Evidence for a Finite Temperature Insulator

In superconductors the zero-resistance current-flow is protected from dissipation at finite temperatures (T) by virtue of the short-circuit condition maintained by the electrons that remain in the condensed state. The recently suggested finite-T insulator and the "superinsulating" phase are different because any residual mechanism of conduction will eventually become dominant as the finite-T insulator sets-in. If the residual conduction is small it may be possible to observe the transition to these intriguing states. We show that the conductivity of the high magnetic-field insulator terminating superconductivity in amorphous indium-oxide exhibits an abrupt drop, and seem to approach a zero conductance at T<0.04 K. We discuss our results in the light of theories that lead to a finite-T insulator

Tunneling Spectroscopy and Vortex Imaging in Boron-Doped Diamond

We present the first scanning tunneling spectroscopy study of single-crystalline boron doped diamond. The measurements were performed below 100 mK with a low temperature scanning tunneling microscope. The tunneling density of states displays a clear superconducting gap. The temperature evolution of the order parameter follows the weak coupling BCS law with $\Delta(0)/k_B T_c \simeq 1.74$. Vortex imaging at low magnetic field also reveals localized states inside the vortex core that are unexpected for such a dirty superconductor.Comment: 4 pages, 4 figures, replaced with revised versio

Approche des phénomènes de conduction-convection dans le cadre des Olympiades de Physique

La participation aux Olympiades de Physique a permis à un groupe d'élèves de terminales STL (Sciences et Techniques de Laboratoire) option Optique de découvrir un domaine de la physique qui leur était peu familier: il s'agit de la thermique. Pour aborder ce domaine ils ont dû se fabriquer des images mentales, pour progresser ils ont choisi d'utiliser la démarche expérimentale la plus rigoureuse possible. C'est cette petite aventure qui est racontée dans les pages qui suivent

Quantum dots and spin qubits in graphene

This is a review on graphene quantum dots and their use as a host for spin qubits. We discuss the advantages but also the challenges to use graphene quantum dots for spin qubits as compared to the more standard materials like GaAs. We start with an overview of this young and fascinating field and will then discuss gate-tunable quantum dots in detail. We calculate the bound states for three different quantum dot architectures where a bulk gap allows for confinement via electrostatic fields: (i) graphene nanoribbons with armchair boundary, (ii) a disc in single-layer graphene, and (iii) a disc in bilayer graphene. In order for graphene quantum dots to be useful in the context of spin qubits, one needs to find reliable ways to break the valley-degeneracy. This is achieved here, either by a specific termination of graphene in (i) or in (ii) and (iii) by a magnetic field, without the need of a specific boundary. We further discuss how to manipulate spin in these quantum dots and explain the mechanism of spin decoherence and relaxation caused by spin-orbit interaction in combination with electron-phonon coupling, and by hyperfine interaction with the nuclear spin system.Comment: 23 pages, 10 figures, topical review prepared for Nanotechnolog

Fractal superconductivity near localization threshold

We develop a semi-quantitative theory of electron pairing and resulting superconductivity in bulk "poor conductors" in which Fermi energy $E_F$ is located in the region of localized states not so far from the Anderson mobility edge $E_c$. We review the existing theories and experimental data and argue that a large class of disordered films is described by this model. Our theoretical analysis is based on the analytical treatment of pairing correlations, described in the basis of the exact single-particle eigenstates of the 3D Anderson model, which we combine with numerical data on eigenfunction correlations. Fractal nature of critical wavefunction's correlations is shown to be crucial for the physics of these systems. We identify three distinct phases: 'critical' superconductive state formed at $E_F=E_c$, superconducting state with a strong pseudogap, realized due to pairing of weakly localized electrons and insulating state realized at $E_F$ still deeper inside localized band. The 'critical' superconducting phase is characterized by the enhancement of the transition temperature with respect to BCS result, by the inhomogeneous spatial distribution of superconductive order parameter and local density of states. The major new feature of the pseudo-gaped state is the presence of two independent energy scales: superconducting gap $\Delta$, that is due to many-body correlations and a new "pseudogap" energy scale $\Delta_P$ which characterizes typical binding energy of localized electron pairs and leads to the insulating behavior of the resistivity as a function of temperature above superconductive $T_c$. Two gap nature of the "pseudo-gaped superconductor" is shown to lead to a number of unusual physical properties.Comment: 110 pages, 39 figures. The revised version corrects a number of typos, adds references and discussion of recent result

Spin-helical transport in normal and superconducting topological insulators

In a topological insulator (TI) the character of electron transport varies from insulating in the interior of the material to metallic near its surface. Unlike, however, ordinary metals, conducting surface states in TIs are topologically protected and characterized by spin helicity whereby the direction of the electron spin is locked to the momentum direction. In this paper we review selected topics regarding recent theoretical and experimental work on electron transport and related phenomena in two-dimensional (2D) and three-dimensional (3D) TIs. The review provides a focused introductory discussion of the quantum spin Hall effect in HgTe quantum wells as well as transport properties of 3DTIs such as surface weak antilocalization, the half-integer quantum Hall effect, s + p-wave induced superconductivity, superconducting Klein tunneling, topological Andreev bound states and related Majorana midgap states. These properties of TIs are of practical interest, guiding the search for the routes towards topological spin electronics.Comment: Invited Topical Review on electron transport in 2D and 3D topological insulators, with focus on the quantum spin-Hall effect, weak antilocalization, half-integer quantum Hall effect, s- and p-wave induced superconductivity and superconducting Klein tunneling; 18 pages, 14 figures; accepted for publication in Physica Status Solidi

