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International audience; We present a joint experimental and theoretical study of the superconductivity in doped silicon clathrates. The critical temperature in Ba8@Si–46 is shown to strongly decrease with applied pressure. These results are corroborated by ab initio calculations using MacMillan's formulation of the BCS theory with the electron-phonon coupling constant lambda calculated from perturbative density functional theory. Further, the study of I8@Si–46 and of gedanken pure silicon diamond and clathrate phases doped within a rigid-band approach show that the superconductivity is an intrinsic property of the sp3 silicon network. As a consequence, carbon clathrates are predicted to yield large critical temperatures with an effective electron-phonon interaction much larger than in C60.

We present a full process based on chemical vapour deposition that allows fabrication and integration at the wafer scale of carbon-nanotube-based field effect transistors. We make a statistical analysis of the integration yield that allows assessment of the parameter fluctuations of the titanium–nanotube contact obtained by self-assembly. This procedure is applied to raw devices without post-process. Statistics at the wafer scale reveal the respective role of semiconducting and metallic connected nanotubes and show that connection yields up to 86% can be reached. For large scale device integration, our process has to implement both wafer-scale self-assembly of the nanotubes and high transistor performances. In order to address this last issue, a gate engineering process has been investigated. We present the improvements obtained using low and high κ dielectrics for the gate oxide.

A nucleotide sequence analysis of a portion of the mitochondrial large ribosomal subunit was performed to define the phylogeography of the threatened crayfish Austropotamobius (Decapoda; Astacidae) in Italy. We collected 61 specimens from 31 localities across the Italian peninsula. For the phylogenetic inference, we combined the 61 Austropotamobius spp sequences obtained from this study with 18 sequences deposited in GenBank and corresponding to Italian, French, Irish, Swiss, and Slovenian locations. Among the analysed sequences, 34 distinct haplotypes were detected. Our results confirmed the presence of both A. pallipes and A. italicus in the Italian peninsula and the existence within the latter species of a strong intraspecific genetic variation, due to the occurrence of four subspecies with a well-defined geographic distribution. From a conservation viewpoint, Italy, with its high haplotype variability, may be considered a 'hot spot' for the genetic diversity of the European native crayfish Austropotamobius. We suggest that re-introduction programs should be conducted with extreme caution in Italy, since not only the two Austropotamobius species but also the four A. italicus subspecies are genetically and taxonomically separate units and require independent conservation plans.

The electronic structure of Cu(111) and Ni(111) free surfaces and of Ni-Cu structures on the Cu (111) surface is studied from ab initio calculations. Systems ending on a Ni layer are found to be magnetic. The spin splitting of the Ni d band raises the position of some minority Ni dxz,yz states which then lie in the energy range where the Shockley state should appear. This results in a strong sp-d hybridization of the spin-down components which furthermore depends on the k momentum. As a result, the Shockley state of Ni-terminated surfaces is spin polarized. The ab initio results are compared to recent STM experiments.

the proceedings editor does not allow on-line preprints (!) link to the paper on the editor site: http://onlinelibrary.wiley.com/doi/10.1002/pssc.200303622/abstract; International audience; Conductance measurements of thin granular aluminium films are reported. They exhibit a symmetrical field effect anomaly and slow relaxation of the conductance after the samples are put out of equilibrium by abruptly changing the temperature or the gate voltage. The results are identical to the ones reported in InOx thin films, suggesting that these phenomena are quite general in disordered insulators. The study of granular systems may help to elucidate the origin of the glassy behaviour.

International audience; Undoped and phosphorus-doped diamond thin films grown by Microwave Plasma-enhanced Chemical Vapour Deposition (MPCVD) on Ib {111}-oriented diamond substrates have been studied by confocal micro-Raman spectroscopy and confocal Raman imaging. A distinct Raman peak, broader and 6 cm-1 lower than the zone-centre optical phonon line of the substrate, was nearly systematically detected showing that in most cases the homoepitaxial layers were under an intense tensile strain. The magnitude of this strain increased with deposited thickness. In the thicker films, it was further observed that a network of oriented cracks could relieve the internal stress. These results suggest that tensile strain is a general feature of defective {111}-oriented CVD diamond homoepitaxial layers.

International audience; Conductivity and field effect measurements in thin insulating Al granular films are reported. The occurrence of a symmetrical field effect and of very slow conductance relaxations is demonstrated. They are identical to the electron glassy behaviours already reported in insulating indium oxide thin films. The results suggest that the phenomena are quite general. The study of structurally discontinuous samples should help to understand the origin and mechanism of the glassy behaviour.

