6,755 research outputs found
Patterson Function from Low-Energy Electron Diffraction Measured Intensities and Structural Discrimination
Surface Patterson Functions have been derived by direct inversion of
experimental Low-Energy Electron Diffraction I-V spectra measured at multiple
incident angles. The direct inversion is computationally simple and can be used
to discriminate between different structural models. 1x1 YSi_2 epitaxial layers
grown on Si(111) have been used to illustrate the analysis. We introduce a
suitable R-factor for the Patterson Function to make the structural
discrimination as objective as possible. From six competing models needed to
complete the geometrical search, four could easily be discarded, achieving a
very significant and useful reduction in the parameter space to be explored by
standard dynamical LEED methods. The amount and quality of data needed for this
analysis is discussed.Comment: 5 pages, 4 figure
One-dimensional potential for image-potential states on graphene
In the framework of dielectric theory the static non-local self-energy of an
electron near an ultra-thin polarizable layer has been calculated and applied
to study binding energies of image-states near free-standing graphene. The
corresponding series of eigenvalues and eigenfunctions have been obtained by
solving numerically the one-dimensional Schr{\"o}dinger equation.
Image-potential-state wave functions accumulate most of their probability
outside the slab. We find that a Random Phase Approximation (RPA) for the
non-local dielectric function yields a superior description for the potential
inside the slab, but a simple Fermi-Thomas theory can be used to get a
reasonable quasi-analytical approximation to the full RPA result that can be
computed very economically. Binding energies of the image-potential states
follow a pattern close to the Rydberg series for a perfect metal with the
addition of intermediate states due to the added symmetry of the potential. The
formalism only requires a minimal set of free parameters; the slab width and
the electronic density. The theoretical calculations are compared to
experimental results for work function and image-potential states obtained by
two-photon photoemission.Comment: 24 pages; 10 figures. arXiv admin note: text overlap with
arXiv:1301.448
Hot electron transport in Ballistic Electron Emission Spectroscopy: band structure effects and k-space currents
Using a Green's function approach, we investigate band structure effects in
the BEEM current distribution in reciprocal space. In the elastic limit, this
formalism provides a 'parameter free' solution to the BEEM problem. At low
temperatures, and for thin metallic layers, the elastic approximation is enough
to explain the experimental I(V) curves at low voltages. At higher voltages
inelastic effects are approximately taken into account by introducing an
effective RPA-electron lifetime, much in similarity with LEED theory. For thick
films, however, additional damping mechanisms are required to obtain agreement
with experiment.Comment: 4 pages, 3 postscript figures, revte
Quantum mechanical analysis of the elastic propagation of electrons in the Au/Si system: application to Ballistic Electron Emission Microscopy
We present a Green's function approach based on a LCAO scheme to compute the
elastic propagation of electrons injected from a STM tip into a metallic film.
The obtained 2D current distribution in real and reciprocal space furnish a
good representation of the elastic component of Ballistic Electron Emission
Microscopy (BEEM) currents. Since this component accurately approximates the
total current in the near threshold region, this procedure allows --in contrast
to prior analyses-- to take into account effects of the metal band structure in
the modeling of these experiments. The Au band structure, and in particular its
gaps appearing in the [111] and [100] directions provides a good explanation
for the previously irreconcilable results of nanometric resolution and
similarity of BEEM spectra on both Au/Si(111) and Au/Si(100).Comment: 12 pages, 9 postscript figures, revte
A theoretical analysis of Ballistic Electron Emission Microscopy: k-space distributions and spectroscopy
We analyze BEEM experiments. At low temperatures and low voltages, near the
threshold value of the Schottky barrier, the BEEM current is dominated by the
elastic component. Elastic scattering by the lattice results in the formation
of focused beams and narrow lines in real space. To obtain the current injected
in the semiconductor, we compute the current distribution in reciprocal space
and, assuming energy and conservation. Our results show an
important focalization of the injected electron beam and explain the similarity
between BEEM currents for Au/Si(111) and Au/Si(100).Comment: 17 pages, 5 figures (postscript), Latex, APS,
http://www.icmm.csic.es/Pandres/pedro.htm. Appl. Surf. Sci. (in press
The VMC Survey. XXII. Hierarchical Star Formation in the 30 Doradus-N158-N159-N160 Star-forming Complex
