468 research outputs found
Le tableur et l'option informatique : vers la programmation par objets
Des difficultés décelées auprès de plusieurs groupes d'élèves dans l'enseignement optionnel de l'informatique en classe de seconde m'ont amené à élaborer puis à affiner par étapes successives une approche particulière du programme. Cette communication présente l'état d'avancement de ma réflexion à ce sujet au cours de l'année scolaire 1987/88. Dans la première partie de l'année scolaire les élèves apprennent à identifier dans un problème les objets significatifs, à reconnaître a priori le type de données informatiques qui permettra le codage de ces objets (valeurs logiques, chaînes de caractères, nombres, objets composés) et la nature de la définition de ces objets (définition simple, définition conditionnelle, définition itérative). La découverte de ces notions s'effectue progressivement, les objets à définition itérative qui constituent l'introduction à l'algorithmique séquentielle classique ne sont abordés que lorsque la notion d'objet est maîtrisée par un nombre suffisant d'élèves. Cette démarche englobe à la fois le cours théorique, les exercices et les travaux pratiques sur ordinateurs. Son originalité principale est de s'appuyer pour les travaux pratiques sur le tableur Multiplan, utilisé dans un contexte de résolution de problème, de simulation et d'aide à la décision au cours de la première moitié de l'année scolaire. La force du tableur par opposition à un langage traditionnel résulte : - de la visualisation permanente de l'état des données (des objets), - de la possibilité de repousser les difficultés liées à la notion de séquence tout en préparant son introduction et son étude systématique, - de la facilité de la prise en main. Les résultats obtenus par des élèves lors d'un exercice d'analyse et lors de deux travaux pratiques sont commentés à titre d'illustration et servent de base à des comparaisons entre le tableur d'une part, et d'autre part les langages impératifs, les langages applicatifs et les langages à objets
FOX: A friendly tool to solve nonmolecular structures from powder diffraction
Structural characterization from powder diffraction of compounds not containing isolated molecules but three-dimensional infinite structure (alloys, intermetallics, framework compounds, extended solids) by direct space methods has been largely improved in the last 15 years. The success of the method depends very much on a proper modeling of the structure from building blocks. The modeling from larger building blocks improves the convergence of the global optimization algorithm by a factor of up to 10. However, care must be taken about the correctness of the building block, like its rigidity, deformation, bonding distances, and ligand identity. Dynamical occupancy correction implemented in the direct space program FOX has shown to be useful when merging excess atoms, and even larger building blocks like coordination polyhedra. It also allows joining smaller blocks into larger ones in the case when the connectivity was not a priori evident from the structural model. We will show in several examples of nonmolecular structures the effect of the modeling by correct structural unit
Coherent Diffraction Imaging of Single 95nm Nanowires
Photonic or electronic confinement effects in nanostructures become
significant when one of their dimension is in the 5-300 nm range. Improving
their development requires the ability to study their structure - shape, strain
field, interdiffusion maps - using novel techniques. We have used coherent
diffraction imaging to record the 3-dimensionnal scattered intensity of single
silicon nanowires with a lateral size smaller than 100 nm. We show that this
intensity can be used to recover the hexagonal shape of the nanowire with a
28nm resolution. The article also discusses limits of the method in terms of
radiation damage.Comment: 5 pages, 5 figure
Fast computing of scattering maps of nanostructures using graphical processing units
Scattering maps from strained or disordered nano-structures around a Bragg
reflection can either be computed quickly using approximations and a (Fast)
Fourier transform, or using individual atomic positions. In this article we
show that it is possible to compute up to 4.10^10 $reflections.atoms/s using a
single graphic card, and we evaluate how this speed depends on number of atoms
and points in reciprocal space. An open-source software library (PyNX) allowing
easy scattering computations (including grazing incidence conditions) in the
Python language is described, with examples of scattering from non-ideal
nanostructures.Comment: 7 pages, 4 figure
Nonresonant microwave absorption in epitaxial La-Sr-Mn-O films and its relation to colossal magnetoresistance
We study magnetic-field-dependent nonresonant microwave absorption and
dispersion in thin LaSrMnO films and show that it
originates from the colossal magnetoresistance. We develop the model for
magnetoresistance of a thin ferromagnetic film in oblique magnetic field. The
model accounts fairly well for our experimental findings, as well as for
results of other researchers. We demonstrate that nonresonant microwave
absorption is a powerful technique that allows contactless measurement of
magnetic properties of thin films, including magnetoresistance, anisotropy
field and coercive field.Comment: 20 pages, 11 figure
Large and uniform optical emission shifts in quantum dots externally strained along their growth axis
We introduce a method which enables to directly compare the impact of elastic
strain on the optical properties of distinct quantum dots (QDs). Specifically,
the QDs are integrated in a cross-section of a semiconductor core wire which is
surrounded by an amorphous straining shell. Detailed numerical simulations show
that, thanks to the mechanical isotropy of the shell, the strain field in a
core section is homogeneous. Furthermore, we use the core material as an in
situ strain gauge, yielding reliable values for the emitter energy tuning
slope. This calibration technique is applied to self-assembled InAs QDs
submitted to incremental tensile strain along their growth axis. In contrast to
recent studies conducted on similar QDs stressed perpendicularly to their
growth axis, optical spectroscopy reveals 5-10 times larger tuning slopes, with
a moderate dispersion. These results highlight the importance of the stress
direction to optimise QD response to applied strain, with implications both in
static and dynamic regimes. As such, they are in particular relevant for the
development of wavelength-tunable single photon sources or hybrid QD
opto-mechanical systems
Local tetragonal distortion in La_{0.7}Sr_{0.3}MnO_3 strained thin films probed by x-ray absorption spectroscopy
We report on an angular resolved X-ray Absorption Spectroscopy study of
thin films epitaxially grown by pulsed laser
deposition on slightly mismatched substrates which induce tensile or
compressive strains. XANES spectra give evidence of tetragonal distortion
within the octahedra, with opposite directions for tensile and
compressive strains. Quantitative analysis has been done and a model of
tetragonal distortion reflecting the strain has been established. EXAFS data
collected in plane for tensile substrate confirm the change in the
average bond distance and the increase of length matching with the
enlargement of the cell parameter. From these results we conclude that there is
no significant change in the angle. Our observations conflict with
the scenarios which this angle is the main driving parameter in the sensitivity
of manganite films properties to external strains and suggest that the
distortion within the octahedra plays a key role in the modification of the
transport and magnetic properties.Comment: 8 pages, 6 figure
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