743 research outputs found
"Double-trace" Deformations, Boundary Conditions and Spacetime Singularities
Double-trace deformations of the AdS/CFT duality result in a new perturbation
expansion for string theory, based on a non-local worldsheet. We discuss some
aspects of the deformation in the low energy gravity approximation, where it
appears as a change in the boundary condition of fields. We relate unique
features of the boundary of AdS to the worldsheet becoming non-local, and
conjecture that non-local worldsheet actions may be generic in other classes of
backgrounds.Comment: 21 pages, 2 figures, harvmac. v2: minor changes, references added,
version sent to JHEP. v3 minor correction
OXIDATIVE-REFORMING OF METHANE AND PARTIAL OXIDATION OF METHANE REACTIONS OVER NiO/PrO2/ZrO2 CATALYSTS: EFFECT OF NICKEL CONTENT
In this work the behavior of NiO-PrO2-ZrO2 catalysts containing various nickel loadings was evaluated in the partial oxidation of methane and oxidative-reforming reactions of methane. The catalysts were characterized by X-Ray Diffraction Analysis (in situ-XRD), Temperature Programmed Reduction (H-2-TPR), Scanning Electron Microscopy (SEM/EDX) and Adsorption-Desorption of nitrogen (BET area). The reactions were carried out at 750 degrees C and 1 atm for 5 hours. The catalysts were studied with different nickel content: 0, 5, 10 and 15% (related to total weight of catalyst, wt%). In both reactions, the catalyst containing the mixture of the three oxides (NiO/PrO2/ZrO2) with 15% nickel (15NiPrZr catalyst) showed the best activity for the conversion of the reactants into Syngas and showed high selectivity for H-2 and CO. The results suggest that the promoter PrO2 and the Ni degrees centers are in a good proportion in the catalyst with 15% Ni. Our results showed that low nickel concentrations in the catalyst led to high metallic dispersion; however, very low nickel concentrations did not favor the methane transformation into Syngas. The catalyst containing only NiO/ZrO2 in the mixture was not sufficient for the catalysis. The presence of the promoter PrO2 was very important for the catalysis of the POM.Univ Fed Sao Paulo, Inst Mar, BR-11070100 Santos, SP, BrazilUniv Sao Paulo, Inst Quim Sao Carlos, BR-13560970 Sao Carlos, SP, BrazilUniv Fed Sao Carlos, Dept Engn Quim, BR-13565905 Sao Carlos, SP, BrazilUniv Fed Sao Paulo, Inst Mar, BR-11070100 Santos, SP, BrazilWeb of Scienc
The Effective Potential of the N=0* Yang-Mills Theory
We study the \N=4 SYM theory with SU(N) gauge group in the large N limit,
deformed by giving equal mass to the four adjoint fermions. With this
modification, a potential is dynamically generated for the six scalars in the
theory, \phi^i. We show that the resulting theory is stable (perturbatively in
the 't Hooft coupling), and that there are some indications that =0 is
the vacuum of the theory. Using the AdS/CFT correspondence, we compare the
results to the corresponding supergravity computation, i.e. brane probing a
deformed AdS_5 x S^5 background, and we find qualitative agreement.Comment: 12 pages, 2 figures, version to appear in JHE
Intensity correlations and mesoscopic fluctuations of diffusing photons in cold atoms
We study the angular correlation function of speckle patterns that result
from multiple scattering of photons by cold atomic clouds. We show that this
correlation function becomes larger than the value given by Rayleigh law for
classical scatterers. These large intensity fluctuations constitute a new
mesoscopic interference effect specific to atom-photon interactions, that could
not be observed in other systems such as weakly disordered metals. We provide a
complete description of this behavior and expressions that allow for a
quantitative comparison with experiments.Comment: 4 pages, 2 figure
Reduction of quantum noise in optical interferometers using squeezed light
We study the photon counting noise in optical interferometers used for
gravitational wave detection. In order to reduce quantum noise a squeezed
vacuum state is injected into the usually unused input port. Here, we
specifically investigate the so called `dark port case', when the beam splitter
is oriented close to 90{\deg} to the incoming laser beam, such that nearly all
photons go to one output port of the interferometer, and only a small fraction
of photons is seen in the other port (`dark port'). For this case it had been
suggested that signal amplification is possible without concurrent noise
amplification [R.Barak and Y.Ben-Aryeh, J.Opt.Soc.Am.B25(361)2008]. We show
that by injection of a squeezed vacuum state into the second input port,
counting noise is reduced for large values of the squeezing factor, however the
signal is not amplified. Signal strength only depends on the intensity of the
laser beam.Comment: 8 pages, 1 figur
On Witten's Instability and Winding Tachyons
We investigate, from a spacetime perspective, some aspects of Horowitz's
recent conjecture that black strings may catalyze the decay of Kaluza-Klein
spacetimes into a bubble of nothing. We identify classical configurations that
interpolate between flat space and the bubble, and discuss the energetics of
the transition. We investigate the effects of winding tachyons on the size and
shape of the barrier and find no evidence at large compactification radius that
tachyons enhance the tunneling rate. For the interesting radii, of order the
string scale, the question is difficult to answer due to the failure of the
expansion.Comment: 15 pages, 2 figures, Late
The Lensed Arc Production Efficiency of Galaxy Clusters: A Comparison of Matched Observed and Simulated Samples
We compare the statistical properties of giant gravitationally lensed arcs
produced in matched simulated and observed cluster samples. The observed sample
consists of 10 X-ray selected clusters at redshifts z ~ 0.2 imaged with HST by
Smith et al. The simulated dataset is produced by lensing the Hubble Deep
Field, which serves as a background source image, with 150 realizations
(different projections and shifts) of five simulated z = 0.2 clusters from a
LambdaCDM N-body simulation. The real and simulated clusters have similar
masses, the real photometric redshift is used for each background source, and
all the observational effects influencing arc detection in the real dataset,
including light from cluster galaxies, are simulated in the artificial dataset.
