45,859 research outputs found
Aggregated Feature Retrieval for MPEG-7 via Clustering
In this paper, we describe an approach to combining text and visual features from MPEG-7 descriptions of video. A video retrieval process is aligned to a text retrieval process based on the TF*IDF vector space model via clustering of low-level visual features. Our assumption is that shots within the same cluster are not only similar visually but also semantically, to a certain extent. Our experiments on the TRECVID2002 and TRECVID2003 collections show that adding extra meaning to a shot based on the shots from the same cluster is useful when each video in a collection contains a high proportion of similar shots, for example in documentaries
Measurement of binding energy of negatively charged excitons in GaAs/AlGaAs quantum wells
We report a photoluminescence study of electron-hole complexes in specially
designed semiconductor heterostructures. Placing a remote dilute layer of
donors at different distances \itshape d \normalfont from the quantum well
leads to the transformation of luminescence spectra of neutral () and
negatively charged () excitons. The onset of an additional spectral line
and its energy dependence on \itshape d \normalfont allows us to unambiguously
relate the so-called trion state with charged excitons bound on charged
donors in a barrier. The results indicate the overestimation in free-trion
binding energies from previous studies of GaAs/AlGaAs quantum
wells, and give their corrected values for QWs of width 200 and 300 \AA \space
in the limiting case of infinitely distant donors.Comment: 5 pages, 4 figure
Complex Dynamics of Correlated Electrons in Molecular Double Ionization by an Ultrashort Intense Laser Pulse
With a semiclassical quasi-static model we achieve an insight into the
complex dynamics of two correlated electrons under the combined influence of a
two-center Coulomb potential and an intense laser field. The model calculation
is able to reproduce experimental data of nitrogen molecules for a wide range
of laser intensities from tunnelling to over-the-barrier regime, and predicts a
significant alignment effect on the ratio of double over single ion yield. The
classical trajectory analysis allows to unveil sub-cycle molecular double
ionization dynamics.Comment: 5 pages, 5 figures. to appear in Phys. Rev. Lett.(2007
Classical Trajectory Perspective on Double Ionization Dynamics of Diatomic Molecules Irradiated by Ultrashort Intense Laser Pulses
In the present paper, we develop a semiclassical quasi-static model
accounting for molecular double ionization in an intense laser pulse. With this
model, we achieve insight into the dynamics of two highly-correlated valence
electrons under the combined influence of a two-center Coulomb potential and an
intense laser field, and reveal the significant influence of molecular
alignment on the ratio of double over single ion yield. Analysis on the
classical trajectories unveils sub-cycle dynamics of the molecular double
ionization. Many interesting features, such as the accumulation of emitted
electrons in the first and third quadrants of parallel momentum plane, the
regular pattern of correlated momentum with respect to the time delay between
closest collision and ionization moment, are revealed and successfully
explained by back analyzing the classical trajectories. Quantitative agreement
with experimental data over a wide range of laser intensities from tunneling to
over-the-barrier regime is presented.Comment: 8 pages, 9 figure
Existence theorems for a nonlinear second-order distributional differential equation
In this work, we are concerned with existence of solutions for a nonlinear
second-order distributional differential equation, which contains measure
differential equations and stochastic differential equations as special cases.
The proof is based on the Leray--Schauder nonlinear alternative and
Kurzweil--Henstock--Stieltjes integrals. Meanwhile, examples are worked out to
demonstrate that the main results are sharp.Comment: This is a preprint of a paper whose final and definite form is with
'Journal of King Saud University - Science', ISSN 1018-3647. Submitted
05-March-2017; revised 24-April-2017; accepted for publication 26-April-201
Time Dependent Saddle Node Bifurcation: Breaking Time and the Point of No Return in a Non-Autonomous Model of Critical Transitions
There is a growing awareness that catastrophic phenomena in biology and
medicine can be mathematically represented in terms of saddle-node
bifurcations. In particular, the term `tipping', or critical transition has in
recent years entered the discourse of the general public in relation to
ecology, medicine, and public health. The saddle-node bifurcation and its
associated theory of catastrophe as put forth by Thom and Zeeman has seen
applications in a wide range of fields including molecular biophysics,
mesoscopic physics, and climate science. In this paper, we investigate a simple
model of a non-autonomous system with a time-dependent parameter and
its corresponding `dynamic' (time-dependent) saddle-node bifurcation by the
modern theory of non-autonomous dynamical systems. We show that the actual
point of no return for a system undergoing tipping can be significantly delayed
in comparison to the {\em breaking time} at which the
corresponding autonomous system with a time-independent parameter undergoes a bifurcation. A dimensionless parameter
is introduced, in which is the curvature
of the autonomous saddle-node bifurcation according to parameter ,
which has an initial value of and a constant rate of change . We
find that the breaking time is always less than the actual point
of no return after which the critical transition is irreversible;
specifically, the relation is analytically obtained. For a system with a small , there exists a significant window of opportunity
during which rapid reversal of the environment can save the system from
catastrophe
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