Electronic continuum states and far infrared absorption of InAs/GaAs quantum dots

The electronic continuum states of InAs/GaAs semiconductor quantum dots embedded in a GaAs/AlAs superlattice are theoretically investigated and the far infrared absorption spectra are calculated for a variety of structures and polarizations. The effect of a strong magnetic field applied parallel to the growth direction is also investigated. We predict that the flatness of the InAs/GaAs dots leads to a far infrared absorption which is almost insensitive to the magnetic field, in spite of the reorganization of the continuum into series of quasi-Landau states. We also predict that it is possible to design InAs/GaAs photoconductors which display very strong in-plane absorption.Comment: 8 pages, 10 figure

Some new results on Lipschitz regularization for parabolic equations

It is well known that the bounded solution u(t, x) of the heat equation posed in RN×(0,T) for any continuous initial condition becomes Lipschitz continuous as soon as t>0, even if the initial datum is not Lipschitz continuous. We investigate this Lipschitz regularization for both strictly and degenerate parabolic equations of Hamilton–Jacobi type. We give proofs avoiding Bernstein’s method which leads to new, less restrictive conditions on the Hamiltonian, i.e., the first-order term. We discuss also whether the Lipschitz constant depends on the oscillation for the initial datum or not. Finally, some important applications of this Lipschitz regularization are presented

Quantum criticality at the superconductor to insulator transition revealed by specific heat measurements

The superconductor-insulator transition (SIT) is considered an excellent example of a quantum phase transition which is driven by quantum fluctuations at zero temperature. The quantum critical point is characterized by a diverging correlation length and a vanishing energy scale. Low energy fluctuations near quantum criticality may be experimentally detected by specific heat, $c_{\rm p}$, measurements. Here, we use a unique highly sensitive experiment to measure $c_{\rm p}$ of two-dimensional granular Pb films through the SIT. The specific heat shows the usual jump at the mean field superconducting transition temperature $T_{\rm c}^{\rm {mf}}$ marking the onset of Cooper pairs formation. As the film thickness is tuned toward the SIT, $T_{\rm c}^{\rm {mf}}$ is relatively unchanged, while the magnitude of the jump and low temperature specific heat increase significantly. This behaviour is taken as the thermodynamic fingerprint of quantum criticality in the vicinity of a quantum phase transition.Comment: 9 pages, 5 figures, 1 tabl

Evaluation morphométrique des chevaux pur-sang Arabe en Algérie: mensurations corporelles et proposition d’équations barymétriques

Cette étude vise à la caractérisation morphobiométrique des chevaux de course pur-sang arabe et à l’estimation d’équations barymétriques adaptées à cette race. La caractérisation a concerné 98 chevaux, dont 44 femelles et 54 mâles, tous âgés de trois ans et plus, auprès de 77 propriétaires-éleveurs dans 3 hippodromes d’Algérie (Zemmouri, Tiaret et Caroubier). Dix-neuf mensurations étaient relevées ainsi que le poids vif (PV). Le poids moyen est de 456,2 +/- 43,0 kg, variant de 335 kg à 545 kg. La sélection des variables à inclure dans les équations barymétriques a été réalisée à l’aide de la procédure stepwise du SAS. Quatre mensurations parmi les 19 réalisées ont été retenues pour la proposition d’équations d’estimation du poids vif des chevaux : le périmètre thoracique (PT), la hauteur à la croupe (HC), la longueur de l’encolure (LE) et le tour de l’encolure (TE). Ainsi, les équations proposées pour les mâles et pour les femelles sont respectivement de : PV= 7,024*PT - 787,119 (R²=0,99); PV=6,207*PT + 0,633*HC + 0,668*TE - 0,878*LE - 746,370 (R²=0,96). Les résultats de cette étude devraient permettre aux propriétaires-éleveurs et entraineurs de suivre aisément le poids de leurs chevaux. Ce suivi est nécessaire pour adapter l’activité et l’alimentation des chevaux et favoriser leur performance en course

