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 $\gamma$-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 $\Gamma =10$ ($\Gamma$ is the Lorentz factor of the blob) and angles larger than $\sim 20^o$, the earlier-used calculation entails an over-prediction (order of magnitude or more) of the proton spectra for $E>\Gamma mc^2$, 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 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

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