953 research outputs found
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 Ăąge crĂ©tacĂ© infĂ©rieur probable pour les palĂ©oaltĂ©rations latĂ©ritiques du graben de Saint-Maixent-l'Ăcole (seuil du Poitou) d'aprĂšs l'Ă©tude 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 dĂ©but du CrĂ©tacĂ© a Ă©tĂ© marquĂ© en Europe occidentale par le dĂ©veloppement de palĂ©oaltĂ©rations de type latĂ©ritique. Les argiles rouges Ă cuirasse ferrugineuse pisolitique du graben de Saint-Maixent-l'Ăcole (seuil du Poitou) peuvent constituer un exemple-tĂ©moin de ce type d'altĂ©ration. Toutefois, leur Ăąge reste inconnu et leur position stratigraphique, au-dessus des calcaires oligocĂšnes du fond du graben, ne permet pas d'exclure un Ăąge de formation beaucoup plus rĂ©cent. Des pisolites de goethite provenant de ces sols latĂ©ritiques ont Ă©tĂ© Ă©chantillonnĂ©s dans la forĂȘt de Fouilloux et ont fait l'objet d'analyses isotopiques Rb-Sr, U-Th et d 18 OdD , afin de tenter de lever l'indĂ©termination sur l'Ăąge de leur formation. Pour Rb-Sr, outre les analyses sur pisolites totaux, des expĂ©riences de lessivages mĂ©nagĂ©s ont Ă©tĂ© effectuĂ©es afin d'essayer d'isoler un composant radiogĂ©nique. Les rĂ©sultats obtenus n'ont pas permis de donner un Ăąge fiable par la mĂ©thode des isochrones, mĂȘme si les donnĂ©es tendraient plutĂŽt Ă faire remonter l'Ă©pisode d'altĂ©ration au dĂ©but du CrĂ©tacĂ©. Les analyses d 18 OdD ont Ă©tĂ© effectuĂ©es sur des pisolites de goethite totaux, ainsi que sur des fragments sous-Ă©chantillonnĂ©s. Les rĂ©sultats plaident en faveur de conditions de formation en zone climatique chaude et/ou aride. Aucun argument en faveur d'un Ăąge fini ou post-oligocĂšne ne peut ĂȘtre tirĂ© de l'Ă©tude des isotopes de l'oxygĂšne et de l'hydrogĂšne. Enfin, les analyses U-Th ont Ă©tĂ© effectuĂ©es sur des goethites totales ainsi que sur des fragments sous-Ă©chantillonnĂ©s. Les rapports d'activitĂ©s 234 U/ 238 U sont tous lĂ©gĂšrement supĂ©rieurs Ă l'Ă©quilibre sĂ©culaire, indiquant que les goethites Ă©voluent actuellement et/ou ont Ă©voluĂ© rĂ©cemment en systĂšme ouvert. Ainsi, les sols latĂ©ritiques apparaissent toujours comme gĂ©ochimiquement actifs. Si cette Ă©tude n'a pas permis d'apporter une rĂ©ponse claire sur l'Ăąge des sols latĂ©ritiques, l'hypothĂšse la plus probable au vu des donnĂ©es isotopiques est que ces sols latĂ©ritiques se sont dĂ©veloppĂ©s au-dessus des calcaires kimmĂ©ridgiens sur les bordures du graben de Saint-Maixent-l'Ăcole, puis remaniĂ©s et finalement resĂ©dimentĂ©s au fond du graben
Global Positioning System (GPS) location accuracy improvement due to Selective Availability removal
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 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.
- âŠ