After a short review of the present stage of knowledge of the Si[100] and β-SiC surface reconstructions and Si carbonisation mechanism, we discuss the possible origin for the change in sign of the interfacial strain recently observed in this strongly mismatched cubic hetero-epitaxial system as a function of the initial carbonisation temperature. Both the interfacial and anisotropic character of this strain are confirmed by micro-Raman analysis of cleaved CVD-grown samples, which reveal also how extended defects relieve the residual strain after the growth of a few micrometers of 3C-SiC. Such pseudo-substrates have been used as a cubic template for the MBE growth of cubic nitrides and Raman scattering results on β-AlN materials and cubic GaN/AlN stacked quantum boxes are presented. Finally, the potential and limitations of such 3C-SiC pseudo-substrates for nitride growth are discussed.

The spontaneous evolution of the structures obtained by submonolayer deposition of Ni on Cu(1 1 1) at room temperature has been studied by scanning tunnelling microscopy. The growth of an additional Cu plane on top of Ni-rich monolayer (ML) islands, and the simultaneous development of a characteristic electron standing wave pattern, have been observed in real time. From these observations we could derive the composition of the surface plane of the two kinds of bilayer islands that coexist in the deposit. A simple nucleation model is presented to explain the different evolution of pure and partially covered ML islands.

On examining the stability of a Wigner Crystal (WC) in an ionic dielectric, two competitive effects due to Polaron formation are found to be important: (i) the screening of the Coulomb forces which destabilizes the crystal, compensated by (ii) the increase of the carrier mass (polaron mass). The quantum melting of the Polaron Wigner Crystal (PWC) is examined. By calculating the quantum fluctuations of both the electrons and the polarization, we show that there is a competition between the dissociation of the Polarons at the insulator-to-metal transition (IMT), and a melting towards a polaron liquid. We find that at strong coupling ($\alpha > \alpha^*$), a liquid state of polarons cannot exist, and the IMT is driven by polaron dissociation. Next, we show that the dipolar interactions between localized polarons are responsible for a phonon instability of the PWC as the density increases. This provides a new mechanism for the IMT in doped dielectrics. Examining the sign of the dielectric constant of the PWC, we conjecture that such an instability could yield an Insulator-to-Superconductor transition., Comment: 4 Pages, 2 Figures included, Int. Conf. M2S-HTSC-VI (Houston 2000) to be published in Physica C

Paper Session: Applications III; International audience; Using Atomic Force Microscopes (AFM) to manipulate nano-objects is an actual challenge for surface scientists. Basic haptic interfacesbetween the AFM and experimentalists have already been implemented. Themulti-sensory renderings (seeing, hearing and feeling) studied from acognitive point of view increase the efficiency of the actual interfaces. Toallow the experimentalist to feel and touch the nano-world, we add mixedrealities between an AFM and a force feedback device, enriching thus thedirect connection by a modeling engine. We present in this paper the firstresults from a real-time remote-control handling of an AFM by our ForceFeedback Gestural Device through the example of the approach-retract curve.

We consider a simple setup in which a small quantum dot is strongly connected to a finite size box. This box can be either a metallic box or a finite size quantum wire.The formation of the Kondo screening cloud in the box strongly depends on the ratio between the Kondo temperature and the box level spacing. By weakly connecting two metallic reservoirs to the quantum dot, a detailed spectroscopic analysis can be performed. Since the transport channels and the screening channels are almost decoupled, such a setup allows an easier access to the measure of finite-size effects associated with the finite extension of the Kondo cloud., Comment: contribution to Les Houches proceeding, ``Quantum magnetism'' 2006

International audience; The paper presents thin SOI NEMS structures for accelerometers based on thin SOI technology and compatible with "In-IC" integration. The goal of this work is to demonstrate the feasibility in terms of concept and technological manufacturing. Modeling of Casimir force, development of hybrid e-beam/DUV lithography and FH-vapor release, specific AFM characterizations have allowed to design, fabricate and characterize first devices.

Traditionally, the understanding of quantum transport, coherent and ballistic1, relies on the measurement of macroscopic properties such as the conductance. While powerful when coupled to statistical theories, this approach cannot provide a detailed image of "how electrons behave down there". Ideally, understanding transport at the nanoscale would require tracking each electron inside the nano-device. Significant progress towards this goal was obtained by combining Scanning Probe Microscopy (SPM) with transport measurements2-7. Some studies even showed signatures of quantum transport in the surrounding of nanostructures4-6. Here, SPM is used to probe electron propagation inside an open quantum ring exhibiting the archetype of electron wave interference phenomena: the Aharonov-Bohm effect8. Conductance maps recorded while scanning the biased tip of a cryogenic atomic force microscope above the quantum ring show that the propagation of electrons, both coherent and ballistic, can be investigated in situ, and even be controlled by tuning the tip potential., Comment: 11 text pages + 3 figures

International audience; Josephson transport through a single molecule or carbon nanotube is considered in the presence of a local vibrational mode coupled to the electronic charge. The ground-state solution is obtained exactly in the limit of a large superconducting gap, and is extended to the general case by variational analysis. Coherent charge fluctuations are entangled with non-classical phonon states. The Josephson current induces squeezing of the phonon mode, which is controlled by the superconducting phase difference and by the junction asymmetry. Optical probes of non-classical phonon states are briefly discussed.

