Search results for "Tact"
showing 10 items of 1313 documents
Hydrodynamic synchronization of pairs of puller type magnetotactic bacteria in a high frequency rotating magnetic field.
2019
Ensembles of magnetotactic bacteria are known to interact hydrodynamically and form swarms under the influence of external magnetic fields. We describe the synchronization of puller type magnetotactic bacteria in a rotating magnetic field by representing the bacteria as hydrodynamic force dipoles. Numerical simulations show that at moderate values of the hydrodynamic interaction parameter large ensembles of asynchronously rotating bacteria randomly eject propagating doublets of synchronized bacteria. We quantitatively analyze the dynamics of the doublets and show that an important role in the formation of these propagating structures is played by the parameters characterizing the possible t…
Dynamical environments of MU69: a state of chaotic clearing
2018
AbstractThe second (after Pluto) plausible target object for the New Horizons mission is 2014 MU69. It is a classical TNO, a primordial contact binary. Identifying any material in the vicinities of a target object is of an especial concern for planning cosmic fly-byes, as it is hazardous for a space probe. Luckily, no such material has been reported for MU69 up to now. The point of our report is that this lucky absence is just a dynamical consequence of the physical nature of MU69. Spinning gravitating dumbbells create zones of dynamical chaos around them, and this has a clearing effect: any material put in orbits around a rotating dumbbell (e.g., any material ejected from its surface) cann…
Unavoidable decoherence in semiconductor quantum dots
2005
Phonon-induced unavoidable decoherence of orbital degrees of freedom in quantum dots is studied and the relevant time scales are estimated. Dephasing of excitons due to acoustic phonons and, in a polar medium, to optical phonons, including anharmonic effects and enhancement of the effective Fr\"ohlich constant due to localization, is assessed for typical self-assembled quantum dots. Temporal inefficiency of Pauli blocking due to lattice inertia is predicted. For quantum dots placed in a diluted magnetic semiconductor medium a magnon-induced dephasing of a spin is also estimated in accordance with experimental results.
Single scatterings in single artificial atoms: Quantum coherence and entanglement
2003
We employ the quantum-jump approach to study single scatterings in single semiconductor quantum dots. Two prototypical situations are investigated. First, we analyze two-photon emissions from the cascade biexciton decay of a dot where the single-exciton states exhibit a fine-structure splitting. We show that this splitting results for appropriately chosen polarization filters in an oscillatory behavior of two-photon correlations, and carefully examine the proper theoretical description of the underlying scattering processes. Secondly, we analyze the decay of a single-electron charged exciton in a quantum dot embedded in a field effect structure. We show how the quantum properties of the cha…
Electronic and magnetic structure of artificial atoms
1999
The concept of shell structure has been found useful in the description of semiconductor quantum dots, which today can be made so small that they contain less than 20 electrons. We review the experimental discovery of magic numbers and spin alignment following Hund’s rules in the addition spectra of vertical quantum dots, and show that these results compare well to model calculations within spin density functional theory. We further discuss the occurrence of spin density waves in quantum dots and quantum wires. For deformable two-dimensional quantum dots (for example, jellium clusters on surfaces), we study the interplay between Hund’s rules and Jahn–Teller deformations and investigate the …
Excited atomic energy levels in protactinium by resonance ionization spectroscopy
2018
We present high-resolution data of the single-excitation spectrum of protactinium, reaching slightly beyond the first-ionization threshold. Within this work, more than 1500 energy levels are recorded in different excitation energy ranges below $50\phantom{\rule{0.16em}{0ex}}000\phantom{\rule{4pt}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$. Our experimental results show that the tabulated data in the literature severely underestimate the density of states particularly regarding the highly excited spectral range.
Dynamics of an active magnetic particle in a rotating magnetic field.
2006
The motion of an active (self-propelling) particle with a permanent magnetic moment under the action of a rotating magnetic field is considered. We show that below a critical frequency of the external field the trajectory of a particle is a circle. For frequencies slightly above the critical point the particle moves on an approximately circular trajectory and from time to time jumps to another region of space. Symmetry of the particle trajectory depends on the commensurability of the field period and the period of the orientational motion of the particle. We also show how our results can be used to study the properties of naturally occurring active magnetic particles, so-called magnetotacti…
Diffusion of magnetotactic bacterium in rotating magnetic field
2011
Swimming trajectory of a magnetotactic bacterium in a rotating magnetic field is a circle. Random reversals of the direction of the bacterium motion induces a random walk of the curvature center of the trajectory. In assumption of the distribution of the switching events according to the Poisson process the diffusion coefficient is calculated in dependence on the frequency of the rotating field and the characteristic time between the switching events. It is confirmed by the numerical simulation of the random walk of the bacterium in the rotating magnetic field.
Diffusion in active magnetic colloids
2013
Abstract Properties of active colloids of circle swimmers are reviewed. As a particular example of active magnetic colloids the magnetotactic bacteria under the action of a rotating magnetic field is considered. The relation for a diffusion coefficient due to the random switching of the direction of rotation of their rotary motors is derived on the basis of the master equation. The obtained relation is confirmed by the direct numerical simulation of random trajectory of a magnetotactic bacterium under the action of the Poisson type internal noise due to the random switching of rotary motors. The results obtained are in qualitative and quantitative agreement with the available experimental r…
Nuclear Contact Times in Dissipative Heavy Ion Collsions Measured Via γ-Ray Spectroscopy
1987
Electron spectra have been measured for elastic and dissipative U + Au collisions at 8.6 MeV/u and analysed within a simple schematic model which describes γ-ray emission in the presence of a nuclear contact time and a total kinetic energy loss (TKEL). A nearly linear dependence of the mean nuclear contact time τ and TKEL was found, reaching τ = 1.1 * 10-21 s with a variance σ = ±0.4 * 10-21 s for a TKEL of (400 ± 50) MeV.