolution of the satellites' surfaces.
We investigate the role of surface plasmons in the electromagnetic Casimir effect at finite temperature, including situations out of global thermal equilibrium. The free energy is calculated analytically and expanded for different regimes of distances and temperatures. Similar to the zero-temperature case, the interaction changes from attraction to repulsion with distance. Thermal effects are shown to be negligible for small plate separations and at room temperature but become dominant and repulsive at large values of these parameters. In configurations out of global thermal equilibrium, we show that the selective excitation of surface plasmons can create a repulsive Casimir force between metal plates.
We have used space-charge limited current measurements to study the mobility of holes and electrons in two fluorene-based copolymers for temperatures from 100 to 300 K. Interpreting the results using the standard analytical model produced an Arrhenius-type temperature dependence for a limited temperature range only and mobility was found to be apparently dependent on the thickness of the polymer film. To improve on this, we have interpreted our data using a numerical model that takes into account the effects of the carrier concentration and energetic disorder on transport. This accounted for the thickness dependence and gave a more consistent temperature dependence across the full range of temperatures, giving support to the extended Gaussian disorder model for transport in disordered polymers. Furthermore, we find that the same model adequately describes both electron and hole transport without the need to explicitly include a distribution of electron traps. Room-temperature mobilities were found to be in the region of 4 x 10(-8) and 2 x 10(- 8) cm(2) V-1 s(-1) in the limit of zero field and zero carrier density with disorders of 110+/-10 and 100+/-10 meV for polymers poly{9,9-dioctylfluorene-co-bis[N,N'-(4-butylphenyl)]bis(N, N'-phenyl-1,4-phenylene)diamine} and poly(9,9-dioctylfluorene-co-benzothiadiazole), respectively.