Large-scale BN tunnel barriers for graphene spintronics

We have fabricated graphene spin-valve devices utilizing scalable materials made from chemical vapor deposition (CVD). Both the spin-transporting graphene and the tunnel barrier material are CVD-grown. The tunnel barrier is realized by h-BN, used either as a monolayer or bilayer and placed over the graphene. Spin transport experiments were performed using ferromagnetic contacts deposited onto the barrier. We find that spin injection is still greatly suppressed in devices with a monolayer tunneling barrier due to resistance mismatch. This is, however, not the case for devices with bilayer barriers. For those devices, a spin relaxation time of 260 ps intrinsic to the CVD graphene material is deduced. This time scale is comparable to those reported for exfoliated graphene, suggesting that this CVD approach is promising for spintronic applications which require scalable materials.Comment: 13 pages, 3 figure

Ballistic one-dimensional holes with strong g-factor anisotropy in germanium

We report experimental evidence of ballistic hole transport in one-dimensional quantum wires gate-defined in a strained SiGe/Ge/SiGe quantum well. At zero magnetic field, we observe conductance plateaus at integer multiples of 2e2/h. At finite magnetic field, the splitting of these plateaus by Zeeman effect reveals largely anisotropic g-factors with absolute values below 1 in the quantum-well plane, and exceeding 10 out-of-plane. This g-factor anisotropy is consistent with a heavy-hole character of the propagating valence-band states, which is in line with a predominant confinement in the growth direction. Remarkably, we observe quantized ballistic conductance in device channels up to 600 nm long. These findings mark an important step toward the realization of novel devices for applications in quantum spintronics

CHOIX METHODOLOGIQUES EN SCIENCES DE GESTION : POURQUOI TANT DE CHIFFRES ?

Nous sommes partis d'un double constat, celui de l'augmentation du degré de formalisation mathématique des méthodes de recherche en sciences de gestion et celui de la place prédominante faite aux travaux formalisés dans les revues de la même discipline. Si le positionnement épistémologique explique une part des choix méthodologiques, l'analyse montre que certaines explications sont techniques et d'autres psychologiques. Les premières correspondent au développement de techniques formalisées pour traiter les données qualitatives et les secondes tiennent à la valeur symbolique des chiffres. Au final, les techniques quantitatives apportent un soutien au chercheur dans ses travaux et jouent le rôle d'objet-frontière dans la communauté scientifique en facilitant les échanges de connaissances.méthodologie, quantification, analyse de données, objet-frontière

Non-local spectroscopy of Andreev bound states

We experimentally investigate Andreev bound states (ABSs) in a carbon nanotube quantum dot (QD) connected to a superconducting Nb lead (S). A weakly coupled normal metal contact acts as a tunnel probe that measures the energy dispersion of the ABSs. Moreover we study the response of the ABS to non-local transport processes, namely Cooper pair splitting and elastic co-tunnelling, that are enabled by a second QD fabricated on the same nanotube on the opposite side of S. We find an appreciable non-local conductance with a rich structure, including a sign reversal at the ground state transition from the ABS singlet to a degenerate magnetic doublet. We describe our device by a simple rate equation model that captures the key features of our observations and demonstrates that the sign of the non-local conductance is a measure for the charge distribution of the ABS, given by the respective Bogoliubov-de Gennes amplitudes $u$ and $v$

Les déterminants des discours managériaux : application au cas des responsables qualité

Les normes qui se diffusent dans les organisations ont été souvent conçues par des ingénieurs et pour le secteur industriel. Il est intéressant de se demander de ce fait si les responsables qualité qui n'ont pas de formation à l'origine scientifique ou technique diffusent et mettent en place ces outils de contrôle organisationnel de façon similaire ou non aux responsables qualité ingénieurs ou techniciens. Nous avons étudié le discours de différents responsables qualité. La méthode de recherche utilisée s'est appuyée sur une analyse sémantique de celui-ci. Il apparaît que, s'il existe une distinction entre les discours de responsables qualité d'origine scientifique et ceux d'origine non scientifique, d'autres facteurs interviennent pour constituer un discours qui demeure en grande partie bien spécifique à chacun.contrôle organisationnel, responsable qualité, analyse de discours

First order 0/π0/\pi quantum phase transition in the Kondo regime of a superconducting carbon nanotube quantum dot

We study a carbon nanotube quantum dot embedded into a SQUID loop in order to investigate the competition of strong electron correlations with proximity effect. Depending whether local pairing or local magnetism prevails, a superconducting quantum dot will respectively exhibit positive or negative supercurrent, referred to as a 0 or $\pi$ Josephson junction. In the regime of strong Coulomb blockade, the 0 to $\pi$ transition is typically controlled by a change in the discrete charge state of the dot, from even to odd. In contrast, at larger tunneling amplitude the Kondo effect develops for an odd charge (magnetic) dot in the normal state, and quenches magnetism. In this situation, we find that a first order 0 to $\pi$ quantum phase transition can be triggered at fixed valence when superconductivity is brought in, due to the competition of the superconducting gap and the Kondo temperature. The SQUID geometry together with the tunability of our device allows the exploration of the associated phase diagram predicted by recent theories. We also report on the observation of anharmonic behavior of the current-phase relation in the transition regime, that we associate with the two different accessible superconducting states. Our results ultimately reveal the spin singlet nature of the Kondo ground state, which is the key process in allowing the stability of the 0-phase far from the mixed valence regime.Comment: 10 pages, 6 figures in main text, 4 figures in appendi

UNE APPROCHE EMPIRIQUE DE LA CLARIFICATION ET DE L'INTEGRATION ORGANISATIONNELLES GENEREES PAR L'ANALYSE DES PROCESSUS

L'hypothèse théorique d'une dimension de clarification et d'intégration de l'organisation suite à la réalisation d'une analyse des processus est tout d'abord établie. L'analyse des processus est ensuite appréhendée d'un point de vue pratique à travers la mise en place d'ERP et à travers les travaux préparatoires à l'obtention de la certification ISO 9000 dans des organisations de tailles différentes et assurant des activités variées. Les conséquences organisationnelles en sont ensuite abordées. Elles confirment l'hypothèse d'une meilleure clarification et intégration de l'organisation et montrent parallèlement l'apparition d'une alternative entre coercition ou apprentissage selon les cas.ERP; ISO; Clarification; Intégration

Snake Trajectories in Ultraclean Graphene p-n Junctions

Snake states are trajectories of charge carriers curving back and forth along an interface. There are two types of snake states, formed by either inverting the magnetic field direction or the charge carrier type at an interface. Whereas the former has been demonstrated in GaAs-AlGaAs heterostructures, the latter has become conceivable only with the advance of ballistic graphene where a gapless p-n interface governed by Klein tunneling can be formed. Such snake states were hidden in previous experiments due to limited sample quality. Here we report on magneto-conductance oscillations due to snake states in a ballistic suspended graphene p-n-junction which occur already at a very small magnetic field of 20mT. The visibility of 30% is enabled by Klein collimation. Our finding is firmly supported by quantum transport simulations. We demonstrate the high tunability of the device and operate it in different magnetic field regimesComment: Accepted for publication in Nature Communication