Measuring many-body effects in carbon nanotubes with a scanning tunneling microscope

Electron-electron interactions and excitons in carbon nanotubes are locally measured by combining Scanning tunneling spectroscopy and optical absorption in bundles of nanotubes. The largest gap deduced from measurements at the top of the bundle is found to be related to the intrinsic quasi-particle gap. From the difference with optical transitions, we deduced exciton binding energies of 0.4 eV for the gap and 0.7 eV for the second Van Hove singularity. This provides the first experimental evidence of substrate-induced gap renormalization on SWNTs

Symmetry-selected spin-split hybrid states in C60_{60}/ferromagnetic interfaces

The understanding of orbital hybridization and spin-polarization at the organic-ferromagnetic interface is essential in the search for efficient hybrid spintronic devices. Here, using first-principles calculations, we report a systematic study of spin-split hybrid states of C$_{60}$ deposited on various ferromagnetic surfaces: bcc-Cr(001), bcc-Fe(001), bcc-Co(001), fcc-Co(001) and hcp-Co(0001). We show that the adsorption geometry of the molecule with respect to the surface crystallographic orientation of the magnetic substrate as well as the strength of the interaction play an intricate role in the spin-polarization of the hybrid orbitals. We find that a large spin-polarization in vacuum above the buckyball can only be achieved if the molecule is adsorbed upon a bcc-(001) surface by its pentagonal ring. Therefore bcc-Cr(001), bcc-Fe(001) and bcc-Co(001) are the optimal candidates. Spin-polarized scanning tunneling spectroscopy measurements on single C$_{60}$ adsorbed on Cr(001) and Co/Pt(111) also confirm that both the symmetry of the substrate and of the molecular conformation have a strong influence on the induced spin polarization. Our finding may give valuable insights for further engineering of spin filtering devices through single molecular orbitals.Comment: 10 pages, 9 figure

Représenter l'information et Web sémantique

National audienceCe petit ouvrage s'adresse à tous les curieux de l'intelligence artificielle (IA). C'est une introduction au sujet, volontairement brève, aussi élémentaire que possible, afin d'être accessible au plus grand nombre. Elle est écrite par un groupe de spécialistes reconnus. Pour conclure Glossaire Quelques références Contributeurs De même que l'IA concerne tous les secteurs, cet ouvrage devrait intéresser tous vos lecteurs

Uncertainty-sensitive reasoning for inferring sameAs facts in linked data

albakri2016aInternational audienceDiscovering whether or not two URIs described in Linked Data -- in the same or different RDF datasets -- refer to the same real-world entity is crucial for building applications that exploit the cross-referencing of open data. A major challenge in data interlinking is to design tools that effectively deal with incomplete and noisy data, and exploit uncertain knowledge. In this paper, we model data interlinking as a reasoning problem with uncertainty. We introduce a probabilistic framework for modelling and reasoning over uncertain RDF facts and rules that is based on the semantics of probabilistic Datalog. We have designed an algorithm, ProbFR, based on this framework. Experiments on real-world datasets have shown the usefulness and effectiveness of our approach for data linkage and disambiguation

Exploiting ontologies and alignments for trust in semantic P2P networks

Les rapports de recherche du LIG - ISSN: 2105-0422In a semantic P2P network, peers use separate ontologies and rely on alignments between their ontologies for translating queries. However, alignments may be limited —unsound or incomplete— and generate flawed translations, and thereby produce unsatisfactory answers. In this paper we propose a trust mechanism that can assist peers to select those in the network that are better suited to answer their queries. The trust that a peer has towards another peer is subject to a specific query and approximates the probability that the latter peer will provide a satisfactory answer. In order to compute trust, we exploit the information provided by peers’ ontologies and alignments, along with the information that comes from peers’ experience. Trust values are refined over time as more queries are sent and answers received, and we prove that these approximations converge

Alignment-Based Trust for Resource Finding in Semantic P2P Networks

Research Track: Policies and TrustInternational audienceIn a semantic P2P network, peers use separate ontologies and rely on alignments between their ontologies for translating queries. Nonetheless, alignments may be limited -unsound or incomplete- and generate flawed translations, leading to unsatisfactory answers. In this paper we present a trust mechanism that can assist peers to select those in the network that are better suited to answer their queries. The trust that a peer has towards another peer depends on a specific query and represents the probability that the latter peer will provide a satisfactory answer. In order to compute trust, we exploit both alignments and peers' direct experience, and perform Bayesian inference. We have implemented our technique and conducted an evaluation. Experimental results showed that trust values converge as more queries are sent and answers received. Furthermore, the use of trust improves both precision and recall

