53,115 research outputs found
Scanning tunneling microscopy of adsorbed molecules on metalic surfaces for nearly localized atomic states
We consider a Hubbard-Anderson model which describes localized orbitals in
five different sites hybridized both among themselves and with a continuum of
extended states. A square planar geometry with an atom at the center is used to
represent TBrPP-Co molecules. When the renormalized effective hopping between
sites is small compared with a Kondo energy scale determined by the
sitecontinuum hybridization, the system can be described as a set of
independent Kondo resonances, rather than molecular states. We study the
crossover between both regimes and analyze the spectral density of conduction
electrons as a function of position. The results are in qualitative agreement
with measurements of the differential conductance in a system with TBrPP-Co
molecules adsorbed on a Cu(111) surface.Comment: 5 pages, 4 figure
La desregulación de la comercialización de electricidad en Inglaterra y Gales
A partir de 1995, las autoridades regulatorias de Reino Unido han comenzado un proceso de introducción de competencia para ventas minoristas de electricidad. Esta política demandó un gran esfuerzo de organización por parte de los agentes de sistema, cuyos costos de lanzamiento se estiman en alrededor de 500 millones de libras. El presente reporte, en primer lugar, describe el programa de apertura llevado a cabo y evalúa los efectos de la apertura sobre el funcionamiento del sector y los niveles de bienestar de distintos estratos sociales de la población. Finalmente, del estudio de la experiencia británica se pueden extraer lecciones para la implementación futura de similares medidas en otros países.desregulación; comercialización de electricidad;
Optimisation of out-vessel magnetic diagnostics for plasma boundary reconstruction in tokamaks
To improve the low frequency spectrum of magnetic field measurements of
future tokamak reactors such as ITER, several steady state magnetic sensor
technologies have been considered. For all the studied technologies it is
always advantageous to place the sensors outside the vacuum vessel and as far
away from the reactor core to minimize radiation damage and temperature
effects, but not so far as to compromise the accuracy of the equilibrium
reconstruction. We have studied to what extent increasing the distance between
out-vessel sensors and plasma can be compensated for sensor accuracy and/or
density before the limit imposed by the degeneracy of the problem is reached.
The study is particularized for the Swiss TCV tokamak, due to the quality of
its magnetic data and its ability to operate with a wide range of plasma shapes
and divertor configurations. We have scanned the plasma boundary reconstruction
error as function of out-vessel sensor density, accuracy and distance to the
plasma. The study is performed for both the transient and steady state phases
of the tokamak discharge. We find that, in general, there is a broad region in
the parameter space where sensor accuracy, density and proximity to the plasma
can be traded for one another to obtain a desired level of accuracy in the
reconstructed boundary, up to some limit. Extrapolation of the results to a
tokamak reactor suggests that a hybrid configuration with sensors inside and
outside the vacuum vessel could be used to obtain a good boundary
reconstruction during both the transient and the flat-top of the discharges, if
out-vessel magnetic sensors of sufficient density and accuracy can be placed
sufficiently far outside the vessel to minimize radiation damage.Comment: 36 pages, 17 figures, Accepted for publication in Nuclear Fusio
On hadronic beam models for quasars and microquasars
Most of the hadronic jet models for quasars (QSOs) and microquasars (MQs)
found in literature represent beams of particles (e.g. protons). These
particles interact with the matter in the stellar wind of the companion star in
the system or with crossing clouds, generating gamma-rays via proton-proton
processes. Our aim is to derive the particle distribution in the jet as seen by
the observer, so that proper computation of the -ray and neutrino
yields can be done. We use relativistic invariants to obtain the transformed
expressions in the case of a power-law and power-law with a cutoff particle
distribution in the beam. We compare with previous expressions used earlier in
the literature. We show that formerly used expressions for the particle
distributions in the beam as seen by the observer are in error, differences
being strongly dependent on the viewing angle. For example, for
( is the Lorentz factor of the blob) and angles larger than , the earlier-used calculation entails an over-prediction (order of
magnitude or more) of the proton spectra for , whereas it always
over-predicts (two orders of magnitude) the proton spectrum at lower energies,
disregarding the viewing angle. All the results for photon and neutrino fluxes
in hadronic models in beams that have made use of the earlier calculation are
affected. Given that correct gamma-ray fluxes will be in almost any case
significantly diminished in comparison with published results, and that the
time of observations in Cherenkov facilities grows with the square of the
flux-reduction factor in a statistically limited result, the possibility of
observing hadronic beams is undermined.Comment: Accepted for publication in A&A Letter
Computing spectral sequences
In this paper, a set of programs enhancing the Kenzo system is presented.
Kenzo is a Common Lisp program designed for computing in Algebraic Topology, in
particular it allows the user to calculate homology and homotopy groups of
complicated spaces. The new programs presented here entirely compute Serre and
Eilenberg-Moore spectral sequences, in particular the groups and differential
maps for arbitrary r. They also determine when the spectral sequence has
converged and describe the filtration of the target homology groups induced by
the spectral sequence
Quantum transport through single and multilayer icosahedral fullerenes
We use a tight-binding Hamiltonian and Green functions methods to calculate
the quantum transmission through single-wall fullerenes and bilayered and
trilayered onions of icosahedral symmetry attached to metallic leads. The
electronic structure of the onion-like fullerenes takes into account the
curvature and finite size of the fullerenes layers as well as the strength of
the intershell interactions depending on to the number of interacting atom
pairs belonging to adjacent shells. Misalignment of the symmetry axes of the
concentric icosahedral shells produces breaking of the level degeneracies of
the individual shells, giving rise some narrow quasi-continuum bands instead of
the localized discrete peaks of the individual fullerenes. As a result, the
transmission function for non symmetrical onions are rapidly varying functions
of the Fermi energy. Furthermore, we found that most of the features of the
transmission through the onions are due to the electronic structure of the
outer shell with additional Fano-like antiresonances arising from coupling with
or between the inner shells.Comment: 16 pages, 5 figur
Quantum interference through gated single-molecule junctions
We discuss the general form of the transmission spectrum through a molec-
ular junction in terms of the Green function of the isolated molecule. By
introducing a tight binding method, we are able to translate the Green func-
tion properties into practical graphical rules for assessing beforehand the
possible existence of antiresonances in an energy range for a given choice of
connecting sites. The analysis is exemplified with a benzene molecule under a
hypothetical local gate, which allows one to continuously tune the on-site
energy of single atoms, for various connection topologies and gate positions.Comment: 17 pages, 5 figure
- …