Spin relaxation of "upstream" electrons in quantum wires: Failure of the drift diffusion model

The classical drift diffusion (DD) model of spin transport treats spin relaxation via an empirical parameter known as the ``spin diffusion length''. According to this model, the ensemble averaged spin of electrons drifting and diffusing in a solid decays exponentially with distance due to spin dephasing interactions. The characteristic length scale associated with this decay is the spin diffusion length. The DD model also predicts that this length is different for ``upstream'' electrons traveling in a decelerating electric field than for ``downstream'' electrons traveling in an accelerating field. However this picture ignores energy quantization in confined systems (e.g. quantum wires) and therefore fails to capture the non-trivial influence of subband structure on spin relaxation. Here we highlight this influence by simulating upstream spin transport in a multi-subband quantum wire, in the presence of D'yakonov-Perel' spin relaxation, using a semi-classical model that accounts for the subband structure rigorously. We find that upstream spin transport has a complex dynamics that defies the simplistic definition of a ``spin diffusion length''. In fact, spin does not decay exponentially or even monotonically with distance, and the drift diffusion picture fails to explain the qualitative behavior, let alone predict quantitative features accurately. Unrelated to spin transport, we also find that upstream electrons undergo a ``population inversion'' as a consequence of the energy dependence of the density of states in a quasi one-dimensional structure.Comment: 13 figures. To appear in Phys. Rev.

Intrinsic Spin Hall Conductivity of MoTe2 and WTe2 Semimetals

We report a comprehensive study on the intrinsic spin Hall conductivity (SHC) of semimetals MoTe2 and WTe2 by ab initio calculation. Large SHC and desirable spin Hall angles have been discovered, due to the strong spin orbit coupling effect and low charge conductivity in semimetals. Diverse anisotropic SHC values, attributed to the unusual reduced-symmetry crystalline structure, have been revealed. We report an effective method on SHC optimization by electron doping, and exhibit the mechanism of SHC variation respect to the energy shifting by the spin Berry curvature. Our work provides insights into the realization of strong spin Hall effects in 2D systems

La professionnalisation dans l’enseignement universitaire : un processus dialogique ?

National audienceProfessionalization in higher education in France is the subject of many debates about the goals and purposes of university degrees. This leads to a wide range of plans and actions, but the mechanisms behind have rarely been studied. After reviewing the questions raised by the main scientific approaches dealing with professionalization, this article analyses how it is implemented for professional bachelor degrees and what professors and programme directors think of, and do about it. Drawing on theory of activity - and in particular directed activity -, our conclusions emphasize the "dialogical universe" of activities, from course design to evaluation. In the same way, most of the activities offered to the students are designed in order to encourage them to analyse situations from different points of views. Professionalization can therefore be regarded as a dialogical process.La professionnalisation des études supérieures constitue, en France, l’objet d’un débat sur les finalités des cursus. Elle se met en place au travers d’un éventail de dispositifs mais alimente encore peu de travaux de recherche lorsqu’il s’agit d’en étudier les processus. Après une revue de questions portant sur les principales approches scientifiques de la professionnalisation, cet article analyse la mise en œuvre de la professionnalisation dans les licences professionnelles, telle que conçue et réalisée par les concepteurs et responsables pédagogiques de ces diplômes. Lus au moyen du modèle de l’activité dirigée, les résultats mettent en évidence l’ « univers dialogique » qui trame toutes les activités des responsables des formations, depuis la conception de la formation jusqu’à l’évaluation finale. En écho, les activités proposées aux étudiants visent pour beaucoup d’entre elles à leur permettre de multiplier les points de vue sur les situations rencontrées. La professionnalisation peut ainsi s’entendre comme un processus dialogique

A Pearson Effective Potential for Monte-Carlo simulation of quantum confinement effects in various MOSFET architectures

A Pearson Effective Potential model for including quantization effects in the simulation of nanoscale nMOSFETs has been developed. This model, based on a realistic description of the function representing the non zero-size of the electron wave packet, has been used in a Monte-Carlo simulator for bulk, single gate SOI and double-gate SOI devices. In the case of SOI capacitors, the electron density has been computed for a large range of effective field (between 0.1 MV/cm and 1 MV/cm) and for various silicon film thicknesses (between 5 nm and 20 nm). A good agreement with the Schroedinger-Poisson results is obtained both on the total inversion charge and on the electron density profiles. The ability of an Effective Potential approach to accurately reproduce electrostatic quantum confinement effects is clearly demonstrated.Comment: 13 pages, 11 figures, 3 table