De l’intérêt des pictogrammes pour figurer les représentations sociocognitives de l’espace résidentiel

Quel rapport émotionnel un individu entretient-il avec son habitat ? Un jeu de simulation de négociation entre les différents critères d’évaluation de l’environnement résidentiel permet de recueillir un corpus de données. Son analyse implique une réflexion sur la formalisation des résultats obtenus. Deux options sont confrontées : une représentation en graphe polaire et une image composée de pictogrammes symbolisant les multiples dimensions de l’espace résidentiel qui peuvent motiver les choix d’installation. Cette dernière image permet une appréhension plus simple du lien entre les structures spatiales, cognitives et sociales.What emotional relationship does an individual have with his habitat? A negotiation simulation game between the different criteria of evaluation of the residential environment makes it possible to collect a corpus of data. Its analysis implies a reflection on the formalization of the results obtained. Two options are confronted: a polar graph representation and an image composed of pictograms symbolizing the multiple dimensions of the residential space that can motivate settlement choices. This latter image allows a simpler apprehension of the link between spatial, cognitive and social structures.¿Cuál es la relación emocional de un individuo con su hábitat?. Un juego de simulación para la negociación pemite recopilar información a partir de criterios de evaluación del entorno residencial. Su análisis implica una reflexión sobre la formalización gráfica de los resultados obtenidos. Se confrontan dos opciones de representación que pueden motivar la elección o no de una ubicación: una gráfica polar y una imagen compuesta por pictogramas que simbolizan las múltiples dimensiones del espacio residencial. Esta última imagen visibiliza mejor las relaciones entre las estructuras espaciales, cognitivas y sociales

On the possible role of hydrogen in the formation of fatigue striation in a moist atmosphere

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Localized state and charge transfer in nitrogen-doped graphene

Nitrogen-doped epitaxial graphene grown on SiC(000?1) was prepared by exposing the surface to an atomic nitrogen flux. Using Scanning Tunneling Microscopy (STM) and Spectroscopy (STS), supported by Density Functional Theory (DFT) calculations, the simple substitution of carbon by nitrogen atoms has been identified as the most common doping configuration. High-resolution images reveal a reduction of local charge density on top of the nitrogen atoms, indicating a charge transfer to the neighboring carbon atoms. For the first time, local STS spectra clearly evidenced the energy levels associated with the chemical doping by nitrogen, localized in the conduction band. Various other nitrogen-related defects have been observed. The bias dependence of their topographic signatures demonstrates the presence of structural configurations more complex than substitution as well as hole-doping.Comment: 5 pages, accepted in PR

Selective control of molecule charge state on graphene using tip-induced electric field and nitrogen doping

The combination of graphene with molecules offers promising opportunities to achieve new functionalities. In these hybrid structures, interfacial charge transfer plays a key role in the electronic properties and thus has to be understood and mastered. Using scanning tunneling microscopy and ab initio density functional theory calculations, we show that combining nitrogen doping of graphene with an electric field allows for a selective control of the charge state in a molecular layer on graphene. On pristine graphene, the local gating applied by the tip induces a shift of the molecular levels of adsorbed molecules and can be used to control their charge state. Ab initio calculations show that under the application of an electric field, the hybrid molecule/graphene system behaves like an electrostatic dipole with opposite charges in the molecule and graphene sub-units that are found to be proportional to the electric field amplitude, which thereby controls the charge transfer. When local gating is combined with nitrogen doping of graphene, the charging voltage of molecules on nitrogen is greatly lowered. Consequently, applying the proper electric field allows one to obtain a molecular layer with a mixed charge state, where a selective reduction is performed on single molecules at nitrogen sites. The local gating applied by a tip induces a shift of the energy levels of molecules adsorbed on graphene. A team led by Jerome Lagoute at Universite Paris Diderot investigated the interplay between the charge state of molecules on pristine and doped-graphene, and the tip-induced electric fields in scanning tunneling microscopy experiments. The tip-induced electric field was found to shift the molecular levels of tetracyanoquinodimethane molecules on graphene, leading to a change of charge state at negative bias. Ab initio calculations indicated that the molecule-on-graphene hybrid structure can be regarded as an electrostatic dipole, hence the charge transfer and associated electronic charge in the molecule and graphene could be tuned by the electric field. Furthermore, inserting nitrogen atom dopants allowed shifting the energy levels of single molecules absorbed directly on the electron-donating point defects