112,023 research outputs found
Giant enhanced optical nonlinearity of colloidal nanocrystals with a graded-index host
The effective linear and third-order nonlinear optical properties of metallic
colloidal crystal immersed in a graded-index host fluid are investigated
theoretically. The local electric fields are extracted self-consistently based
on the layer-to-layer interactions, which are readily given by the Lekner
summation method. The resultant optical absorption and nonlinearity enhancement
show a series of sharp peaks, which merge in a broadened resonant band. The
sharp peaks become a continuous band for increasing packing density and number
of layers. We believe that the sharp peaks arise from the in-plane dipolar
interactions and the surface plasmon resonance, whereas the continuous band is
due to the presence of the gradient in the host refractive index. These results
have not been observed in homogeneous and randomly-dispersed colloids, and thus
would be of great interest in optical nanomaterial engineering.Comment: Submitted to Applied Physics Letter
The Deuteron Spin Structure Functions in the Bethe-Salpeter Approach and the Extraction of the Neutron Structure Function
The nuclear effects in the spin-dependent structure functions and
are calculated in the relativistic approach based on the Bethe-Salpeter
equation with a realistic meson-exchange potential.
The results of calculations are compared with the non-relativistic
calculations. The problem of extraction of the neutron spin structure function,
, from the deuteron data is discussed.Comment: (Talk given at the SPIN'94 International Symposium, September 15-22,
1994, Bloomington, Indiana), 6 pages, 5 figures, Preprint Alberta Thy 29-9
Current-Phase Relation of a Bose-Einstein Condensate Flowing Through a Weak Link
We study the current-phase relation of a Bose-Einstein condensate flowing
through a repulsive square barrier by solving analytically the one dimensional
Gross-Pitaevskii equation. The barrier height and width fix the current-phase
relation , which tends to for weak
barriers and to the Josephson sinusoidal relation for
strong barriers. Between these two limits, the current-phase relation depends
on the barrier width. In particular, for wide enough barriers, we observe two
families of multivalued current-phase relations. Diagrams belonging to the
first family, already known in the literature, can have two different positive
values of the current at the same phase difference. The second family, new to
our knowledge, can instead allow for three different positive currents still
corresponding to the same phase difference. Finally, we show that the
multivalued behavior arises from the competition between hydrodynamic and
nonlinear-dispersive components of the flow, the latter due to the presence of
a soliton inside the barrier region.Comment: 6 pages, 5 figure
Three-dimensional, transonic rotor flow field reconstructed from holographic interferogram data
Holographic interferometry and computer-assisted tomography (CAT) are used to determine the transonic flow field of a model rotor blade in hover. A pulsed ruby laser records 40 interferograms with a 61 cm-diam view field near the model rotor-blade tip operating at a tip Mach number of 0.90. After digitizing the interferograms and extracting fringe-order functions, the data are transferred to a CAT code. The CAT code then calculates pressure coefficients in several planes above the blade surface. The values from the holography-CAT method compare favorably with previously obtained numerical computations and laser velocimeter measurements at most locations near the blade tip. The results demonstrate the technique's potential for three-dimensional transonic rotor flow studies
Reconstruction of a 3-dimensional transonic rotor flow field from holographic interferogram data
Holographic interferometry and computer-assisted tomography (CAT) are used to determine the transonic velocity field of a model rotor blade in hover. A pulsed ruby laser recorded 40 interferograms with a 2-ft-diam view field near the model rotor-blade tip operating at a tip Mach number of 0.90. After digitizing the interferograms and extracting fringe-order functions, the data are transferred to a CAT code. The CAT code then calculates the perturbation velocity in seeral planes above the blade surface. The values from the holography-CAT method compare favorably with previously obtained numerical computations in most locations near the blade tip. The results demonstrate the technique's potential for three-dimensional transonic rotor flow studies
Emission Optics of the Steigerwald Type Electron Gun
The emission optics of a Steigerwald type electron gun is re-examined. The
virtual and real points of divergence, divergence angles and beam-widths of the
electron beams at different telefocusing strength are measured in detail for
first time . Two different Wehnelt cylinders are used to establish a
contrasting viewpoint. The original `focusing' curves measured by Braucks are
reconstructed and will be explained only through a `new' interpretation which
is different from the conventional views. While the image of the emitting
surface in front of the filament is indeed telefocused beyond the anode, the
envelope of the beam does not `focus' as expected. A new model for the emission
mechanism is established based on our results.Comment: 14 pages, 10 figure
An unexpectedly low-redshift excess of Swift gamma-ray burst rate
Gamma-ray bursts (GRBs) are the most violent explosions in the Universe and
can be used to explore the properties of high-redshift universe. It is believed
that the long GRBs are associated with the deaths of massive stars. So it is
possible to use GRBs to investigate the star formation rate (SFR). In this
paper, we use Lynden-Bell's method to study the luminosity function and
rate of \emph{Swift} long GRBs without any assumptions. We find that the
luminosity of GRBs evolves with redshift as with
. After correcting the redshift evolution through
, the luminosity function can be expressed as
for dim GRBs and for bright GRBs, with the break point
. We also find that the formation
rate of GRBs is almost constant at for the first time, which is
remarkably different from the SFR. At , the formation rate of GRB is
consistent with the SFR. Our results are dramatically different from previous
studies. Some possible reasons for this low-redshift excess are discussed. We
also test the robustness of our results with Monte Carlo simulations. The
distributions of mock data (i.e., luminosity-redshift distribution, luminosity
function, cumulative distribution and distribution) are in good
agreement with the observations. Besides, we also find that there are
remarkable difference between the mock data and the observations if long GRB
are unbiased tracers of SFR at .Comment: 33 pages, 10 figures, 1 table, accepted by ApJ
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