Le développement de l’expérience des acteurs du bilan de compétences

Il est question dans cet article d’une intervention réalisée avec une équipe de professionnels d’un centre de « bilan de compétences ». Précisément, c’est la rédaction de la synthèse, à la fin du bilan qui est l’objet d’une analyse commune. On distingue le dispositif juridique et le réel de l’activité des conseillers, leurs multiples questionnements. La méthodologie mobilisée ici est celle des autoconfrontations croisées, développée par l’équipe de Clinique de l’Activité. On l’y emploie sous une forme particulière, à partir des textes de synthèse écrits par les conseillers. L’analyse des dialogues des conseillers sur ces textes montre le travail d’élaboration collective de techniques.This article focuses on an operation carried out using a team of professionals in a Skills Assessment Test Center. More specifically, it addresses their task of drafting the post-assessment report. We observed how the legal framework differed from the counselor’s actual activity and numerous questions. The methodology used was a variant of the crossed self-confrontation method developed by the “Clinique de l’Activité” research team, based on the counselors’ written reports. An applied discourse analysis of the counselors’ dialogues highlighted the collective aspect of developing assessment techniques

Regard sur l’activité d’accompagnement dans le domaine de l’insertion des jeunes

Les jeunes dits de « bas niveau de qualification » ont en France, comme principaux interlocuteurs pour les « accompagner » vers l’insertion, les conseillers qui œuvrent dans les missions locales. Après avoir retracé, sur un plan historique, les étapes de l’institutionnalisation de ces structures, l’article précise les attendus qui organisent l’activité des conseillers. En se basant sur les concepts de la clinique de l'activité, et notamment sur celui "d'activité dirigée", l'auteur décrit et analyse les acquis d'une expérience de conduite régulière de groupe d'analyse de l'activité avec des conseillers. Elle identifie quelques composantes du « genre professionnel » des conseillers en mission locale et discerne les questions les plus vives que ces conseillers mettent au travail dans les espaces d’analyse collective de leur activité. Enfin, l’article se termine sur différentes voies possibles pour développer le genre professionnel des conseillers.In France,young people with low qualifications are mainly supported toward professional integration by counselors working in local government agencies. After tracing the successive stages of the institutionalization of these agencies from a historical perspective, the article specifies the requirements and expectations guiding the counselors’ activity. Using as a basis the concepts of the Clinic of activity, notably that of “oriented activity”, the writer then proceeds by drawing both on her own sustained experience of leading activity analysis groups. She identifies a few components of the “professional genre” of local government counselors and distinguishes the major issues examined during their collective analysis of activity. Finally, the article concludes by stressing the need for these professionals to diversify frameworks so they can develop their professional genre.En Francia, los jóvenes considerados “insuficientemente cualificados” tienen como principales interlocutores para “acompañarlos” hacia la inserción a consejeros que trabajan en misiones locales. El texto del artículo, tras reconstruir en un plano histórico las etapas de la institucionalización de esas estructuras, se explaya sobre los considerandos en torno a los cuales se organiza la actividad de los consejeros. Basándose en conceptos de la clínica de la actividad, en especial el de "actividad dirigida", la autora describe y analiza las enseñanzas de una experiencia de conducta regular de un grupo de análisis de la actividad con consejeros. Sobre esta base, la autora identifica algunos componentes del “género profesional” de los consejeros en misión local y pone en perspectiva las cuestiones más importantes que estos consejeros utilizan en los espacios de análisis colectivo de su actividad. Por último, el artículo termina con diferentes maneras posibles para desarrollar el género profesional de los consejeros

Effect of discrete impurities on electron transport in ultra-short MOSFET using 3D Monte Carlo simulation

This paper discusses the influence of the channel impurity distribution on the transport and the drive current in short-gate MOSFET. In this purpose, a careful description of electron-ion interaction suitable for the case of discrete impurities has been implemented in a 3D particle Monte Carlo simulator. This transport model is applied to the investigation of 50 nm MOSFET operation. The results show that a small change in the number of doping impurities or in the position of a single discrete impurity in the inversion layer may significantly influence the drain current. This effect is not only related to threshold voltage fluctuations but also to variations in transport properties in the inversion layer, especially at high drain voltage. The results are analyzed in terms of local fluctuations of electron velocity and current density. In a set of fifteen simulated devices the drive current Ion, determined at VGS = VDS = 0.6 V, is found to vary in a range of 23% according to the position of channel impurities.Comment: 31 pages, 13 figures, revised version: discussions and references added, to be published in IEEE Trans. Electron. Device