Corporate collaborative innovation spaces and exploration of new business models : the case of Altran

Pour faire face à leurs enjeux concurrentiels, les entreprises existantes sont incitées à renouveler leur business model. Parallèlement, de plus en plus de lieux d’innovation se développent en leur sein. Dans cette thèse, nous étudions la manière dont ces lieux- que nous appelons les espaces collaboratifs d’innovation (ECI) corporate- contribuent à l’exploration de nouveaux business models.Une étude de cas longitudinale à caractère ethnographique au sein de la société Altran explore la place des ECI dans l’organisation, leurs activités réelles autour des clients ainsi que l’usage des artefacts/ équipements mis à disposition des utilisateurs.Les résultats montrent la dimension d’innovation managériale - fruit de l’ambidextrie organisationnelle- de ces ECI, décryptent les microprocessus conduisant à l’émergence d’une nouvelle proposition de valeur / relation client et situent le rôle des démonstrateurs dans la construction d’un discours proposant une nouvelles expérience client aux visiteurs.Nous nourrissons ainsi la compréhension du fonctionnement des ECI et contribuons à enrichir les théories sur l’innovation en Business model, notamment dans sa perspective multidimensionnelle et systémique.Nous concluons en proposant des pistes pour optimiser l’utilisation des ECI à travers une meilleure appropriation par les ressources internes, notamment les forces de vente.To face their competitive stakes, existing companies are incentivized to renew their business model. Meanwhile, more and more innovation venues are being developed within them. In this thesis, we study how this places- which we call corporate collaborative innovation spaces (CIS)- contribute to the exploration of new business models.A lengthwise ethnographic case study within Altran company explores CIS’s place in the organisation, their real activities around customers and the use of artifacts/equipments made available to users.The results show the managerial innovation dimension- result of the organisational ambidextrous nature-of these CIS, decipher the micro processes leading to the emergence of a new value proposition / customer relationship and situate the demonstrators’ role in the construction of a discourse offering a new customer experience to visitors.We thus foster an understanding of CIS’s functioning and contribute to the enrichment of Business Model’s innovation theories, particularly in their multidimensional and systemic perspectives. We conclude by proposing ways to optimize the use of CIS through better appropriation by internal resources, particularly the sales force

Importance de l'hygiène alimentaire chez l'enfant (élaboration d'une plaquette informative)

LILLE2-BU Santé-Recherche (593502101) / SudocLILLE2-UFR Odontologie (593502202) / SudocPARIS-BIUM (751062103) / SudocSudocFranceF

Spectroscopie tunnel dans les films minces proches de la transition supraconducteur-isolant

Jans cette thèse, nous avons étudié par spectroscopie tunnel STM associée à des mesures de transport, les propriétés supraconductrices de films ultra-minces de nitrure de titane (TiN) et d'oxyde d'indium amorphe. Ces deux matériaux fortement désordonnés présentent. à température nulle, une transition de phase Quantique entre un état supraconducteur et un état isolant lorsque l'épaisseur diminue ou lorsqu'un champ magnétique perpendiculaire au film est appliaqué. Bien que le désordre de ces films soit considéré comme homogène, notre étude de spectroscopie tunnel effectuée à une température de 50 mK a révélé de fortes inhomogénéités du gap supraconducteur qui sont apparues d'autant plus fortes que le désordre est proche du désordre critique. Cette observation permet de décrire les films proches de la transition supraconducteur-isolant par un système d'îlots supraconducteurs percolant par couplage Josephson. L'application d'un champ maanétiaue dans un tel système détruit ces liens faibles et restaure l'état isolant sous-jacent sous forme d'une magnétorésistance géante. Nous avons aussi étudié le régime de fluctuations thermodvnamiques du paramètre d'ordre supraconducteur au-dessus de la température critique Tc. Dans les films minces, ces fluctuations se traduisent par un élargissement de la transition et par la formation d'un pseudogap dans la densité d'états à un électron, Nous avons mesuré dans les films de TiN un fort pseudogap qui s'étend sur une grandegamme de température au-dessus de Tc. Ce pseudogap résulte de l'effet des fluctuations supraconductrices combinées à l'anomalie de Coulomb renforcée par le désordreThis thesis presents a scannina tunnelina spectrocsopv studv of thin disordered superconductina films, namelv titanium nitride and amorphous indium oxide films, close to the superconductor-insulator transition (SIT). ln the superconductina phase, both films can be driven into an insulator bv increasina disorder or bV applvina a perpendicular maanetic field. Our tunnellina spectroscopv studv performed at a tempe rature of 50 mK revealed strona disorder-enhanced inhomogeneities of the superconducting gap. homoaeneous disorder. Close to the SIT such inhomoaeneities vield a picture of superconductina islands connected bv Josephson couplina. Conseauentlv, an applied maanetic field breaks these weak links and restores the underlvina insu latin a states. This thesis presents also a studv of the superconductina fluctuations reaime above the critical temperature. Fluctuations broaden the superconductina transition of thin films and ODen a pseudoaap in the one-electron densitv of states. ln TiN films, we measured a strona pseudoaaa in a wide temperature ranae. This pseudoaap is in aood aareement with a superconductina fluctuation pseudoaap combined with a disorder-enhanced Coulomb anomaly.GRENOBLE1-BU Sciences (384212103) / SudocSudocFranceF