We develop, and apply to both datasets, an objective automatic arc-finding
algorithm. We find consistent arc statistics in the real and in the simulated
sample, with an average of ~ 1 detected giant (length to width ratio >= 10) arc
per cluster and ~ 0.2 giant luminous (R<22.3 mag) arc per cluster. Thus, taking
into account a realistic source population and observational effects, the
clusters predicted by LambdaCDM have the same arc-production efficiency as the
observed clusters. If, as suggested by other studies, there is a discrepancy
between the predicted and the observed total number of arcs on the sky, it must
be the result of differences between the redshift dependent cluster mass
functions, and not due to differences in the lensing efficiency of the most
massive clusters.Comment: 13 pages, Accepted by ApJ, High resolution version of the paper can
be found at: ftp://wise3.tau.ac.il/pub/assafh/horesh_arcs_stat_2005.ps.gz,
Arc-finding algorithm available at: http://wise-obs.tau.ac.il/~assafh/ ; A
comment was added ; A missing x-axis label in Fig. 7 was adde
The Moduli Space and M(atrix) Theory of 9d N=1 Backgrounds of M/String Theory
We discuss the moduli space of nine dimensional N=1 supersymmetric
compactifications of M theory / string theory with reduced rank (rank 10 or
rank 2), exhibiting how all the different theories (including M theory
compactified on a Klein bottle and on a Mobius strip, the Dabholkar-Park
background, CHL strings and asymmetric orbifolds of type II strings on a
circle) fit together, and what are the weakly coupled descriptions in different
regions of the moduli space. We argue that there are two disconnected
components in the moduli space of theories with rank 2. We analyze in detail
the limits of the M theory compactifications on a Klein bottle and on a Mobius
strip which naively give type IIA string theory with an uncharged orientifold
8-plane carrying discrete RR flux. In order to consistently describe these
limits we conjecture that this orientifold non-perturbatively splits into a
D8-brane and an orientifold plane of charge (-1) which sits at infinite
coupling. We construct the M(atrix) theory for M theory on a Klein bottle (and
the theories related to it), which is given by a 2+1 dimensional gauge theory
with a varying gauge coupling compactified on a cylinder with specific boundary
conditions. We also clarify the construction of the M(atrix) theory for
backgrounds of rank 18, including the heterotic string on a circle.Comment: 43 pages, 7 figures, JHEP format. v3: typos correcte
Structural and nuclear characterizations of defects created by noble gas implantation in silicon oxide
Thermally grown silicon oxide layer was implanted at room temperature with 300keV Xe at fluences ranging from 0.5 to 5x10Xe/cm. Bubbles created after Xe-implantation provided a low-k silicon oxide that has potential use as a dielectric material for interconnects in Si integrated circuits. Transmission Electron Microscopy (TEM), Rutherford Backscattering Spectrometry (RBS) and Positron Annihilation Spectroscopy (PAS) were used to provide a comprehensive characterization of defects (bubbles, vacancy, gas atoms and other types of defects) created by Xe implantation in layer. These measurements suggest that the bubbles observed with TEM for all fluences were a consequence of the interaction between Xe and vacancies (V), with complexes created in the zone where V and Xe profiles overlap. Negatively charged defects such as (, and ) are also created after implantation
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