In order to achieve single crystalline {111} diamond p/n junction, studies of Ib diamond substrate, n-type and p-type CVD diamond epilayers were carried out.Surface and internal defects of the diamond bulk substrates were optically studied using Nomarski microscope, AFM and X-Ray measurements. In particular, the polishing defects of the diamond substrate were shown to be able to damage the crystalline quality of the CVD films. A Reactive Ion Etching pre-treatment of the substrate surface was performed before the growth in order to remove the defects induced by mechanical polishing. Sets of phosphorus-doped samples simultaneously grown on substrates without and with RIE pre-treatment were optically analysed and characterized using cathodoluminescence and Hall Effect measurements. The positive effect of the substrate surface etching on the optical and electrical quality of diamond layers was proved with the decrease of Band A emission related to dislocations and the increase of the low field Hall mobility for the pre-treated sample. Experimental data of the electron mobility were compared with theoretical calculations based on the relaxation time approximation allowing us to determine the impurities and phonon scattering mechanisms and the acoustic deformation potential of diamond close to 17.7 eV. The maximum value of the Hall mobility achievable at thermodynamical equilibrium in n-type diamond was determined. A temperature dependence of a new scattering was proposed for the untreated sample. The crystalline, optical and electronic properties of CVD {111} diamond thin films with boron ranging from 8x1015 cm-3 to 3x1021 cm-3 were studied as a function of boron incorporation measured by SIMS. Finally, two diamond p/n junctions, one deposited on heavy boron-doped substrate, the other grown upon an insulating RIE pre-treated substrate were prepared. I(V), cathodoluminescence, electroluminescence and EBIC measurements of each diamond diode were carried out, compared and discussed., Dans le but de réaliser des jonctions p/n monocristallines en diamant orienté selon {111}, des études ont été menées sur le substrat de diamant et sur les couches de diamant CVD dopées de type n au Phosphore et de type p au Bore. Les défauts de surface et internes au substrat de diamant ont été analysés par des observations optiques, des mesures AFM et par des mesures en diffraction de rayons X. Les défauts dus au polissage de la surface du substrat ont en particulier été identifiés comme pouvant détériorer la qualité cristalline des couches CVD.Un prétraitement par gravure RIE de la surface du substrat avant la croissance a été utilisé dans le but de supprimer les défauts induits par le polissage mécanique. Des paires d'échantillons dopés phosphore simultanément homoépitaxiés sur substrats non traités et prétraités RIE ont été caractérisées par cathodoluminescence et effet Hall. L'efficacité de la gravure de la surface du substrat a été prouvée par la diminution de l'émission de bande A reliée aux dislocations et par l'augmentation de la mobilité électronique de Hall pour les échantillons prétraités. Les données expérimentales ont été comparées à un modèle de mobilité basé sur l'approximation du temps de relaxation permettant de mettre en évidence la dépendance en température des différents modes de diffusion par les impuretés et les phonons ainsi que la détermination du potentiel de déformation acoustique du diamant de 17.7 eV. La valeur maximum de mobilité électronique possible à l'équilibre thermodynamique a ainsi pu être déterminée. Un nouveau mode de diffusion a été mis en évidence pour les échantillons non traités.Les propriétés cristallines, optiques et électroniques d'une série de couches minces diamant CVD {111} couvrant une large gamme de dopage de 2x1015 à 3x1021 atomes de bore par cm3 ont été examinées en fonction de l'incorporation d'atomes dopants mesurés par SIMS.Deux jonctions p/n, l'une réalisée sur un substrat fortement dopé bore, l'autre sur un substrat non dopé ont finalement été réalisées. Les caractéristiques I(V), les spectres de cathodoluminescence et d'électroluminescence et les résultats d'EBIC ont été comparés puis discutés.

