6,664 research outputs found
A spectroscopic study of the cycling transition 4s[3/2]_2-4p[5/2]_3 at 811.8 nm in Ar-39: Hyperfine structure and isotope shift
Doppler-free saturated absorption spectroscopy is performed on an enriched
radioactive Ar-39 sample. The spectrum of the 3s^2 3p^5 4s [3/2]_2 - 3s^2 3p^5
4p [5/2]_3 cycling transition at 811.8 nm is recorded, and its isotope shift
between Ar-39 and Ar-40 is derived. The hyperfine coupling constants A and B
for both the 4s [3/2]_2 and 4p [5/2]_3 energy levels in Ar-39 are also
determined. The results partially disagree with a recently published
measurement of the same transition. Based on earlier measurements as well as
the current work, the isotope shift and hyperfine structure of the
corresponding transition in Ar-37 are also calculated. These spectroscopic data
are essential for the realization of laser trapping and cooling of Ar-37 and
Ar-39
GPU-based Low Dose CT Reconstruction via Edge-preserving Total Variation Regularization
High radiation dose in CT scans increases a lifetime risk of cancer and has
become a major clinical concern. Recently, iterative reconstruction algorithms
with Total Variation (TV) regularization have been developed to reconstruct CT
images from highly undersampled data acquired at low mAs levels in order to
reduce the imaging dose. Nonetheless, TV regularization may lead to
over-smoothed images and lost edge information. To solve this problem, in this
work we develop an iterative CT reconstruction algorithm with edge-preserving
TV regularization to reconstruct CT images from highly undersampled data
obtained at low mAs levels. The CT image is reconstructed by minimizing an
energy consisting of an edge-preserving TV norm and a data fidelity term posed
by the x-ray projections. The edge-preserving TV term is proposed to
preferentially perform smoothing only on non-edge part of the image in order to
avoid over-smoothing, which is realized by introducing a penalty weight to the
original total variation norm. Our iterative algorithm is implemented on GPU to
improve its speed. We test our reconstruction algorithm on a digital NCAT
phantom, a physical chest phantom, and a Catphan phantom. Reconstruction
results from a conventional FBP algorithm and a TV regularization method
without edge preserving penalty are also presented for comparison purpose. The
experimental results illustrate that both TV-based algorithm and our
edge-preserving TV algorithm outperform the conventional FBP algorithm in
suppressing the streaking artifacts and image noise under the low dose context.
Our edge-preserving algorithm is superior to the TV-based algorithm in that it
can preserve more information of fine structures and therefore maintain
acceptable spatial resolution.Comment: 21 pages, 6 figures, 2 table
A Multi-Phase Transport model for nuclear collisions at RHIC
To study heavy ion collisions at energies available from the Relativistic
Heavy Ion Collider, we have developed a multi-phase transport model that
includes both initial partonic and final hadronic interactions. Specifically,
the parton cascade model ZPC, which uses as input the parton distribution from
the HIJING model, is extended to include the quark-gluon to hadronic matter
transition and also final-state hadronic interactions based on the ART model.
Predictions of the model for central Au on Au collisions at RHIC are reported.Comment: 7 pages, 4 figure
Intrinsic response time of graphene photodetectors
Graphene-based photodetectors are promising new devices for high-speed
optoelectronic applications. However, despite recent efforts, it is not clear
what determines the ultimate speed limit of these devices. Here, we present
measurements of the intrinsic response time of metal-graphene-metal
photodetectors with monolayer graphene using an optical correlation technique
with ultrashort laser pulses. We obtain a response time of 2.1 ps that is
mainly given by the short lifetime of the photogenerated carriers. This time
translates into a bandwidth of ~262 GHz. Moreover, we investigate the
dependence of the response time on gate voltage and illumination laser power
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A review of the role of the Atlantic meridional overturning circulation in Atlantic multidecadal variability and associated climate impacts
By synthesizing recent studies employing a wide range of approaches (modern observations, paleo reconstructions, and climate model simulations), this paper provides a comprehensive review of the linkage between multidecadal Atlantic Meridional Overturning Circulation (AMOC) variability and Atlantic Multidecadal Variability (AMV) and associated climate impacts. There is strong observational and modeling evidence that multidecadal AMOC variability is a crucial driver of the observed AMV and associated climate impacts and an important source of enhanced decadal predictability and prediction skill. The AMOCâAMV linkage is consistent with observed key elements of AMV. Furthermore, this synthesis also points to a leading role of the AMOC in a range of AMVârelated climate phenomena having enormous societal and economic implications, for example, Intertropical Convergence Zone shifts; Sahel and Indian monsoons; Atlantic hurricanes; El NiñoâSouthern Oscillation; Pacific Decadal Variability; North Atlantic Oscillation; climate over Europe, North America, and Asia; Arctic sea ice and surface air temperature; and hemisphericâscale surface temperature. Paleoclimate evidence indicates that a similar linkage between multidecadal AMOC variability and AMV and many associated climate impacts may also have existed in the preindustrial era, that AMV has enhanced multidecadal power significantly above a red noise background, and that AMV is not primarily driven by external forcing. The role of the AMOC in AMV and associated climate impacts has been underestimated in most stateâofâtheâart climate models, posing significant challenges but also great opportunities for substantial future improvements in understanding and predicting AMV and associated climate impacts