Closed combination of context-embedding iterative strategies

This work is motivated by the challenging problem of the computer-aided generation of approximations (viewed as a series of transformations) of partial derivative equations. In this framework, the approximations posed over complex settings are incrementally constructed by extending an approximation posed on a simple setting and combining these extensions. In order to formalize these extensions and their combination, we introduce a class of rewriting strategies, called context-embedding iterative strategies (CE-strategies, for short). Roughly speaking, the class of CE-strategies is constructed by means of adding contexts and an iteration operator allowing the definition of recursive strategies. We show that the class of CE-strategies is closed under combination with respect to a correctness-completeness criterion. It turns out that the class CE-strategies enjoy nice algebraic properties, namely, the combination is associative, has a neutral element, and all the elements are idempotents

Anatomy and giant enhancement of the perpendicular magnetic anisotropy of cobalt-graphene heterostructures

We report strongly enhanced perpendicular magnetic anisotropy (PMA) of Co films by graphene coating from both first-principles and experiments. Our calculations show that graphene can dramatically boost the surface anisotropy of Co films up to twice the value of its pristine counterpart and can extend the out-of-plane effective anisotropy up to unprecedented thickness of 25~\AA. These findings are supported by our experiments on graphene coating on Co films grown on Ir substrate. Furthermore, we report layer-resolved and orbital-hybridization-resolved anisotropy analysis which help understanding the physical mechanisms of PMA and more practically can help design structures with giant PMA. As an example, we propose super-exchange stabilized Co-graphene heterostructures with a robust out-of-plane constant effective PMA and linearly increasing interfacial anisotropy as a function of film thickness. These findings point towards possibilities to engineer graphene/ferromagnetic metal heterostructures with giant magnetic anisotropy more than 20 times larger compared to conventional multilayers, which constitutes a hallmark for future graphene and traditional spintronic technologies.Comment: 17 pages, 4 figure

Statistical tolerance analysis of over-constrained mechanisms with gaps using system reliability methods

One of the aims of statistical tolerance analysis is to evaluate a predicted quality level at the design stage. One method consists of computing the defect probability PD expressed in parts per million (ppm). It represents the probability that a functional requirement will not be satisfied in mass production. This paper focuses on the statistical tolerance analysis of over-constrained mechanisms containing gaps. In this case, the values of the functional characteristics depend on the gap situations and are not explicitly formulated with respect to part deviations. To compute PD, an innovative methodology using system reliability methods is presented. This new approach is compared with an existing one based on an optimization algorithm and Monte Carlo simulations. The whole approach is illustrated using two industrial mechanisms: one inspired by a producer of coaxial connectors and one prismatic pair. Its major advantage is to considerably reduce computation time

Alloy effects in GaInN/GaN heterostructures

We show that the large band offsets between GaN and InN and the heavy carrier effective masses preclude the use of the Virtual Crystal Approximation to describe the electronic structure of Ga_(1-x)In_(x)N/GaN heterostructures while this approximation works very well for the Ga_(1-x)In_(x)As/GaAs heterostructures.Comment: submitted to Applied Physics Lette

Statistical tolerance analysis of a hyperstatic mechanism, using system reliability methods

The quality level of a mechanism can be evaluated a posteriori after several months by following the number of warranty returns. However, it is more interesting to evaluate a predicted quality level in the design stage: this is one of the aims of statistical tolerance analysis. A possible method consists of computing the defect probability (PD) expressed in ppm. It represents the probability that a functional requirement will not be satisfied in mass production. For assembly reasons, many hyperstatic mechanisms require gaps, which their functional requirements depend on. The defect probability assessment of such mechanisms is not straightforward, and requires advanced numerical methods. This problem particularly interests the VALEO W.S. company, which experiences problems with an assembly containing gaps. This paper proposes an innovative methodology to formulate and compute the defect probability of hyperstatic mechanisms with gaps in two steps. First, a complex feasibility problem is converted into a simpler problem. Then the defect probability is efficiently computed thanks to system reliability methods and the m-dimensional multivariate normal distribution Um. Finally, a sensitivity analysis is provided to improve the original design. The whole approach is illustrated with an industrial case study, but can be adapted to other similar problems