Quantum confinement effects in Pb Nanocrystals grown on InAs

In the recent work of Ref.\cite{Vlaic2017-bs}, it has been shown that Pb nanocrystals grown on the electron accumulation layer at the (110) surface of InAs are in the regime of Coulomb blockade. This enabled the first scanning tunneling spectroscopy study of the superconducting parity effect across the Anderson limit. The nature of the tunnel barrier between the nanocrystals and the substrate has been attributed to a quantum constriction of the electronic wave-function at the interface due to the large Fermi wavelength of the electron accumulation layer in InAs. In this manuscript, we detail and review the arguments leading to this conclusion. Furthermore, we show that, thanks to this highly clean tunnel barrier, this system is remarkably suited for the study of discrete electronic levels induced by quantum confinement effects in the Pb nanocrystals. We identified three distinct regimes of quantum confinement. For the largest nanocrystals, quantum confinement effects appear through the formation of quantum well states regularly organized in energy and in space. For the smallest nanocrystals, only atomic-like electronic levels separated by a large energy scale are observed. Finally, in the intermediate size regime, discrete electronic levels associated to electronic wave-functions with a random spatial structure are observed, as expected from Random Matrix Theory.Comment: Main 12 pages, Supp: 6 page

Real Space Observation of Electronic Coupling between Self-Assembled Quantum Dots

International audienceThe control of quantum coupling between nano-objects is essential to quantum technologies. Confined nanostructures, such as cavities, resonators, or quantum dots, are designed to enhance interactions between electrons, photons, or phonons, giving rise to new properties, on which devices are developed. The nature and strength of these interactions are often measured indirectly on an assembly of dissimilar objects. Here, we adopt an innovative point of view by directly mapping the coupling of single nanostructures using scanning tunneling microscopy and spectroscopy (STM and STS). We take advantage of the unique capabilities of STM/STS to map simultaneously the nano-object's morphology and electronic density in order to observe in real space the electronic coupling of pairs of In(Ga)As/GaAs self-assembled quantum dots (QDs), forming quantum dot molecules (QDMs). Differential conductance maps dI/dV (E, x, y) demonstrate the presence of an effective electronic coupling, leading to bonding and antibonding states, even for dissymmetric QDMs. The experimental results are supported by numerical simulations. The actual geometry of the QDMs is taken into account to determine the strength of the coupling, showing the crucial role of quantum dot size and pair separation for device growth optimizatio

Synthesis of Functionalized Few-Layer Graphene Through Fast Electrochemical Expansion of Graphite

International audienc

Un âge crétacé inférieur probable pour les paléoaltérations latéritiques du graben de Saint-Maixent-l'École (seuil du Poitou) d'après l'étude isotopique (Rb-Sr, d 18 O-dD, U-Th) de pisolites ferrugineux

