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Search Results: 1 - 10 of 45426 matches for " Michael Stark "
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Natural Orifice Surgery (NOS)-the next step in the evolution of minimally invasive surgery
Tahar Benhidjeb,Michael Stark
Journal of the Turkish-German Gynecological Association , 2012,
Abstract: Endoscopy, which was introduced in the 20th century, changed the outcome of surgery by reducing the need for analgesia and shortening hospital stay. Any new surgical method should improve safety and outcome. At the beginning of the 21st century, the use of natural orifice surgery is a promising progress. The transgastric and transdouglas approaches are currently being investigated and evaluated. The transgastric approach still has a long way to go due to objective problems such as infections, stomach acidity, and the optimal way to ensure the safe closure of gastrotomy. The transdouglas approach, however, is already starting to establish itself and it seems that with the construction of designated instruments it will prevail in the 21st century.
Coherent Quantum Ratchets Driven by Tunnel Oscillations
Michael Stark,Sigmund Kohler
Physics , 2009, DOI: 10.1209/0295-5075/91/20007
Abstract: We demonstrate that the tunnel oscillations of a biased double quantum dot can be employed as driving source for a quantum ratchet. As a model, we use two capacitively coupled double quantum dots. One double dot is voltage biased and provides the ac force, while the other experiences the ac force and acts as coherent quantum ratchet. The current is obtained from a Bloch-Redfield master equation which ensures a proper equilibrium limit. We find that the two-electron states of the coupled ratchet-drive Hamiltonian lead to unexpected steps in the ratchet current.
Circling particles and drafting in optical vortices
Michael Reichert,Holger Stark
Physics , 2004, DOI: 10.1088/0953-8984/16/38/023
Abstract: Particles suspended in a viscous fluid circle in optical vortices generated by holographic optical-tweezer techniques [Curtis J E and Grier D G 2003 Phys. Rev. Lett. 90 133901]. We model this system and show that hydrodynamic interactions between the circling particles determine their collective motion. We perform a linear-stability analysis to investigate the stability of regular particle clusters and illustrate the limit cycle to which the unstable modes converge. We clarify that drafting of particle doublets is essential for the understanding of the limit cycle.
Superdiffusion in a Honeycomb Billiard
Michael Schmiedeberg,Holger Stark
Physics , 2005, DOI: 10.1103/PhysRevE.73.031113
Abstract: We investigate particle transport in the honeycomb billiard that consists of connected channels placed on the edges of a honeycomb structure. The spreading of particles is superdiffusive due to the existence of ballistic trajectories which we term perfect paths. Simulations give a time exponent of 1.72 for the mean square displacement and a starlike, i.e., anisotropic particle distribution. We present an analytical treatment based on the formalism of continuous-time random walks and explain both the time exponent and the anisotropic distribution. In billiards with randomly distributed channels, conventional diffusion is always observed in the long-time limit, although for small disorder transient superdiffusional behavior exists. Our simulation results are again supported by an analytical analysis.
Hydrodynamic coupling of two rotating spheres trapped in harmonic potentials
Michael Reichert,Holger Stark
Physics , 2003, DOI: 10.1103/PhysRevE.69.031407
Abstract: We theoretically study in detail the hydrodynamic coupling of two equal-sized colloidal spheres at low Reynolds numbers assuming the particles to be harmonically trapped with respect to both their positions and orientations. By taking into account the rotational motion, we obtain a rich spectrum of collective eigen modes whose properties we determine on the basis of pure symmetry arguments. Extending recent investigations on translational correlations [J.-C. Meiners and S. R. Quake, Phys. Rev. Lett. 82, 2211 (1999)], we derive the complete set of auto- and cross-correlation functions emphasizing the coupling of rotation to translation which we illustrate in a few examples. An important feature of our system is the self-coupling of translation and rotation of one particle mediated by the neighboring particle that is clearly visible in the appropriate auto-correlation function. This coupling is a higher-order effect and therefore not included in the widely used Rotne-Prager approximation for the hydrodynamic mobilities.
Kinetic description of thermalization dynamics in weakly interacting quantum systems
Michael Stark,Marcus Kollar
Physics , 2013,
Abstract: After a sudden disruption, weakly interacting quantum systems first relax to a prethermalized state that can be described by perturbation theory and a generalized Gibbs ensemble. Using these properties of the prethermalized state we perturbatively derive a kinetic equation which becomes a quantum Boltzmann equation in the scaling limit of vanishing interaction. Applying this to interaction quenches in the fermionic Hubbard model we find that the momentum distribution relaxes to the thermal prediction of statistical mechanics. For not too large interaction, this two-stage scenario provides a quantitative understanding of the time evolution leading from the initial pure via a metastable prethermal to the final thermal state.
