10,470 research outputs found
Hydrodynamic synchronization of flagellar oscillators
We survey the theory synchronization in collections of noisy oscillators.
This framework is applied to flagellar synchronization by hydrodynamic
interactions. The time-reversibility of hydrodynamics at low Reynolds numbers
prompts swimming strokes that break symmetry to facilitate hydrodynamic
synchronization. We discuss different physical mechanisms for flagellar
synchronization, which break this symmetry in different ways.Comment: 15 pages, 3 figures; accepted for publication in EPJ Special Topics
Issue,Lecture Notes of the Summer School "Microswimmers -- From Single
Particle Motion to Collective Behaviour'', organised by the DFG Priority
Programme SPP 1726 (Forschungszentrum J\"ulich, J\"ulich, 2015
Spectral properties of Bunimovich mushroom billiards
Properties of a quantum mushroom billiard in the form of a superconducting
microwave resonator have been investigated. They reveal unexpected nonuniversal
features such as, e.g., a supershell effect in the level density and a dip in
the nearest-neighbor spacing distribution. Theoretical predictions for the
quantum properties of mixed systems rely on the sharp separability of phase
space - an unusual property met by mushroom billiards. We however find
deviations which are ascribed to the presence of dynamic tunneling.Comment: 4 pages, 7 .eps-figure
The Energy-Momentum tensor on manifolds
On manifolds, we study the Energy-Momentum tensor associated with a
spinor field. First, we give a spinorial Gauss type formula for oriented
hypersurfaces of a manifold. Using the notion of generalized
cylinders, we derive the variationnal formula for the Dirac operator under
metric deformation and point out that the Energy-Momentum tensor appears
naturally as the second fundamental form of an isometric immersion. Finally, we
show that generalized Killing spinors for Codazzi Energy-Momentum
tensor are restrictions of parallel spinors.Comment: To appear in IJGMMP (International Journal of Geometric Methods in
Modern Physics), 22 page
Collisions of paramagnetic molecules in magnetic ļ¬elds: An analytic model based on Fraunhofer diļ¬raction of matter waves
We investigate the eļ¬ects of a magnetic ļ¬eld on the dynamics of rotationally inelastic collisions of open-shell molecules (Ā² Ī£, Ā³Ī£, and Ā²Ī ) with closed-shell atoms. Our treatment makes use of the Fraunhofer model of matter wave scattering and its recent extension to collisions in electric [M. Lemeshko and B. Friedrich, J. Chem. Phys. 129, 024301 (2008)] and radiative ļ¬elds [M. Lemeshko and B. Friedrich, Int. J. Mass. Spec. in press (2008)]. A magnetic ļ¬eld aligns the molecule in the space-ļ¬xed frame and thereby alters the eļ¬ective shape of the diļ¬raction target. This signiļ¬cantly aļ¬ects the diļ¬erential and integral scattering cross sections. We exemplify our treatment by evaluating the magnetic-ļ¬eld-dependent scattering characteristics of the He ā CaH (XĀ² Ī£āŗ), He ā Oā (XĀ³Ī£ā») and He ā OH (XĀ²Ī Ī© ) systems at thermal collision energies. Since the cross sections can be obtained fordiļ¬erent orientations of the magnetic ļ¬eld with respect to the relative velocity vector, the model also oļ¬ers predictions about the frontal-versus-lateral steric asymmetry of the collisions. The steric asymmetry is found to be almost negligible for the He ā OH system, weak for the He ā CaH collisions, and strong for the He ā Oā . While odd āM transitions dominate the He ā OH (J = 3/2, f ā Jā² , e/f ) integral cross sections in a magnetic ļ¬eld parallel to the relative velocity vector, even āM transitions prevail in the case of the He ā CaH (XĀ²Ī£āŗ) and He ā Oā (XĀ³Ī£ā») collision systems. For the latter system, the magnetic ļ¬eld opens inelastic channels that are closed in the absence of the ļ¬eld. These involve the transitions N = 1, J = 0 ā Nā², Jā² with Jā² = Nā²
Much Polyphony but Little Harmony: Otto Sackur's Groping for a Quantum Theory of Gases
The endeavor of Otto Sackur (1880-1914) was driven, on the one hand, by his interest in Nernstās theorem, statistical mechanics, and the problem of chemical equilibrium, and, on the other, his goal to shed light on classical mechanics from the quantum vantage point. Inspired by the interplay between classical physics and quantum theory, Sackur chanced to expound his personal take on the role of the quantum in the changing landscape of physics in the turbulent 1910s. In this paper, we tell the story of this enthusiastic practitioner of the old quantum theory and early contributor to quantum statistical mechanics, whose scientific ontogenesis provides a telling clue about the phylogeny of his contemporaries
An analytic model of the stereodynamics of rotationally inelastic molecular collisions
We develop an analytic model of vector correlations in rotationally inelastic atom-diatom collisions and test it against the much examined Ar--NO (XĀ²Ī ) system. Based on the Fraunhofer scattering of matter waves, the model furnishes complex scattering amplitudes needed to evaluate the polarization moments characterizing the quantum stereodynamics. The analytic polarization moments are found to be in an excellent agreement with experimental results and with close-coupling calculations available at thermal energies. The model reveals that the stereodynamics is governed by diffraction from the repulsive core of the Ar--NO potential, which can be characterized by a single Legendre moment
- ā¦