18,820 research outputs found
Ag and N acceptors in ZnO: ab initio study of acceptor pairing, doping efficiency, and the role of hydrogen
Efficiency of ZnO doping with Ag and N shallow acceptors, which substitute
respectively cations and anions, was investigated. First principles
calculations indicate a strong tendency towards formation of nearest neighbor
Ag-N pairs and N-Ag-N triangles. Binding of acceptors stems from the formation
of quasi-molecular bonds between dopants, and has a universal character in
semiconductors. The pairing increases energy levels of impurities, and thus
lowers doping efficiency. In the presence of donors, pairing is weaker or even
forbidden. However, hydrogen has a tendency to form clusters with Ag and N,
which favors the Ag-N aggregation and lowers the acceptor levels of such
complexes.Comment: 10 pages, 4 figure
Absence of reflection as a function of the coupling constant
We consider solutions of the one-dimensional equation where is locally integrable, is integrable with supp, and
is a coupling constant. Given a family of solutions
which satisfy for all , we prove that the zeros of , the Wronskian of and , form a discrete set
unless . Setting , one sees that a particular
consequence of this result may be stated as: if the fixed energy scattering
experiment gives rise to a reflection coefficient
which vanishes on a set of couplings with an accumulation point, then .Comment: To appear in Journal of Mathematical Physic
Classical instability of Kerr-AdS black holes and the issue of final state
It is now established that small Kerr-Anti-de Sitter (Kerr-AdS) black holes
are unstable against scalar perturbations, via superradiant amplification
mechanism. We show that small Kerr-AdS black holes are also unstable against
gravitational perturbations and we compute the features of this instability. We
also describe with great detail the evolution of this instability. In
particular, we identify its endpoint state. It corresponds to a Kerr-AdS black
hole whose boundary is an Einstein universe rotating with the light velocity.
This black hole is expected to be slightly oblate and to co-exist in
equilibrium with a certain amount of outside radiation.Comment: 11 pages, RevTex4. v2: small typos corrected. Version to appear in
Phys. Rev.
Resonant Cyclotron Radiation Transfer Model Fits to Spectra from Gamma-Ray Burst GRB870303
We demonstrate that models of resonant cyclotron radiation transfer in a
strong field (i.e. cyclotron scattering) can account for spectral lines seen at
two epochs, denoted S1 and S2, in the Ginga data for GRB870303. Using a
generalized version of the Monte Carlo code of Wang et al. (1988,1989b), we
model line formation by injecting continuum photons into a static
plane-parallel slab of electrons threaded by a strong neutron star magnetic
field (~ 10^12 G) which may be oriented at an arbitrary angle relative to the
slab normal. We examine two source geometries, which we denote "1-0" and "1-1,"
with the numbers representing the relative electron column densities above and
below the continuum photon source plane. We compare azimuthally symmetric
models, i.e. models in which the magnetic field is parallel to the slab normal,
with models having more general magnetic field orientations. If the bursting
source has a simple dipole field, these two model classes represent line
formation at the magnetic pole, or elsewhere on the stellar surface. We find
that the data of S1 and S2, considered individually, are consistent with both
geometries, and with all magnetic field orientations, with the exception that
the S1 data clearly favor line formation away from a polar cap in the 1-1
geometry, with the best-fit model placing the line-forming region at the
magnetic equator. Within both geometries, fits to the combined (S1+S2) data
marginally favor models which feature equatorial line formation, and in which
the observer's orientation with respect to the slab changes between the two
epochs. We interpret this change as being due to neutron star rotation, and we
place limits on the rotation period.Comment: LaTeX2e (aastex.cls included); 45 pages text, 17 figures (on 21
pages); accepted by ApJ (to be published 1 Nov 1999, v. 525
X-ray time variability across the atoll source states of 4U 1636--53
We have studied the rapid X-ray time variability in 149 pointed observations
with the \textit{Rossi X-ray Timing Explorer} (RXTE)'s Proportional Counter
Array of the atoll source 4U~1636--53 in the banana state and, for the first
time with RXTE, in the island state. We compare the frequencies of the
variability components of 4U~1636--53 with those in other atoll and Z-sources
and find that 4U~1636--53 follows the universal scheme of correlations
previously found for other atoll sources at (sometimes much) lower
luminosities. Our results on the hectohertz QPO suggest that the mechanism that
sets its frequency differs from that for the other components, while the
amplitude setting mechanism is common. A previously proposed interpretation of
the narrow low-frequency QPO frequencies in different sources in terms of
harmonic mode switching is not supported by our data, nor by some previous data
on other sources and the frequency range that this QPO covers is found not to
be related to spin, angular momentum or luminosity.Comment: 16 pages, 13 figures, accepted for publication in Ap
Magnetization Process of Kagome-Lattice Heisenberg Antiferromagnet
The magnetization process of the isotropic Heisenberg antiferromagnet on the
kagome lattice is studied. Data obtained from the numerical-diagonalization
method are reexamined from the viewpoint of the derivative of the magnetization
with respect to the magnetic field. We find that the behavior of the derivative
at approximately one-third of the height of the magnetization saturation is
markedly different from that for the cases of typical magnetization plateaux.