Photocurrent measurements of supercollision cooling in graphene
The cooling of hot electrons in graphene is the critical process underlying
the operation of exciting new graphene-based optoelectronic and plasmonic
devices, but the nature of this cooling is controversial. We extract the hot
electron cooling rate near the Fermi level by using graphene as novel
photothermal thermometer that measures the electron temperature () as it
cools dynamically. We find the photocurrent generated from graphene
junctions is well described by the energy dissipation rate , where the heat capacity is and is the
base lattice temperature. These results are in disagreement with predictions of
electron-phonon emission in a disorder-free graphene system, but in excellent
quantitative agreement with recent predictions of a disorder-enhanced
supercollision (SC) cooling mechanism. We find that the SC model provides a
complete and unified picture of energy loss near the Fermi level over the wide
range of electronic (15 to 3000 K) and lattice (10 to 295 K) temperatures
investigated.Comment: 7pages, 5 figure
Cosmological Solutions in String Theories
We obtain a large class of cosmological solutions in the
toroidally-compactified low energy limits of string theories in dimensions.
We consider solutions where a -dimensional subset of the spatial
coordinates, parameterising a flat space, a sphere, or an hyperboloid,
describes the spatial sections of the physically-observed universe. The
equations of motion reduce to Liouville or Toda equations, which
are exactly solvable. We study some of the cases in detail, and find that under
suitable conditions they can describe four-dimensional expanding universes. We
discuss also how the solutions in dimensions behave upon oxidation back to
the string theory or M-theory.Comment: Latex, 21 pages, a reference adjuste
Heavy ion collisions and AdS/CFT
We review some recent applications of the AdS/CFT correspondence to heavy ion
collisions including a calculation of the jet quenching parameter in N=4
super-Yang-Mills theory and quarkonium suppression from velocity scaling of the
screening length for a heavy quark-antiquark pair. We also briefly discuss
differences and similarities between QCD and N=4 Super-Yang-Mills theory.Comment: Plenary talk given at Quark Matter 2006, Shanghai, China, 14-20 Nov
2006; to appear in the conference proceedin
Photoconductivity of biased graphene
Graphene is a promising candidate for optoelectronic applications such as
photodetectors, terahertz imagers, and plasmonic devices. The origin of
photoresponse in graphene junctions has been studied extensively and is
attributed to either thermoelectric or photovoltaic effects. In addition, hot
carrier transport and carrier multiplication are thought to play an important
role. Here we report the intrinsic photoresponse in biased but otherwise
homogeneous graphene. In this classic photoconductivity experiment, the
thermoelectric effects are insignificant. Instead, the photovoltaic and a
photo-induced bolometric effect dominate the photoresponse due to hot
photocarrier generation and subsequent lattice heating through electron-phonon
cooling channels respectively. The measured photocurrent displays polarity
reversal as it alternates between these two mechanisms in a backgate voltage
sweep. Our analysis yields elevated electron and phonon temperatures, with the
former an order higher than the latter, confirming that hot electrons drive the
photovoltaic response of homogeneous graphene near the Dirac point
Electrically Tunable Excitonic Light Emitting Diodes based on Monolayer WSe2 p-n Junctions
Light-emitting diodes are of importance for lighting, displays, optical
interconnects, logic and sensors. Hence the development of new systems that
allow improvements in their efficiency, spectral properties, compactness and
integrability could have significant ramifications. Monolayer transition metal
dichalcogenides have recently emerged as interesting candidates for
optoelectronic applications due to their unique optical properties.
Electroluminescence has already been observed from monolayer MoS2 devices.
However, the electroluminescence efficiency was low and the linewidth broad due
both to the poor optical quality of MoS2 and to ineffective contacts. Here, we
report electroluminescence from lateral p-n junctions in monolayer WSe2 induced
electrostatically using a thin boron nitride support as a dielectric layer with
multiple metal gates beneath. This structure allows effective injection of
electrons and holes, and combined with the high optical quality of WSe2 it
yields bright electroluminescence with 1000 times smaller injection current and
10 times smaller linewidth than in MoS2. Furthermore, by increasing the
injection bias we can tune the electroluminescence between regimes of
impurity-bound, charged, and neutral excitons. This system has the required
ingredients for new kinds of optoelectronic devices such as spin- and
valley-polarized light-emitting diodes, on-chip lasers, and two-dimensional
electro-optic modulators.Comment: 13 pages main text with 4 figures + 4 pages upplemental material
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