191 research outputs found
Finite rank perturbations and solutions to the operator Riccati equation
We consider an off-diagonal self-adjoint finite rank perturbation of a
self-adjoint operator in a complex separable Hilbert space , where is finite dimensional. We
describe the singular spectrum of the perturbed operator and establish a
connection with solutions to the operator Riccati equation. In particular, we
prove existence results for solutions in the case where the whole Hilbert space
is finite dimensional.Comment: 13 pages, added Preliminaries, added more detail
Wave packet approach to periodically driven scattering
For autonomous systems it is well known how to extract tunneling
probabilities from wavepacket calculations. Here we present a corresponding
approach for periodically time-dependent Hamiltonians, valid at all
frequencies, field strengths, and transition orders. After mapping the
periodically driven system onto a time-independent one with an additional
degree of freedom, use is made of the correlation function formulation of
scattering [J. Chem. Phys. {\bf 98}, 3884 (1993)]. The formalism is then
applied to study the transmission properties of a resonant tunneling double
barrier structure under the influence of a sinusoidal laser field, revealing an
unexpected antiresonance in the zero photon transition for large field
strengths.Comment: 4 pages, 2 figure
Active Brownian particles with velocity-alignment and active fluctuations
We consider a model of active Brownian particles with velocity-alignment in
two spatial dimensions with passive and active fluctuations. Hereby, active
fluctuations refers to purely non-equilibrium stochastic forces correlated with
the heading of an individual active particle. In the simplest case studied
here, they are assumed as independent stochastic forces parallel (speed noise)
and perpendicular (angular noise) to the velocity of the particle. On the other
hand, passive fluctuations are defined by a noise vector independent of the
direction of motion of a particle, and may account for example for thermal
fluctuations.
We derive a macroscopic description of the active Brownian particle gas with
velocity-alignment interaction. Hereby, we start from the individual based
description in terms of stochastic differential equations (Langevin equations)
and derive equations of motion for the coarse grained kinetic variables
(density, velocity and temperature) via a moment expansion of the corresponding
probability density function.
We focus here in particular on the different impact of active and passive
fluctuations on the onset of collective motion and show how active fluctuations
in the active Brownian dynamics can change the phase-transition behaviour of
the system. In particular, we show that active angular fluctuation lead to an
earlier breakdown of collective motion and to emergence of a new bistable
regime in the mean-field case.Comment: 5 figures, 22 pages, submitted to New Journal of Physic
Dissociation and ionization of small molecules steered by external noise
We show that ionization and dissociation can be influenced separately in a
molecule with appropriate external noise. Specifically we investigate the
hydrogen molecular ion under a stochastic force quantum mechanically beyond the
Born-Oppenheimer approximation. We find that up to 30% of dissociation without
ionization can be achieved by suitably tuning the forcing parameters.Comment: 13 pages, 6 figure
Mixed methodology in human brain research: integrating MRI and histology
Postmortem magnetic resonance imaging (MRI) can provide a bridge between histological observations and the in vivo anatomy of the human brain. Approaches aimed at the co-registration of data derived from the two techniques are gaining interest. Optimal integration of the two research fields requires detailed knowledge of the tissue property requirements for individual research techniques, as well as a detailed understanding of the consequences of tissue fixation steps on the imaging quality outcomes for both MRI and histology. Here, we provide an overview of existing studies that bridge between state-of-the-art imaging modalities, and discuss the background knowledge incorporated into the design, execution and interpretation of postmortem studies. A subset of the discussed challenges transfer to animal studies as well. This insight can contribute to furthering our understanding of the normal and diseased human brain, and to facilitate discussions between researchers from the individual disciplines
Coulomb blockade effects in driven electron transport
We study numerically the influence of strong Coulomb repulsion on the current
through molecular wires that are driven by external electromagnetic fields. The
molecule is described by a tight-binding model whose first and last site is
coupled to a respective lead. The leads are eliminated within a perturbation
theory yielding a master equation for the wire. The decomposition into a
Floquet basis enables an efficient treatment of the driving field. For the
electronic excitations in bridged molecular wires, we find that strong Coulomb
repulsion significantly sharpens resonance peaks which broaden again with
increasing temperature. By contrast, Coulomb blockade has only a small
influence on effects like non-adiabatic electron pumping and coherent current
suppression.Comment: 9 pages, 7 figures. Added a plot for temperature dependence of
resonance peaks. Published versio
Einleitung
Bulst N, Rüthing H. Einleitung. In: Krutisch P, Großmann GU, eds. Der Weserraum zwischen 1500 und 1650: Gesellschaft, Wirtschaft und Kultur in der frühen Neuzeit. Materialien zur Kunst- und Kulturgeschichte in Nord- und Westdeutschland ; 4. Marburg: Jonas-Verl.; 1993: 9-14
Semiclassical description of multiphoton processes
We analyze strong field atomic dynamics semiclassically, based on a full
time-dependent description with the Hermann-Kluk propagator. From the
properties of the exact classical trajectories, in particular the accumulation
of action in time, the prominent features of above threshold ionization (ATI)
and higher harmonic generation (HHG) are proven to be interference phenomena.
They are reproduced quantitatively in the semiclassical approximation.
Moreover, the behavior of the action of the classical trajectories supports the
so called strong field approximation which has been devised and postulated for
strong field dynamics.Comment: 10 pages, 11 figure
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