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Search Results: 1 - 10 of 5038 matches for " Helmut Kr?ger "
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Universality in Statistical Measures of Trajectories in Classical Billiard Systems  [PDF]
Jean-Fran?ois Laprise, Ahmad Hosseinizadeh, Helmut Krger
Applied Mathematics (AM) , 2015, DOI: 10.4236/am.2015.68132
Abstract: For classical billiards, we suggest that a matrix of action or length of trajectories in conjunction with statistical measures, level spacing distribution and spectral rigidity, can be used to distinguish chaotic from integrable systems. As examples of 2D chaotic billiards, we considered the Bunimovich stadium billiard and the Sinai billiard. In the level spacing distribution and spectral rigidity, we found GOE behaviour consistent with predictions from random matrix theory. We studied transport properties and computed a diffusion coefficient. For the Sinai billiard, we found normal diffusion, while the stadium billiard showed anomalous diffusion behaviour. As example of a 2D integrable billiard, we considered the rectangular billiard. We found very rigid behaviour with strongly correlated spectra similar to a Dirac comb. These findings present numerical evidence for universality in level spacing fluctuations to hold in classically integrable systems and in classically fully chaotic systems.
The Massive Schwinger Model - a Hamiltonian Lattice Study in a Fast Moving Frame
Helmut Krger,Norbert Scheu
Physics , 1998, DOI: 10.1016/S0370-2693(98)00449-3
Abstract: We present a non-perturbative study of the massive Schwinger model. We use a Hamiltonian approach, based on a momentum lattice corresponding to a fast moving reference frame, and equal time quantization. We present numerical results for the mass spectrum of the vector and scalar particle. We find good agreement with chiral perturbation theory in the strong coupling regime and also with other non-perturbative studies (Hamer et al., Mo and Perry) in the non-relativistic regime. The most important new result is the study of the $\theta$-action, and computation of vector and scalar masses as a function of the $\theta$-angle. We find excellent agreement with chiral perturbation theory. Finally, we give results for the distribution functions. We compare our results with Bergknoff's variational study from the infinite momentum frame in the chiral region.
Nonequilibrium Superconductor-Normal Metal Tunnel Contact and the Phonon Deficit Effect
Gurgen Melkonyan,Helmut Krger,Armen M. Gulian
Physics , 1999, DOI: 10.1016/S0921-4526(99)02823-9
Abstract: We consider tunnel microrefrigerators at low temperature. There is a number of experimental studies performed on microrefrigeration in tunneling superconductor--normal metal (SN) structures. Related to these experiments, only the electron subsystem has been considered theoretically. Independently, the phonon deficit effect has been studied a while ago in superconductor-superconductor tunnel junctions. It can be regarded as a possible prototype scheme for superconducting microrefrigerators. We try to provide the missing link between experiments on the SN tunnel junction refrigerators and the theory which includes microscopically phonons in combination with the mechanism of the phonon deficit effect.
Modeling extracellular field potentials and the frequency-filtering properties of extracellular space
Claude Bédard,Helmut Krger,Alain Destexhe
Physics , 2003, DOI: 10.1016/S0006-3495(04)74250-2
Abstract: Extracellular local field potentials (LFP) are usually modeled as arising from a set of current sources embedded in a homogeneous extracellular medium. Although this formalism can successfully model several properties of LFPs, it does not account for their frequency-dependent attenuation with distance, a property essential to correctly model extracellular spikes. Here we derive expressions for the extracellular potential that include this frequency-dependent attenuation. We first show that, if the extracellular conductivity is non-homogeneous, there is induction of non-homogeneous charge densities which may result in a low-pass filter. We next derive a simplified model consisting of a punctual (or spherical) current source with spherically-symmetric conductivity/permittivity gradients around the source. We analyze the effect of different radial profiles of conductivity and permittivity on the frequency-filtering behavior of this model. We show that this simple model generally displays low-pass filtering behavior, in which fast electrical events (such as Na$^+$-mediated action potentials) attenuate very steeply with distance, while slower (K$^+$-mediated) events propagate over larger distances in extracellular space, in qualitative agreement with experimental observations. This simple model can be used to obtain frequency-dependent extracellular field potentials without taking into account explicitly the complex folding of extracellular space.
