Poincar\'e series for surfaces with boundary

We show that the Poincar\'e series counting orthogeodesics of a negatively curved surface with totally geodesic boundary extends meromorphically to the whole complex plane, as well as the series counting geodesic arcs linking two points; we also give their value at zero

Conduction electrons localized by charged magneto-acceptors A2−^{2-} in GaAs/GaAlAs quantum wells

A variational theory is presented of A$^{1-}$ and A$^{2-}$ centers, i.e. of a negative acceptor ion localizing one and two conduction electrons, respectively, in a GaAs/GaAlAs quantum well in the presence of a magnetic field parallel to the growth direction. A combined effect of the well and magnetic field confines conduction electrons to the proximity of the ion, resulting in discrete repulsive energies above the corresponding Landau levels. The theory is motivated by our experimental magneto-transport results which indicate that, in a heterostructure doped in the GaAs well with Be acceptors, one observes a boil-off effect in which the conduction electrons in the crossed-field configuration are pushed by the Hall electric field from the delocalized Landau states to the localized acceptor states and cease to conduct. A detailed analysis of the transport data shows that, at high magnetic fields, there are almost no conducting electrons left in the sample. It is concluded that one negative acceptor ion localizes up to four conduction electrons.Comment: 8 pages, 5 figure

Heating process in the pre-Breakdown regime of the Quantum Hall Efect : a size dependent effect

Our study presents experimental measurements of the contact and longitudinal voltage drops in Hall bars, as a function of the current amplitude. We are interested in the heating phenomenon which takes place before the breakdown of the quantum Hall effect, i.e. the pre-breakdown regime. Two types of samples has been investigated, at low temperature (4.2 and 1.5K) and high magnetic field (up to 13 T). The Hall bars have several different widths, and our observations clearly demonstrate that the size of the sample influences the heating phenomenon. By measuring the critical currents of both contact and longitudinal voltages, as a function of the filling factor (around $i=2$), we highlight the presence of a high electric field domain near the source contact, which is observable only in samples whose width is smaller than 400 microns.Comment: 4 pages, 5 igures, 7th International Symposium of Research in High Magnetic Fields, to be published in physica

La réflexion, processus déclenché et constructeur : cas d’enseignants de FLS en formation ou en exercice et d’aspirants coopérants internationaux

Depuis les années 1990, les formations à visée professionnelle, comme l’enseignement, adoptent le paradigme du praticien réflexif. Au Québec, le référentiel de compétences proposé par le ministère de l’Éducation introduit l’idée que tout futur enseignant doit apprendre à « réfléchir sur sa pratique » (MEQ, 2001). Malgré de nombreuses études sur la réflexion, le concept reste flou et polysémique. Comment, dans ces conditions, « faire réfléchir » ? Des chercheurs contemporains, dans la mouvance éducative, humaniste et pragmatique de Dewey (1933), aboutissent à des conceptions convergentes de l’apprentissage par réflexion sur l’expérience (Osterman et Kottkamp, 2004; Brouwer et Korthagen, 2005; Loughran, 2006; Brockbank et McGill, 2007; Donnay et Charlier, 2008, entre autres). De leurs points communs est synthétisée une définition de la réflexion qui peut aider à clarifier son rôle en formation. La recherche se donne comme objectif de « saisir » des événements réflexifs pour élucider comment des formations universitaires contribuent à développer des mécanismes de réflexion favorables à un autorenouvellement professionnel à long terme. La démarche est qualitative, l’approche interprétative-compréhensive. Des entrevues semi-structurées ont permis de recueillir des données auprès de finissants en enseignement du français langue seconde (FLS), en coopération internationale, à l’Université de Montréal, ainsi que d’enseignants de FLS expérimentés d’une université québécoise. Du corpus d’« occurrences de réflexion » ont émergé les significations que les acteurs donnaient à leur expérience d’apprentissage ou de travail. Les résultats sont présentés en trois articles. Le premier décrit la méthodologie construite pour repérer des occurrences de réflexion. Le second révèle deux grandes caractéristiques de dispositifs qui la stimulent particulièrement: 1) l’agir en situation de travail authentique ou vraisemblable; 2) la confrontation interactive à l’altérité (pairs, clientèle). Le troisième article aborde les représentations plus riches, nuancées et critiques de la profession, l’Autre et soi-même sur lesquelles débouche la réflexion. L’étude documente aussi les effets de ces reconceptualisations sur l’acteur et l’action, et produit des typologies des préoccupations des (futurs) professionnels et des objets réfléchis Des pistes de recherche et d’application sont dégagées pour les formations professionnalisantes et le développement professionnel en milieu de travail.Since the 1990’s, the paradigm of the reflective practitioner entered higher education curricula. In Quebec, the Ministry of Education introduced in its list of skills required to become a teacher the idea that one should learn ‘to reflect on one’s practice’ (MEQ, 2001). Despite many studies about reflection, the concept of ‘reflection’ itself remains vague. So the effectiveness of the pedagogical and organizational tools used to “make people reflect” is highly questionable. Nevertheless, some researchers, following the seminal educative humanistic and pragmatic ideas of Dewey (1933) come to similar conceptualizations of what learning is through the reflection on practice (Osterman & Kottkamp, 2004; Brouwer & Korthagen, 2005; Loughran, 2006; Brockbank & McGill, 2007; Donnay & Charlier, 2008, among others). An operational definition of reflection is synthetized. It can help clarify the role of reflection in professional curricula. Going beyond the challenge of ‘capturing’ reflection events, this research aims at understanding how higher education programs contribute to develop long term professional self-renewal mecanisms. This study is based on qualitative interviews, within an interpretive approach. Data came from prospective teachers in French as a second language (FSL) and in international co-operation at Université de Montréal, as well as experienced FSL teachers working in a Quebec university. The method first isolated ‘occurrences of reflection’, then extracted the meanings emerging from what the interviewees had said about their experience of learning or working. Results provide insights into the process and outcomes of reflection. Two characteristics of the higher education programs appear to stimulate it the most : 1) ‘doing’ things in an authentic workplace or in simulated situations in the classroom; 2) interactive confrontation to others (peers and clients). Different types of ‘triggers’, conditions, concerns and objects of reflection are also brought to light. It appears that reflection stimulates changes in perspective in the mind of learners or workers, which makes them see things ‘differently’, that is, in a richer, more precise, more critical way, about their work, their colleagues or clients, and themselves as professionals. The study proposes some ways of improving teacher education programs and professional development in the workplace

