481 research outputs found
Van der Waals contribution to the inelastic atom-surface scattering
A calculation of the inelastic scattering rate of Xe atoms on Cu(111) is
presented. We focus in the regimes of low and intermediate velocities, where
the energy loss is mainly associated to the excitation electron-hole pairs in
the substrate. We consider trajectories parallel to the surface and restrict
ourselves to the Van der Waals contribution. The decay rate is calculated
within a self-energy formulation. The effect of the response function of the
substrate is studied by comparing the results obtained with two different
approaches: the Specular Reflection Model and the Random Phase Approximation.
In the latter, the surface is described by a finite slab and the wave functions
are obtained from a one-dimensional model potential that describes the main
features of the surface electronic structure while correctly retains the
image-like asymptotic behaviour. We have also studied the influence of the
surface state on the calculation, finding that it represents around 50% of the
total probability of electron-hole pairs excitation.Comment: 7 pages, 4 figure
Ab initio study of the double row model of the Si(553)-Au reconstruction
Using x-ray diffraction Ghose et al. [Surf. Sci. {\bf 581} (2005) 199] have
recently produced a structural model for the quantum-wire surface Si(553)-Au.
This model presents two parallel gold wires located at the step edge. Thus, the
structure and the gold coverage are quite different from previous proposals. We
present here an ab initio study using density functional theory of the
stability, electronic band structure and scanning tunneling microscopy images
of this model.Comment: Submitted to Surface Science on December 200
INSA scientific activities in the space astronomy area
Support to Astronomy operations is an important and long-lived activity
within INSA. Probably the best known (and traditional) INSA activities are
those related with real-time spacecraft operations: Ground station maintenance
and operation (Ground station engineers and operators); spacecraft and payload
real-time operation (spacecraft and instruments controllers); computing
infrastructure maintenance (operators, analysts) and general site services.In
this paper, we'll show a different perspective, probably not so well-known,
presenting some INSA recent activities at the European Space Astronomy Centre
(ESAC) and NASA Madrid Deep Space Communication Complex (MDSCC) directly
related to scientific operations. Basic lines of activity involved include:
Operations support for science operations; system and software support for real
time systems; technical administration and IT support; R \& D activities,
radioastronomy (at MDSCC and ESAC) and scientific research projects. This paper
is structured as follows: first, INSA activities in two ESA cornerstone
astrophysics missions, XMM-Newton and Herschel, will be outlined. Then, our
activities related to Science infrastructure services, represented by the
Virtual Observatory (VO) framework and the Science Archives development
facilities are briefly shown. Radio Astronomy activities will be described
afterwards, and finally, a few research topics in which INSA scientists are
involved will be also described.Comment: 6 pages. Highlights of Spanish Astrophysics V Proceedings of the VIII
Scientific Meeting of the Spanish AstronomicalSociety (SEA) held in
Santander, 7-11 July, 200
Calculation of the optical response of C60 and Na8 using time-dependent density functional theory and local orbitals
We report on a general method for the calculation of the frequency-dependent
optical response of clusters based upon time-dependent density functional
theory (TDDFT). The implementation is done using explicit propagation in the
time domain and a self-consistent program that uses a linear combination of
atomic orbitals (LCAO). Our actual calculations employ the SIESTA program,
which is designed to be fast and accurate for large clusters. We use the
adiabatic local density approximation to account for exchange and correlation
effects. Results are presented for the imaginary part of the linear
polarizability, Im [\alpha(w)], and the dipole strength function, S(w), of C60
and Na8, compared to previous calculations and to experiment. We also show how
to calculate the integrated frequency-dependent second order non-linear
polarizability for the case of a step function electric field,
\gamma_{step}(w), and present results for C60.Comment: 11 pages with 6 postscript figures. Submitted for publicatio
Magnetism of Covalently Functionalized Carbon Nanotubes
We investigate the electronic structure of carbon nanotubes functionalized by
adsorbates anchored with single C-C covalent bonds. We find that, despite the
particular adsorbate, a spin moment with a universal value of 1.0 per
molecule is induced at low coverage. Therefore, we propose a mechanism of
bonding-induced magnetism at the carbon surface. The adsorption of a single
molecule creates a dispersionless defect state at the Fermi energy, which is
mainly localized in the carbon wall and presents a small contribution from the
adsorbate. This universal spin moment is fairly independent of the coverage as
long as all the molecules occupy the same graphenic sublattice. The magnetic
coupling between adsorbates is also studied and reveals a key dependence on the
graphenic sublattice adsorption site.Comment: final version, improved discussion about calculations and defect
concentratio
Magnetism of Substitutional Co Impurities in Graphene: Realization of Single -Vacancies
We report {\it ab initio} calculations of the structural, electronic and
magnetic properties of a graphene monolayer substitutionally doped with Co
(Co) atoms. We focus in Co because among traditional ferromagnetic
elements (Fe, Co and Ni), only Co atoms induce spin-polarization in
graphene. Our results show the complex magnetism of Co substitutional impurites
in graphene, which is mapped into simple models such as the -vacancy and
Heisenberg model. The links established in our work can be used to bring into
contact the engineering of nanostructures with the results of -models in
defective graphene. In principle, the structures considered here can be
fabricated using electron irradiation or Ar ion bombardment to create
defects and depositing Co at the same time
Scanning tunneling microscopy of monoatomic gold chains on vicinal Si(335) surface: experimental and theoretical study
We study electronic and topographic properties of the Si(335) surface,
containing Au wires parallel to the steps. We use scanning tunneling microscopy
(STM) supplemented by reflection of high energy electron diffraction (RHEED)
technique. The STM data show the space and voltage dependent oscillations of
the distance between STM tip and the surface which can be explained within one
band tight binding Hubbard model. We calculate the STM current using
nonequilibrium Keldysh Green function formalism.Comment: 6 pages, 4 figure
- …