242 research outputs found
Photodetachment of H near a partial reflecting surface
Theoretical and interpretative study on the subject of photodetachment of
H near a partial reflecting surface is presented, and the absorption
effect of the surface is investigated on the total and differential cross
sections using a theoretical imaging method. To understand the absorption
effect, a reflection parameter is introduced as a multiplicative factor to
the outgoing detached-electron wave of H propagating toward the wall. The
reflection parameter measures, how much electron wave would reflect from the
surface; K=0 corresponds to no reflection and K=1 corresponds to the total
reflection.Comment: 8 pages, 4 figure
On the Complexity of Local Search for Weighted Standard Set Problems
In this paper, we study the complexity of computing locally optimal solutions
for weighted versions of standard set problems such as SetCover, SetPacking,
and many more. For our investigation, we use the framework of PLS, as defined
in Johnson et al., [JPY88]. We show that for most of these problems, computing
a locally optimal solution is already PLS-complete for a simple neighborhood of
size one. For the local search versions of weighted SetPacking and SetCover, we
derive tight bounds for a simple neighborhood of size two. To the best of our
knowledge, these are one of the very few PLS results about local search for
weighted standard set problems
Nanoelectromechanics of Piezoresponse Force Microscopy
To achieve quantitative interpretation of Piezoresponse Force Microscopy
(PFM), including resolution limits, tip bias- and strain-induced phenomena and
spectroscopy, analytical representations for tip-induced electroelastic fields
inside the material are derived for the cases of weak and strong indentation.
In the weak indentation case, electrostatic field distribution is calculated
using image charge model. In the strong indentation case, the solution of the
coupled electroelastic problem for piezoelectric indentation is used to obtain
the electric field and strain distribution in the ferroelectric material. This
establishes a complete continuum mechanics description of the PFM contact
mechanics and imaging mechanism. The electroelastic field distribution allows
signal generation volume in PFM to be determined. These rigorous solutions are
compared with the electrostatic point charge and sphere-plane models, and the
applicability limits for asymptotic point charge and point force models are
established. The implications of these results for ferroelectric polarization
switching processes are analyzed.Comment: 81 pages, 19 figures, to be published in Phys. Rev.
Active feedback scheme for minimization of helicity-dependent instrumental asymmetries
A method for the active feedback reduction of optical instrumental intensity asymmetries is presented. It is based on the fast chopping of two spatially separated beams of light with orthogonal linear polarizations that are recombined and passed through a quarter-wave plate to yield a single beam with rapidly flipping helicity. Active electro-optic feedback has been successfully employed to maintain this asymmetry below 10−5
Active feedback scheme for minimization of helicity-dependent instrumental asymmetries
A method for the active feedback reduction of optical instrumental intensity asymmetries is presented. It is based on the fast chopping of two spatially separated beams of light with orthogonal linear polarizations that are recombined and passed through a quarter-wave plate to yield a single beam with rapidly flipping helicity. Active electro-optic feedback has been successfully employed to maintain this asymmetry below 10−5
Propagation of charged particle waves in a uniform magnetic field
This paper considers the probability density and current distributions
generated by a point-like, isotropic source of monoenergetic charges embedded
into a uniform magnetic field environment. Electron sources of this kind have
been realized in recent photodetachment microscopy experiments. Unlike the
total photocurrent cross section, which is largely understood, the spatial
profiles of charge and current emitted by the source display an unexpected
hierarchy of complex patterns, even though the distributions, apart from
scaling, depend only on a single physical parameter. We examine the electron
dynamics both by solving the quantum problem, i. e., finding the energy Green
function, and from a semiclassical perspective based on the simple cyclotron
orbits followed by the electron. Simulations suggest that the semiclassical
method, which involves here interference between an infinite set of paths,
faithfully reproduces the features observed in the quantum solution, even in
extreme circumstances, and lends itself to an interpretation of some (though
not all) of the rich structure exhibited in this simple problem.Comment: 39 pages, 16 figure
Ballistic matter waves with angular momentum: Exact solutions and applications
An alternative description of quantum scattering processes rests on
inhomogeneous terms amended to the Schroedinger equation. We detail the
structure of sources that give rise to multipole scattering waves of definite
angular momentum, and introduce pointlike multipole sources as their limiting
case. Partial wave theory is recovered for freely propagating particles. We
obtain novel results for ballistic scattering in an external uniform force
field, where we provide analytical solutions for both the scattering waves and
the integrated particle flux. Our theory directly applies to p-wave
photodetachment in an electric field. Furthermore, illustrating the effects of
extended sources, we predict some properties of vortex-bearing atom laser beams
outcoupled from a rotating Bose-Einstein condensate under the influence of
gravity.Comment: 42 pages, 8 figures, extended version including photodetachment and
semiclassical theor
Greedy Selfish Network Creation
We introduce and analyze greedy equilibria (GE) for the well-known model of
selfish network creation by Fabrikant et al.[PODC'03]. GE are interesting for
two reasons: (1) they model outcomes found by agents which prefer smooth
adaptations over radical strategy-changes, (2) GE are outcomes found by agents
which do not have enough computational resources to play optimally. In the
model of Fabrikant et al. agents correspond to Internet Service Providers which
buy network links to improve their quality of network usage. It is known that
computing a best response in this model is NP-hard. Hence, poly-time agents are
likely not to play optimally. But how good are networks created by such agents?
We answer this question for very simple agents. Quite surprisingly, naive
greedy play suffices to create remarkably stable networks. Specifically, we
show that in the SUM version, where agents attempt to minimize their average
distance to all other agents, GE capture Nash equilibria (NE) on trees and that
any GE is in 3-approximate NE on general networks. For the latter we also
provide a lower bound of 3/2 on the approximation ratio. For the MAX version,
where agents attempt to minimize their maximum distance, we show that any
GE-star is in 2-approximate NE and any GE-tree having larger diameter is in
6/5-approximate NE. Both bounds are tight. We contrast these positive results
by providing a linear lower bound on the approximation ratio for the MAX
version on general networks in GE. This result implies a locality gap of
for the metric min-max facility location problem, where n is the
number of clients.Comment: 28 pages, 8 figures. An extended abstract of this work was accepted
at WINE'1
Near-threshold collisional dynamics in the \u3ci\u3ee\u3csup\u3e−\u3c/sup\u3ee\u3csup\u3e+\u3c/sup\u3ep\u3c/i\u3e system
We study e+-H(n) and Ps(n)-p collisions near the three-body breakup threshold and thresholds for the charge-transfer processes.We show that classical trajectoryMonte Carlo (CTMC) simulations for the three-body breakup agree reasonably well in this energy region with quantum-mechanical convergent close-coupling (CCC) calculations even if the initial hydrogen atom or positronium atom is in the ground state. The threshold behavior of the three-body breakup cross section in e+-H(1s) and Ps(1s)-p collisions agrees with the Wannier law with Klar’s exponent and obeys the classical scaling laws, although some deviation from the Klar-Wannier behavior is observed in the CCC results. Below the threshold the agreement between CTMC and CCC disappears. In particular the CTMC method fails completely for the processes of H formation in Ps(1s)-p collisions and Ps formation in e+-H collisions well below the three-body breakup threshold. For higher initial states the CTMC results below the threshold improve substantially, in accordance with the correspondence principle. This is explained by comparing the quantum-mechanical threshold laws with the classical laws
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