Anharmonicity and asymmetry of Landau levels for a two-dimensional electron gas

We calculate the density of states of a two dimensional electron gas located at the interface of a GaAlAs/GaAs heterojunction. The disorder potential which is generally created by a single doping layer behind a spacer, is here enhanced by the presence of a second delta doped layer of scatterers which can be repulsive or attractive impurities. We have calculated the density of states by means of the Klauder's approximation, in the presence of a magnetic field of arbitrary strength. At low field either band tails or impurity bands are observed for attractive potentials, depending on the impurity concentration. At higher field, impurity bands are observed for both repulsive and attractive potentials. We discuss the effect of such an asymmetrical density of states on the transport properties in the quantum Hall effect regime.Comment: 22 pages, 12 figures. submitted to Phys. Rev.

Geodesic L\'evy flights and expected stopping time for random searches

We give an analytic description for the infinitesimal generator constructed by Applebaum-Estrade for L\'evy flights on a broad class of closed Riemannian manifolds including all negatively-curved manifolds, the flat torus and the sphere. Various properties of the associated semigroup and the asymptotics of the expected stopping time for L\'evy flight based random searches for small targets, also known as the narrow capture problem, are then obtained using our newfound understanding of the infinitesimal generator. Our study also relates to the L\'evy flight foraging hypothesis in the field of biology as we compute the expected time for finding a small target by using the L\'evy flight random search. A similar calculation for Brownian motion on surfaces was done in [arXiv:2209.12425].Comment: 40 page

Anne-Marie Duranton-Cabrol, Nicole Racine et Rémy Rieffel (dir.), Pontigny, Royaumont, Cerisy : au miroir du genre

Femmes intellectuelles, l’épithète, longtemps, parut incongrue : André Gide, quand s’ouvrirent en 1910 les Décades de Pontigny, rassemblement annuel d’intellectuels français et de toutes les nations, aurait en effet souhaité les bannir de cette sociabilité choisie. Cet ouvrage collectif entend précisément interroger les modalités (épouses suivant leurs maris ou le secondant, auditrices plus ou moins connues, conférencières, directrices de colloques) et surtout le sens de la présence féminine ..

Design and realisation of a 100MHz synthesis chain from an X-band reference signal

LPMO has undertaken the building of a cryogenic sapphire oscillator with the support of th french Space and Metrolgy agencies (CNES and BNM). The aim of this project is to provide a reference oscillator presenting short frequency stability better than 5.10-14 in order to fulfill reference tests requirements for spacial and metrological applications. The cryogenic oscillator can operate on different frequencies ranging from 8 to 13GHz depending on the sapphire resonator mode chosen as reference. The exact output signal frequency is not ‘a priori' known with a great accuracy due to the large relative uncertainties (of the order of 10-4) affecting the resonator frequencies theoritical determination. Then a special synthesis chain has to be designed in order to transfert the cryogenic oscillator performances to a reference signal whose frequency is fully compatible with most of Time and Frequency instrumentation. In this paper, we present the design, realization and preliminary tests of a synthesis chain generating a 100MHz signal from an X-band reference. The performances of the two realyzed prototypes will enable to transfert better than 5.10-14 short term frequency stability

Zener Tunneling Between Landau Orbits in a High-Mobility Two-Dimensional Electron Gas

Magnetotransport in a laterally confined two-dimensional electron gas (2DEG) can exhibit modified scattering channels owing to a tilted Hall potential. Transitions of electrons between Landau levels with shifted guiding centers can be accomplished through a Zener tunneling mechanism, and make a significant contribution to the magnetoresistance. A remarkable oscillation effect in weak field magnetoresistance has been observed in high-mobility 2DEGs in GaAs-AlGa$_{0.3}$As$_{0.7}$ heterostructures, and can be well explained by the Zener mechanism.Comment: 5 pages, 4 figures. Text slightly shortened, figures resize