Efficacy of Synaptic Inhibition Depends on Multiple, Dynamically Interacting Mechanisms Implicated in Chloride Homeostasis
Nicolas Doyon,Steven A. Prescott,Annie Castonguay,Antoine G. Godin,Helmut Krger,Yves De Koninck
PLOS Computational Biology , 2011, DOI: 10.1371/journal.pcbi.1002149
Abstract: Chloride homeostasis is a critical determinant of the strength and robustness of inhibition mediated by GABAA receptors (GABAARs). The impact of changes in steady state Cl? gradient is relatively straightforward to understand, but how dynamic interplay between Cl? influx, diffusion, extrusion and interaction with other ion species affects synaptic signaling remains uncertain. Here we used electrodiffusion modeling to investigate the nonlinear interactions between these processes. Results demonstrate that diffusion is crucial for redistributing intracellular Cl? load on a fast time scale, whereas Cl?extrusion controls steady state levels. Interaction between diffusion and extrusion can result in a somato-dendritic Cl? gradient even when KCC2 is distributed uniformly across the cell. Reducing KCC2 activity led to decreased efficacy of GABAAR-mediated inhibition, but increasing GABAAR input failed to fully compensate for this form of disinhibition because of activity-dependent accumulation of Cl?. Furthermore, if spiking persisted despite the presence of GABAAR input, Cl? accumulation became accelerated because of the large Cl? driving force that occurs during spikes. The resulting positive feedback loop caused catastrophic failure of inhibition. Simulations also revealed other feedback loops, such as competition between Cl? and pH regulation. Several model predictions were tested and confirmed by [Cl?]i imaging experiments. Our study has thus uncovered how Cl? regulation depends on a multiplicity of dynamically interacting mechanisms. Furthermore, the model revealed that enhancing KCC2 activity beyond normal levels did not negatively impact firing frequency or cause overt extracellular K? accumulation, demonstrating that enhancing KCC2 activity is a valid strategy for therapeutic intervention.
Constraining Forces Causing the Meissner Effect  [PDF]
Ekkehard Krüger
Journal of Modern Physics (JMP) , 2017, DOI: 10.4236/jmp.2017.88074
Abstract: As shown in former papers, the nonadiabatic Heisenberg model presents a novel mechanism of Cooper pair formation which is not the result of an attractive electron-electron interaction but can be described in terms of quantum mechanical constraining forces. This mechanism operates in narrow, roughly half-filled superconducting bands of special symmetry and is evidently responsible for the formation of Cooper pairs in all superconductors. Here we consider this new mechanism within an outer magnetic field. We show that in the magnetic field the constraining forces produce Cooper pairs of non-vanishing total momentum with the consequence that an electric current flows within the superconductor. This current satisfies the London equations and, consequently, leads to the Meissner effect. This theoretical result is confirmed by the experimental observation that all superconductors, whether conventional or unconventional, exhibit the Meissner effect.
Mast Cells: The Key to Multiple Sclerosis?  [PDF]
Per G?ran Krüger
World Journal of Neuroscience (WJNS) , 2014, DOI: 10.4236/wjns.2014.42014

Mast cells are present in high numbers in the border-zones of the multiple sclerosis-plaques. They are located in small clusters along capillaries and venules, and they are more abundant in females than in men. Mast cells can be stimulated to release specific mediators such as histamine, resulting in oedema formation, as well as proteases that may cause demyelination, by several different activation mechanisms. We hypothesize that a putative mast cell activation may be induced by diet factor(s) as well as long lasting mental stress that may lead to the release of catestatin, as well as ACTH released from the pituitary gland. Given a natural flux of mast cell recovery and activation, a putative phenomenon of massive release of mediators and “silent” reload periods may explain the relapsing-remitting phases of multiple sclerosis.

Investigation of the Hyperfine Structure of Atomic Niobium (Nb I) Spectral Lines Based on the Lower Energy Levels at 22936, 23010, and 23048 cm-1  [PDF]
L. Windholz, S. Krger
Spectral Analysis Review (SAR) , 2018, DOI: 10.4236/sar.2018.62004
Abstract: The hyperfine (hf) structure constants of three atomic niobium energy levels in the energy range around 23000 cm-1 (at 22936.90, 23010.58, and 23048.58 cm-1) are known with only limited accuracy, and the constants of combining levels are sometimes even unknown. Thus we performed laser spectroscopic investigations in the wavelength range between 5600 and 6500 Å, and we excited altogether 16 transitions in which these lower levels are involved. Beside a more precise determination of the hf structure constants of the three lower levels (which were determined on several lines sharing a common upper level), these experiments led to the knowledge of the hf constants of nine levels with previously unknown constants. Beside these results, also the hf constants of 13 further energy levels are reported. For six of these levels, the constants were previously unknown.
Multiple Sclerosis: A Mast Cell Mediated Psycho-Somatic Disease?  [PDF]
Per G?ran Krüger
World Journal of Neuroscience (WJNS) , 2018, DOI: 10.4236/wjns.2018.84035
Abstract: This paper reviews evidence that the presence of mast cells in specific sites of central nervous system, suggesting inflammatory processes, may explain all the symptoms observed in multiple sclerosis. This hypothesis would be relatively easy to test.
Improved lattice QCD with quarks: the 2 dimensional case
Jun-Qin Jiang,Xiang-Qian Luo,Zhong-Hao Mei,Hamza Jirari,Helmut Kr{?}ger,Chi-Min Wu
Physics , 1998, DOI: 10.1103/PhysRevD.60.014501
Abstract: QCD in two dimensions is investigated using the improved fermionic lattice Hamiltonian proposed by Luo, Chen, Xu, and Jiang. We show that the improved theory leads to a significant reduction of the finite lattice spacing errors. The quark condensate and the mass of lightest quark and anti-quark bound state in the strong coupling phase (different from t'Hooft phase) are computed. We find agreement between our results and the analytical ones in the continuum.
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