2,486 research outputs found
Velocity measurements by laser resonance fluorescence
The photonburst correlation method was used to detect single atoms in a buffer gas. Real time flow velocity measurements with laser induced resonance fluorescence from single or multiple atoms was demonstrated and this method was investigated as a tool for wind tunnel flow measurement. Investigations show that single atoms and their real time diffusional motion on a buffer gas can be measured by resonance fluorescence. By averaging over many atoms, flow velocities up to 88 m/s were measured in a time of 0.5 sec. It is expected that higher flow speeds can be measured and that the measurement time can be reduced by a factor of 10 or more by careful experimental design. The method is clearly not ready for incorporation in high speed wind tunnels because it is not yet known whether the stray light level will be higher or lower, and it is not known what detection efficiency can be obtained in a wind tunnel situation
Nonperiodic oscillation of bright solitons in the condensates with a periodically oscillating harmonic potential
Considering a periodically oscillating harmonic potential, we explore the
dynamics properties of bright solitons in a Bose-Einstein condensate. It is
found that under a slower oscillating potential, soliton movement exhibits a
nonperiodic oscillation while it is hardly affected under a fast oscillating
potential. Furthermore, the head-on and/or "chase" collisions of two solitons
have been obtained, which can be controlled by the oscillating frequency of
potential.Comment: 4 pages, 2 figure
Lymph node removal enhances corneal graft survival in mice at high risk of rejection
Peer reviewedPublisher PD
Characterization of membranes with X-ray ultramicroscopy
Non-invasive characterization and observation of synthetic membranes is an important practice to monitor the performance of membrane process. Primarily there are two techniques—optical and non-optical for this purpose. Among them, X-ray computed tomography, as a non-optical technique, has been extensively used for the measurement of fibre distribution and air pockets trapped in the modules. However, the micro resolution of most commercial systems has limited its application which can hardly be used for the sub-micro characterization of membrane processes. A novel micro and nano characterization method is introduced in the current work by exploring an innovative development of the X-ray ultramicroscope (XuM) and micro-tomographic techniques. The XuM, based on using a scanning electron microscope as host, provides a new approach to X-ray projection microscopy. It has demonstrated the ability to characterize very small features in objects, down to of order 100 nm, including the use for dry, wet and even liquid samples. It can also distinguish objects with very subtle difference in density.<br /
Interrelationship Between Nutrients and Chlorophyll-a in an Urban Stormwater Lake During the Ice-covered Period
Urban stormwater lakes in cold regions are ice-covered for substantial parts of the winter. It has long been considered that the ice-covered period is the “dormant season,” during which ecological processes are inactive. However, little is known about this period due to the historical focus on the open-water season. Recent pioneering research on ice-covered natural lakes has suggested that some critical ecological processes play out on the ice. The objective of this study was to investigate the active processes in ice-covered stormwater lakes. Data collected during a two-year field measurement program at a stormwater lake located in Edmonton, Alberta, Canada were analyzed. The lake was covered by ice from November to mid-April of the following year. The mean value of chlorophyll-a during the ice-covered period was 22.09% of the mean value for the open-water season, suggesting that primary productivity under ice can be important. Nitrogen and phosphorus were remarkably higher during the ice-covered period, while dissolved organic carbon showed little seasonal variation. Under ice-covered conditions, the total phosphorus was the major nutrient controlling the ratio of total nitrogen to total phosphorus, and a significant positive correlation existed between total phosphorus and chlorophyll-a when the ratio was smaller than 10. The results provide preliminary evidence of the critical nutrient processes in the Stormwater Lake during the ice-covered period
D-instantons and Closed String Tachyons in Misner Space
We investigate closed string tachyon condensation in Misner space, a toy
model for big bang universe. In Misner space, we are able to condense tachyonic
modes of closed strings in the twisted sectors, which is supposed to remove the
big bang singularity. In order to examine this, we utilize D-instanton as a
probe. First, we study general properties of D-instanton by constructing
boundary state and effective action. Then, resorting to these, we are able to
show that tachyon condensation actually deforms the geometry such that the
singularity becomes milder.Comment: 24 pages, 1 figure, minor change
Universally valid reformulation of the Heisenberg uncertainty principle on noise and disturbance in measurement
The Heisenberg uncertainty principle states that the product of the noise in
a position measurement and the momentum disturbance caused by that measurement
should be no less than the limit set by Planck's constant, hbar/2, as
demonstrated by Heisenberg's thought experiment using a gamma-ray microscope.
Here I show that this common assumption is false: a universally valid trade-off
relation between the noise and the disturbance has an additional correlation
term, which is redundant when the intervention brought by the measurement is
independent of the measured object, but which allows the noise-disturbance
product much below Planck's constant when the intervention is dependent. A
model of measuring interaction with dependent intervention shows that
Heisenberg's lower bound for the noise-disturbance product is violated even by
a nearly nondisturbing, precise position measuring instrument. An experimental
implementation is also proposed to realize the above model in the context of
optical quadrature measurement with currently available linear optical devices.Comment: Revtex, 6 page
Aluminum Oxide Layers as Possible Components for Layered Tunnel Barriers
We have studied transport properties of Nb/Al/AlOx/Nb tunnel junctions with
ultrathin aluminum oxide layers formed by (i) thermal oxidation and (ii) plasma
oxidation, before and after rapid thermal post-annealing of the completed
structures at temperatures up to 550 deg C. Post-annealing at temperatures
above 300 deg C results in a significant decrease of the tunneling conductance
of thermally-grown barriers, while plasma-grown barriers start to change only
at annealing temperatures above 450 deg C. Fitting the experimental I-V curves
of the junctions using the results of the microscopic theory of direct
tunneling shows that the annealing of thermally-grown oxides at temperatures
above 300 deg C results in a substantial increase of their average tunnel
barriers height, from ~1.8 eV to ~2.45 eV, versus the practically unchanged
height of ~2.0 eV for plasma-grown layers. This difference, together with high
endurance of annealed barriers under electric stress (breakdown field above 10
MV/cm) may enable all-AlOx and SiO2/AlOx layered "crested" barriers for
advanced floating-gate memory applications.Comment: 7 pages, 6 figure
Toward the End of Time
The null-brane space-time provides a simple model of a big crunch/big bang
singularity. A non-perturbative definition of M-theory on this space-time was
recently provided using matrix theory. We derive the fermion couplings for this
matrix model and study the leading quantum effects. These effects include
particle production and a time-dependent potential. Our results suggest that as
the null-brane develops a big crunch singularity, the usual notion of
space-time is replaced by an interacting gluon phase. This gluon phase appears
to constitute the end of our conventional picture of space and time.Comment: 31 pages, reference adde
Turbulence and Multiscaling in the Randomly Forced Navier Stokes Equation
We present an extensive pseudospectral study of the randomly forced
Navier-Stokes equation (RFNSE) stirred by a stochastic force with zero mean and
a variance , where is the wavevector and the dimension . We present the first evidence for multiscaling of velocity structure
functions in this model for . We extract the multiscaling exponent
ratios by using extended self similarity (ESS), examine their
dependence on , and show that, if , they are in agreement with those
obtained for the deterministically forced Navier-Stokes equation (NSE). We
also show that well-defined vortex filaments, which appear clearly in studies
of the NSE, are absent in the RFNSE.Comment: 4 pages (revtex), 6 figures (postscript
- …