Doppler Modulation of X-ray lines in Cygnus X-3
Michael J. Stark,Malinda Saia
Physics , 2003, DOI: 10.1086/375287
Abstract: We measured Doppler shifts of three bright spectral lines in the X-ray emission from Cygnus X-3 as recorded by the Chandra X-Ray Observatory. Doppler shifts of lines associated with Si XIV and S XVI exhibit orbital modulation. The magnitude and phasing of this modulation relative to the orbital ephemeris indicate the location of the source of this emission within the wind emanating from the compact object's companion. These observations enable us to make an indirect measurement of the separation of the two stars. Under certain assumptions our observation of a line associated with Fe XXV also limits the mass of the compact object M < 3.6 MSun.
Snow Control - An RCT protocol for a web-based self-help therapy to reduce cocaine consumption in problematic cocaine users
Michael Schaub, Robin Sullivan, Lars Stark
BMC Psychiatry , 2011, DOI: 10.1186/1471-244x-11-153
Abstract: This paper presents the protocol of a randomised clinical trial to test the effectiveness of a web-based self-help therapy to reduce cocaine use in problematic cocaine users. The primary outcome is severity of cocaine dependence. Secondary outcome measures include cocaine craving, consumption of cocaine and other substances of abuse in the past month, and changes in depression characteristics. The therapy group will receive a 6-week self-help therapy to reduce cocaine consumption based on methods of Cognitive Behavioural Therapy, principles of Motivational Interviewing and self-control practices. The control group will be presented weekly psycho-educative information with a quiz. The predictive validity of participant characteristics on treatment retention and outcome will be explored.To the best of our knowledge, this will be the first randomised clinical trial to test the effectiveness of online self-help therapy to reduce or abstain from cocaine use. It will also investigate predictors of outcome and retention. This trial is registered at Current Controlled Trials and is traceable as NTR-ISRCTN93702927.Although data on the prevalence of problematic cocaine use and addiction are lacking in Switzerland and many other developed countries, there is no doubt that, in line with other countries, cocaine use has increased in Switzerland in recent years [1,2]. Over the past ten years, the number of cocaine-related disorder treatments has quintupled in outpatient treatment and advisory services [2]. In 2005, resident institutions reported that, for the first time in history, cocaine outstripped opiates as the main substance used [3]. This trend has also been observed in outpatient units [3]. Further evidence of increased cocaine consumption has been found by quantifying cocaine concentrations in sewage effluents [4] and in recent HBSC student surveys [5]. The abovementioned increase in treatment requests likely reflects only a minority of cocaine users. Presumably, the maj
Rotational Diffusion in a Chain of Particles
Holger Stark,Michael Reichert,Jerome Bibette
Physics , 2005, DOI: 10.1088/0953-8984/17/45/058
Abstract: We study the coupled rotational diffusion in a two-particle chain on the basis of a Smoluchowski equation and calculate time-correlation functions that are measurable in an experiment. This might be used to explore hydrodynamic interactions in the limit where lubrication theory is valid.
Photon Channelling in Foams
Michael Schmiedeberg,MirFaez Miri,Holger Stark
Physics , 2005, DOI: 10.1140/epje/i2005-10034-6
Abstract: Experiments by Gittings, Bandyopadhyay, and Durian [Europhys. Lett.\ \textbf{65}, 414 (2004)] demonstrate that light possesses a higher probability to propagate in the liquid phase of a foam due to total reflection. The authors term this observation photon channelling which we investigate in this article theoretically. We first derive a central relation in the work of Gitting {\em et al.} without any free parameters. It links the photon's path-length fraction $f$ in the liquid phase to the liquid fraction $\epsilon$. We then construct two-dimensional Voronoi foams, replace the cell edges by channels to represent the liquid films and simulate photon paths according to the laws of ray optics using transmission and reflection coefficients from Fresnel's formulas. In an exact honeycomb foam, the photons show superdiffusive behavior. It becomes diffusive as soon as disorder is introduced into the foams. The dependence of the diffusion constant on channel width and refractive index is explained by a one-dimensional random-walk model. It contains a photon channelling state that is crucial for the understanding of the numerical results. At the end, we shortly comment on the observation that photon channelling only occurs in a finite range of $\epsilon$.
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