26,320 research outputs found
X-ray based extensometry
A totally new method of extensometry using an X-ray beam was proposed. The intent of the method is to provide a non-contacting technique that is immune to problems associated with density variations in gaseous environments that plague optical methods. X-rays are virtually unrefractable even by solids. The new method utilizes X-ray induced X-ray fluorescence or X-ray induced optical fluorescence of targets that have melting temperatures of over 3000 F. Many different variations of the basic approaches are possible. In the year completed, preliminary experiments were completed which strongly suggest that the method is feasible. The X-ray induced optical fluorescence method appears to be limited to temperatures below roughly 1600 F because of the overwhelming thermal optical radiation. The X-ray induced X-ray fluorescence scheme appears feasible up to very high temperatures. In this system there will be an unknown tradeoff between frequency response, cost, and accuracy. The exact tradeoff can only be estimated. It appears that for thermomechanical tests with cycle times on the order of minutes a very reasonable system may be feasible. The intended applications involve very high temperatures in both materials testing and monitoring component testing. Gas turbine engines, rocket engines, and hypersonic vehicles (NASP) all involve measurement needs that could partially be met by the proposed technology
Active Learning with Statistical Models
For many types of machine learning algorithms, one can compute the
statistically `optimal' way to select training data. In this paper, we review
how optimal data selection techniques have been used with feedforward neural
networks. We then show how the same principles may be used to select data for
two alternative, statistically-based learning architectures: mixtures of
Gaussians and locally weighted regression. While the techniques for neural
networks are computationally expensive and approximate, the techniques for
mixtures of Gaussians and locally weighted regression are both efficient and
accurate. Empirically, we observe that the optimality criterion sharply
decreases the number of training examples the learner needs in order to achieve
good performance.Comment: See http://www.jair.org/ for any accompanying file
Energy transfer from an individual quantum dot to a carbon nanotube
A detailed understanding of energy transduction is crucial for achieving
precise control of energy flow in complex, integrated systems. In this context,
carbon nanotubes (CNTs) are intriguing model systems due to their rich,
chirality-dependent electronic and optical properties. Here, we study the
quenching of fluorescence from isolated quantum dots (QDs) upon approach of
individual CNTs attached to atomic force microscope probes. Precision
measurements of many different CNT/QD pairs reveal behavior consistent with
resonant energy transfer between QD and CNT excitons via a Fohrster-like
dipole-dipole coupling. The data reveal large variations in energy transfer
length scales even though peak efficiencies are narrowly distributed around
96%. This saturation of efficiency is maintained even when energy transfer must
compete with elevated intrinsic non-radiative relaxation rates during QD aging.
These observations suggest that excitons can be created at different locations
along the CNT length, thereby resulting in self-limiting behavior.Comment: 8 pages, 8 figures, with supplementary informatio
An astronomical search for evidence of new physics: Limits on gravity-induced birefringence from the magnetic white dwarf RE J0317-853
The coupling of the electromagnetic field directly with gravitational gauge
fields leads to new physical effects that can be tested using astronomical
data. Here we consider a particular case for closer scrutiny, a specific
nonminimal coupling of torsion to electromagnetism, which enters into a
metric-affine geometry of space-time. We show that under the assumption of this
nonminimal coupling, spacetime is birefringent in the presence of such a
gravitational field. This leads to the depolarization of light emitted from
extended astrophysical sources. We use polarimetric data of the magnetic white
dwarf to set strong constraints on the essential coupling
constant for this effect, giving k^2 \lsim (19 {m})^2 .Comment: Statements about Moffat's NGT modified. Accepted for publication in
Phys.Rev.
Thermal correlators of anyons in two dimensions
The anyon fields have trivial -commutator for not integer.
For integer the commutators become temperature-dependent operator
valued distributions. The -point functions do not factorize as for quasifree
states.Comment: 14 pages, LaTeX (misprints corrected, a reference added
Affine maps of density matrices
For quantum systems described by finite matrices, linear and affine maps of
matrices are shown to provide equivalent descriptions of evolution of density
matrices for a subsystem caused by unitary Hamiltonian evolution in a larger
system; an affine map can be replaced by a linear map, and a linear map can be
replaced by an affine map. There may be significant advantage in using an
affine map. The linear map is generally not completely positive, but the linear
part of an equivalent affine map can be chosen to be completely positive and
related in the simplest possible way to the unitary Hamiltonian evolution in
the larger system.Comment: 4 pages, title changed, sentence added, reference update
Localized gap soliton trains of Bose-Einstein condensates in an optical lattice
We develop a systematic analytical approach to study the linear and nonlinear
solitary excitations of quasi-one-dimensional Bose-Einstein condensates trapped
in an optical lattice. For the linear case, the Bloch wave in the energy
band is a linear superposition of Mathieu's functions and ;
and the Bloch wave in the band gap is a linear superposition of
and . For the nonlinear case, only solitons inside the band gaps are
likely to be generated and there are two types of solitons -- fundamental
solitons (which is a localized and stable state) and sub-fundamental solitons
(which is a lacalized but unstable state). In addition, we find that the
pinning position and the amplitude of the fundamental soliton in the lattice
can be controlled by adjusting both the lattice depth and spacing. Our
numerical results on fundamental solitons are in quantitative agreement with
those of the experimental observation [Phys. Rev. Lett. {\bf92}, 230401
(2004)]. Furthermore, we predict that a localized gap soliton train consisting
of several fundamental solitons can be realized by increasing the length of the
condensate in currently experimental conditions.Comment: 9 pages, 6 figures, accepted for publicaiton in PR
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