The magnetization process of the kagome-lattice antiferromagnet reveals a new
phenomenon, which we call the "magnetization ramp".Comment: 4 pages, 5figures, accepted in J. Phys. Soc. Jpn
MOTIFATOR: detection and characterization of regulatory motifs using prokaryote transcriptome data
Summary: Unraveling regulatory mechanisms (e.g. identification of motifs in cis-regulatory regions) remains a major challenge in the analysis of transcriptome experiments. Existing applications identify putative motifs from gene lists obtained at rather arbitrary cutoff and require additional manual processing steps. Our standalone application MOTIFATOR identifies the most optimal parameters for motif discovery and creates an interactive visualization of the results. Discovered putative motifs are functionally characterized, thereby providing valuable insight in the biological processes that could be controlled by the motif.
The effect of the presence of silicone films on the corrosion resistance of Zn and ZnFe coated steel
Chromatation pre-treatments have been widely used to improve galvanized steel corrosion resistance. However, due to the high toxicity of chromate ions, chromatation pre-treatments tend to be banned and, in last years, alternative coating systems are under investigation.
Recently, polysiloxanes have been developed for application as coatings. Among them, and due to their specific properties, such as hardness, chemical resistance and hydrophobicity, silicone resins may be considered as promising substitutes for chromatation pre-treatments.
In this work silicone films, obtained from the hydrolysis of a methoxy functional silicone
reactive intermediate, were applied on galvanized steel and on steel electroplated with a ZnFe alloy. Electrochemical techniques were used to characterise the degradation behaviour of the samples. These consisted on the monitoring of the open circuit potential (OCP), and on the potentiodynamic polarization of the samples, which was performed in a 3% NaCl aqueous
solution. Additionally, electrochemical impedance spectroscopy (EIS) was used as a complementary technique for the evaluation of the corrosion mechanisms of the coating
system. SEM and EDS were employed to inspect the surface of the samples before and after the electrochemical tests. EIS data was fitted to an equivalent circuit from which the electrochemical parameters were obtained.
Results show the protective character of the resin films, when compared with uncovered
specimens. The capacitance of the films increased with the immersion time, in accordance to the behaviour expected for an organic film. The overall performance of the coating systems appears to be highly dependent on the type of metallic coating applied to the steel. During the first three days of immersion the coatings applied upon galvanized steel showed larger |Z|
values when compared with those applied to the electroplated steel, indicating a superior
corrosion resistance of the former. However, after that time, an abrupt drop of |Z| is observed in the film applied on galvanized steel. In comparison, the coating system involving ZnFe alloy evidences a better stability throughout the immersion time.CAPES - Brasil
CNPq - Brasi
Organic-inorganic hybrid coatings for corrosion protection of galvanized steel and electroplated znfe steel
The development of hybrids materials by the sol-gel process has been extensively investigated in recent years. The combination a wide variety of compositions and production
processes had permit the use of these aterials in different applications like coatings for corrosion protection of metals and abrasion resistant coatings. In this work organic-inorganic hybrid materials
have been prepared from the hydrolysis of tetraethylorthosilicate (TEOS) and silanol-terminated polidymetilmetoxysilane (PDMS) using a sol-gel process. The composition evaluated was 50% TEOS- 50% PDMS. These materials have been applied on galvanized steel and on steel electroplated with a ZnFe alloy. In order to evaluate the degradation behaviour of these coatings, electrochemical techniques were used. They consisting in the monitoring of the open-circuit potential (Ecorr), and in the realization of potentiodynamic polarization measurements of the samples, which was performed in a
3% NaCl aqueous solution. Additionally, electrochemical impedance spectroscopy (EIS) was used as a complementary technique for the evaluation of the corrosion mechanisms of the coating system. The
surface of the samples, before and after the electrochemical tests, have been inspected by SEM and EDS. EIS data was fitted to an equivalent circuit from which the lectrochemical parameters were obtained. Results show a good protective character of the hybrid films, when compared with
uncovered specimens. The capacitance of the films increased with the immersion time, in accordance to the behaviour expected for an organic film. The overall performance of the coating systems appears to be highly dependent on the kind of metallic coating applied to the steel. Coatings applied upon
galvanized steel showed larger |Z| values when compared with those applied to the electroplated steel, indicating a superior corrosion resistance of the former and a better stability throughout the immersion time
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