7,413 research outputs found
Tangential Extremal Principles for Finite and Infinite Systems of Sets, I: Basic Theory
In this paper we develop new extremal principles in variational analysis that
deal with finite and infinite systems of convex and nonconvex sets. The results
obtained, unified under the name of tangential extremal principles, combine
primal and dual approaches to the study of variational systems being in fact
first extremal principles applied to infinite systems of sets. The first part
of the paper concerns the basic theory of tangential extremal principles while
the second part presents applications to problems of semi-infinite programming
and multiobjective optimization
Tangential Extremal Principles for Finite and Infinite Systems of Sets, II: Applications to Semi-infinite and Multiobjective Optimization
This paper contains selected applications of the new tangential extremal
principles and related results developed in Part I to calculus rules for
infinite intersections of sets and optimality conditions for problems of
semi-infinite programming and multiobjective optimization with countable
constraint
Rated Extremal Principles for Finite and Infinite Systems
In this paper we introduce new notions of local extremality for finite and
infinite systems of closed sets and establish the corresponding extremal
principles for them called here rated extremal principles. These developments
are in the core geometric theory of variational analysis. We present their
applications to calculus and optimality conditions for problems with infinitely
many constraints
Linear response within the projection-based renormalization method: Many-body corrections beyond the random phase approximation
The explicit evaluation of linear response coefficients for interacting
many-particle systems still poses a considerable challenge to theoreticians. In
this work we use a novel many-particle renormalization technique, the so-called
projector-based renormalization method, to show how such coefficients can
systematically be evaluated. To demonstrate the prospects and power of our
approach we consider the dynamical wave-vector dependent spin susceptibility of
the two-dimensional Hubbard model and also determine the subsequent magnetic
phase diagram close to half-filling. We show that the superior treatment of
(Coulomb) correlation and fluctuation effects within the projector-based
renormalization method significantly improves the standard random phase
approximation results.Comment: 17 pages, 7 figures, revised versio
Magnetocaloric effect in nano- and polycrystalline manganite
samples were prepared in nano- and polycrystalline
forms by sol-gel and solid state reaction methods, respectively, and
structurally characterized by synchrotron X-ray diffraction. The magnetic
properties determined by ac susceptibility and dc magnetization measurements
are discussed. The magnetocaloric effect in this nanocrystalline manganite is
spread over a broader temperature interval than in the polycrystalline case.
The relative cooling power of the poly- and nanocrystalline manganites is used
to evaluate a possible application for magnetic cooling below room temperature.Comment: 6 pages, 5 (double) figures, 1 table, 16 references; submitted to
Appl. Phys.
On the Cause of Supra-Arcade Downflows in Solar Flares
A model of supra-arcade downflows (SADs), dark low density regions also known
as tadpoles that propagate sunward during solar flares, is presented. It is
argued that the regions of low density are flow channels carved by
sunward-directed outflow jets from reconnection. The solar corona is
stratified, so the flare site is populated by a lower density plasma than that
in the underlying arcade. As the jets penetrate the arcade, they carve out
regions of depleted plasma density which appear as SADs. The present
interpretation differs from previous models in that reconnection is localized
in space but not in time. Reconnection is continuous in time to explain why
SADs are not filled in from behind as they would if they were caused by
isolated descending flux tubes or the wakes behind them due to temporally
bursty reconnection. Reconnection is localized in space because outflow jets in
standard two-dimensional reconnection models expand in the normal (inflow)
direction with distance from the reconnection site, which would not produce
thin SADs as seen in observations. On the contrary, outflow jets in spatially
localized three-dimensional reconnection with an out-of-plane (guide) magnetic
field expand primarily in the out-of-plane direction and remain collimated in
the normal direction, which is consistent with observed SADs being thin.
Two-dimensional proof-of-principle simulations of reconnection with an
out-of-plane (guide) magnetic field confirm the creation of SAD-like depletion
regions and the necessity of density stratification. Three-dimensional
simulations confirm that localized reconnection remains collimated.Comment: 16 pages, 5 figures, accepted to Astrophysical Journal Letters in
August, 2013. This version is the accepted versio
Electron Acceleration by Multi-Island Coalescence
Energetic electrons of up to tens of MeV are created during explosive
phenomena in the solar corona. While many theoretical models consider magnetic
reconnection as a possible way of generating energetic electrons, the precise
roles of magnetic reconnection during acceleration and heating of electrons
still remain unclear. Here we show from 2D particle-in-cell simulations that
coalescence of magnetic islands that naturally form as a consequence of tearing
mode instability and associated magnetic reconnection leads to efficient
energization of electrons. The key process is the secondary magnetic
reconnection at the merging points, or the `anti-reconnection', which is, in a
sense, driven by the converging outflows from the initial magnetic reconnection
regions. By following the trajectories of the most energetic electrons, we
found a variety of different acceleration mechanisms but the energization at
the anti-reconnection is found to be the most important process. We discuss
possible applications to the energetic electrons observed in the solar flares.
We anticipate our results to be a starting point for more sophisticated models
of particle acceleration during the explosive energy release phenomena.Comment: 14 pages, 12 figures (degraded figure quality), 1 table. Accepted for
publication in ApJ
Faxen relations in solids - a generalized approach to particle motion in elasticity and viscoelasticity
A movable inclusion in an elastic material oscillates as a rigid body with
six degrees of freedom. Displacement/rotation and force/moment tensors which
express the motion of the inclusion in terms of the displacement and force at
arbitrary exterior points are introduced. Using reciprocity arguments two
general identities are derived relating these tensors. Applications of the
identities to spherical particles provide several new results, including simple
expressions for the force and moment on the particle due to plane wave
excitation.Comment: 11 pages, 4 figure
Frequency Bin Entangled Photons
A monochromatic laser pumping a parametric down conversion crystal generates
frequency entangled photon pairs. We study this experimentally by addressing
such frequency entangled photons at telecommunication wavelengths (around 1550
nm) with fiber optics components such as electro-optic phase modulators and
narrow band frequency filters. The theory underlying our approach is developed
by introducing the notion of frequency bin entanglement. Our results show that
the phase modulators address coherently up to eleven frequency bins, leading to
an interference pattern which can violate a Bell inequality adapted to our
setup by more than five standard deviations.Comment: 10 pages, 4 figures (extended version
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