International audienceIt has been previously established that Western Europe underwent an important episode of lateritic weathering during early cretaceous times. In this context, the red clays and ferrugineous pisoids deposits occurring in the graben of Saint-Maixent-l'Ecole (Poitou saddle) may represent remnants of lateritic formations associated with this paleoweathering episode. However, their exact age is unknown, and since they occur on top of the Oligocene limestones filling the graben, it cannot be ruled out that these lateritic soils formed more recently. In order to better constrain the age of these lateritic formations, goethite pisoids were sampled in the Fouilloux forest and investigated for Rb-Sr, U-Th and delta O-delta D isotopic systems. Rb and Sr have been measured on bulk pisoids, and also on leachates and residues from leaching experiments carried out in the attempt to isolate a radiogenic component. No reliable Rb-Sr isochron age could be derived, although the isotopic data would favor an early Cretaceous age for these lateritic formations. delta O-18-delta D measurements have been done on bulk goethite pisoids, and also on sub-fragments. The results indicate that the pisoids likely formed under a warm and/or arid climate. No argument for a late of post - oligocene age can be found from the oxygen and hydrogen isotope data. Finally, U-Th analyses have been carried on bulk samples of goethites, and on sub-fragments as well. U-234/U-238 activity ratios are typically slightly higher than the equilibrium value, which indicates that goethites have recently evolved as open systems (if not still open). Hence, pedogenesis processes are still active from a geochemical point of view in these lateritic soils. This study has not allowed to derive a reliable age for these lateritic soils. However, when the results from the different isotopic systems investigated here are considered together, the best interpretation is that these lateritic soils developed most probably over the kimmeridgian limestones along the edge of the Saint-Maixent-l'Ecole graben, and were reworked and redeposited at the bottom of the graben later on.-Le début du Crétacé a été marqué en Europe occidentale par le développement de paléoaltérations de type latéritique. Les argiles rouges à cuirasse ferrugineuse pisolitique du graben de Saint-Maixent-l'École (seuil du Poitou) peuvent constituer un exemple-témoin de ce type d'altération. Toutefois, leur âge reste inconnu et leur position stratigraphique, au-dessus des calcaires oligocènes du fond du graben, ne permet pas d'exclure un âge de formation beaucoup plus récent. Des pisolites de goethite provenant de ces sols latéritiques ont été échantillonnés dans la forêt de Fouilloux et ont fait l'objet d'analyses isotopiques Rb-Sr, U-Th et d 18 OdD , afin de tenter de lever l'indétermination sur l'âge de leur formation. Pour Rb-Sr, outre les analyses sur pisolites totaux, des expériences de lessivages ménagés ont été effectuées afin d'essayer d'isoler un composant radiogénique. Les résultats obtenus n'ont pas permis de donner un âge fiable par la méthode des isochrones, même si les données tendraient plutôt à faire remonter l'épisode d'altération au début du Crétacé. Les analyses d 18 OdD ont été effectuées sur des pisolites de goethite totaux, ainsi que sur des fragments sous-échantillonnés. Les résultats plaident en faveur de conditions de formation en zone climatique chaude et/ou aride. Aucun argument en faveur d'un âge fini ou post-oligocène ne peut être tiré de l'étude des isotopes de l'oxygène et de l'hydrogène. Enfin, les analyses U-Th ont été effectuées sur des goethites totales ainsi que sur des fragments sous-échantillonnés. Les rapports d'activités 234 U/ 238 U sont tous légèrement supérieurs à l'équilibre séculaire, indiquant que les goethites évoluent actuellement et/ou ont évolué récemment en système ouvert. Ainsi, les sols latéritiques apparaissent toujours comme géochimiquement actifs. Si cette étude n'a pas permis d'apporter une réponse claire sur l'âge des sols latéritiques, l'hypothèse la plus probable au vu des données isotopiques est que ces sols latéritiques se sont développés au-dessus des calcaires kimméridgiens sur les bordures du graben de Saint-Maixent-l'École, puis remaniés et finalement resédimentés au fond du graben

X-shooter, NACO, and AMBER observations of the LBV Pistol Star \footnote{Based on ESO runs 85.D-0182A, 085.D-0625AC}

We present multi-instruments and multi-wavelengths observations of the famous LBV star Pistol Star. These observations are part of a larger program about early O stars at different metallicities. The Pistol star has been claimed as the most massive star known, with 250 solar masses. We present the preliminary results based on X-Shooter spectra, as well as the observations performed with the VLTI-AMBER and the VLT-NACO adaptive optics. The X-shooter spectrograph allows to obtain simultaneously a spectrum from the UV to the K-band with a resolving power of $\sim$15000. The preliminary results obtained indicate that Pistol Star has similar properties of Eta Car, including shells of matter, but also the binarity. Other objects of the program, here briefly presented, were selected for their particular nature: early O stars with mass discrepancies between stellar evolution models and observations, discrepancies with the wind momentum luminosity relation.Comment: Poster at the 39th LIAC, submitted version of the proceeding

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

Polarization state of the optical near-field

The polarization state of the optical electromagnetic field lying several nanometers above complex dielectric structures reveals the intricate light-matter interaction that occurs in this near-field zone. This information can only be extracted from an analysis of the polarization state of the detected light in the near-field. These polarization states can be calculated by different numerical methods well-suited to near--field optics. In this paper, we apply two different techniques (Localized Green Function Method and Differential Theory of Gratings) to separate each polarisation component associated with both electric and magnetic optical near-fields produced by nanometer sized objects. The analysis is carried out in two stages: in the first stage, we use a simple dipolar model to achieve insight into the physical origin of the near-field polarization state. In the second stage, we calculate accurate numerical field maps, simulating experimental near-field light detection, to supplement the data produced by analytical models. We conclude this study by demonstrating the role played by the near-field polarization in the formation of the local density of states.Comment: 9 pages, 11 figures, accepted for publication in Phys. Rev.