We study the hierarchical stellar structures in a ∼1.5 deg2 area covering the 30 Doradus-N158-N159-N160 star-forming complex with the VISTA Survey of Magellanic Clouds. Based on the young upper main-sequence stars, we find that the surface densities cover a wide range of values, from log(pc2) ≲ -2.0 to log(pc2) ≳ 0.0. Their distributions are highly non-uniform, showing groups that frequently have subgroups inside. The sizes of the stellar groups do not exhibit characteristic values, and range continuously from several parsecs to more than 100 pc; the cumulative size distribution can be well described by a single power law, with the power-law index indicating a projected fractal dimension D2 = 1.6 ± 0.3. We suggest that the phenomena revealed here support a scenario of hierarchical star formation. Comparisons with other star-forming regions and galaxies are also discussed.Fil: Sun, Ning-Chen. Peking University; ChinaFil: Grijs, Richard De. Peking University; ChinaFil: Subramanian, Smitha. Peking University; ChinaFil: Cioni, Maria-Rosa L.. Universita Zu Berlin. Universita Postdam; AlemaniaFil: Rubele, Stefano. Università di Padova; ItaliaFil: Bekki, Kenji. University of Western Australia; AustraliaFil: Ivanov, Valentin D.. European Southern Observatory; ChileFil: Piatti, Andres Eduardo. Consejo Nacional de Investigaciones CientÃficas y Técnicas. Centro CientÃfico Tecnológico Conicet - Córdoba; Argentina. Universidad Nacional de Córdoba. Observatorio Astronómico de Córdoba; ArgentinaFil: Ripepi, Vincenzo. Osservatorio Astronomico di Capodimonte; Itali
Dark matter candidates in the NMSSM with RH neutrino superfields
R-parity conserving supersymmetric models with right-handed (RH) neutrinos
are very appealing since they could naturally explain neutrino physics and also
provide a good dark matter (DM) candidate such as the lightest supersymmetric
particle (LSP). In this work we consider the next-to-minimal supersymmetric
standard model (NMSSM) plus RH neutrino superfields, with effective Majorana
masses dynamically generated at the electroweak scale (EW). We perform a scan
of the relevant parameter space and study both possible DM candidates: RH
sneutrino and neutralino. Especially for the case of RH sneutrino DM we analyse
the intimate relation between both candidates to obtain the correct amount of
relic density. Besides the well-known resonances, annihilations through scalar
quartic couplings and coannihilation mechanisms with all kind of neutralinos,
are crucial. Finally, we present the impact of current and future direct and
indirect detection experiments on both DM candidates.Comment: Version published in JCAP, 40 pages, 8 figures, 6 table
Incidência de plantas daninhas e desempenho produtivo do sorgo sob influência de doses de nitrogênio e de atrazine.
bitstream/item/30252/1/boletim-12.pd
Hospital volume and outcomes for acute pulmonary embolism: Multinational population based cohort study
[Objectives] To evaluate the association between experience in the management of acute pulmonary embolism, reflected by hospital case volume, and mortality.[Design] Multinational population based cohort study using data from the Registro Informatizado de la Enfermedad TromboEmbólica (RIETE) registry between 1 January 2001 and 31 August 2018.[Setting] 353 hospitals in 16 countries.[Participants] 39 257 consecutive patients with confirmed diagnosis of acute symptomatic pulmonary embolism.[Main] outcome measure Pulmonary embolism related mortality within 30 days after diagnosis of the condition.[Results] Patients with acute symptomatic pulmonary embolism admitted to high volume hospitals (>40 pulmonary embolisms per year) had a higher burden of comorbidities. A significant inverse association was seen between annual hospital volume and pulmonary embolism related mortality. Admission to hospitals in the highest quarter (that is, >40 pulmonary embolisms per year) was associated with a 44% reduction in the adjusted odds of pulmonary embolism related mortality at 30 days compared with admission to hospitals in the lowest quarter (<15 pulmonary embolisms per year; adjusted risk 1.3% v 2.3%; adjusted odds ratio 0.56 (95% confidence interval 0.33 to 0.95); P=0.03). Results were consistent in all sensitivity analyses. All cause mortality at 30 days was not significantly reduced between the two quarters (adjusted odds ratio 0.78 (0.50 to 1.22); P=0.28). Survivors showed little change in the odds of recurrent venous thromboembolism (odds ratio 0.76 (0.49 to 1.19)) or major bleeding (1.07 (0.77 to 1.47)) between the low and high volume hospitals.[Conclusions] In patients with acute symptomatic pulmonary embolism, admission to high volume hospitals was associated with significant reductions in adjusted pulmonary embolism related mortality at 30 days. These findings could have implications for management strategies.Peer reviewe
Machine-Learned Exclusion Limits without Binning
Machine-Learned Likelihoods (MLL) is a method that, by combining modern
machine-learning classification techniques with likelihood-based inference
tests, allows to estimate the experimental sensitivity of high-dimensional data
sets. We extend the MLL method by including the exclusion hypothesis tests and
show that the addition of Kernel Density Estimators avoids the need to bin the
classifier output in order to extract the resulting one-dimensional signal and
background probability density functions. We first test our method on toy
models generated with multivariate Gaussian distributions, where the true
probability distribution functions are known. We then apply it to a case of
interest in the search for new physics at the HL-LHC, in which a
boson decays into lepton pairs, comparing the performance of our method for
estimating 95\% CL exclusion limits to the results obtained applying a binned
likelihood to the machine-learning classifier output.Comment: 14 pages, 3 figures. MLL+KDE code will be available from
https://github.com/AndresDanielPere
- …