Electrostatic Force Microscopy and electric charges in semiconducting nanostructures

Capacitive coupling is most commonly used in NEMS/MEMS or in Scanning Probe Techniques to either induce a displacement or to detect an external interaction applied to the micro/nanosystem. A parabolic elastic deformation of the sensor signs a capacitive interaction as the applied voltage is varied. In this paper, we present detailed force measurements performed in the submicron range with a picoNewton sensitivity, and using a UHV AFM and a silicon microlever equipped with a metallized microsphere,. A complete treatment of the sensor sphere/plane geometry enables one to detect quantitatively a V4 departure from the parabolic behaviour. This shows that using this setup allows to detect the increased deformation of a lever much before the mechanical instability. More importantly, this effect must be carefully taken into account if sphere/plane absolute separations are to be quantitatively measured on the basis of electrostatic calibration. To our knowledge, this behaviour in AFM is reported for the first time. We believe that it is relevant for the description of NEMS/MEMS behaviour. Moreover, it provides a method to accurately measure the lever spring constant without any direct contact.

International audience; The point-contact spectroscopy is used to address the evolution of two superconducting energy gaps in the Al- and C-doped magnesium diboride polycrystals and single crystals with Tc's from 39 to 22 K prepared by different techniques. The obtained evolution of two gaps does not show any anomalous behavior but can be consistently described by the combination of the (prevailing) band filling effect and a (minor) increased interband scattering as proposed by Kortus et al. [Kortus et al., Phys. Rev. Lett. 94 (2005) 027002]. The approaching of two gaps is stronger in the Al-doped systems but interband scattering is still not large enough to merge two gaps. The full merging can expected only for higher dopings with Tc's below 10-15 K. In-magnetic-field measurements are used to analyze the intraband scatterings introduced by these two substitutions. It is shown that the carbon doping introduces significant disorder mainly by decreasing the diffusion coefficient in the p band while the Al substitution leaves the samples in the clean limit.

We investigate the optical response of two sub-wavelength grooves on a metallic screen, separated by a sub-wavelength distance. We show that the Fabry-Perot-like mode, already observed in one-dimensional periodic gratings and known for a single slit, splits into two resonances in our system : a symmetrical mode with a small Q-factor, and an antisymmetric one which leads to a much stronger light enhancement. This behavior results from the near-field coupling of the grooves. Moreover, the use of a second incident wave allows to control the localization of the photons in the groove of our choice, depending on the phase difference between the two incident waves. The system exactly acts as a sub-wavelength optical switch operated from far-field., 5 pages, 4 figures, submitted to PRL

The goal of this thesis is to contribute to the study of the ordering of localized classical spins coupled to itinerant electrons via the Zener double exchange model. The tendency to ferromagnetism resulting from electron motion is thwarted by the competition with antiferromagnetic superexchange interactions, leading to a variety of complex magnetic structures. First, the one dimensional model is studied as a function of the electronic density and a ferromagnetic “polaron” phase, each polaron containing three spins only, is shown to exist in a large part of the diagram even at low density. The tendency towards phase separation is studied in detail. In a second part we consider spin ordering in 3-leg ladders present in the structure of the Fe Ludwigite: Fe(3)O(2)BO(3). We examine the interplay between magnetic ordering, charge ordering and structural instability observed in this system. Finally, we consider the controversy between charge ordering and “Zener Polaron” ordering in the manganite La(0.5)Ca(0.5)MnO(3). It is shown that this two types of ordering are exclusive, except within an extremely reduced range of parameters.; L'objectif de cette thèse est de contribuer à l'étude de l'ordre de spins classiques localisés couplés à des électrons itinérants par un modèle de double échange de Zener. La tendance au ferromagnétisme due au mouvement électronique est contrecarrée par la compétition avec des interactions de super-échange antiferromagnétiques, ce qui conduit à une variété de structures magnétiques complexes. Nous étudions tout d'abord le modèle à une dimension en fonction de la densité électronique, et montrons qu'une phase de « polarons » ferromagnétiques, chaque polaron contenant seulement trois spins, existe dans une grande partie du diagramme de phase même à faible densité. La tendance vers une séparation de phases est étudiée en détail. Dans une deuxième partie nous considérons l'ordre de spins dans les échelles de spins à trois montants présentes dans la structure de la ludwigite de Fe : Fe(3)O(2)BO(3). Nous examinons la relation entre l'ordre magnétique, l'ordre de charge et l'instabilité structurale observée dans ce système. Finalement, nous abordons la controverse entre l'ordre de charge et l'ordre de « polarons de Zener » dans la manganite La(0.5)Ca(0.5)MnO(3). Nous montrons que ces deux types d'ordre sont exclusifs, sauf pour une gamme extrêmement réduite des paramètres.

The goal of this thesis is to contribute to the study of the ordering of localized classical spins coupled to itinerant electrons via the Zener double exchange model. The tendency to ferromagnetism resulting from electron motion is thwarted by the competition with antiferromagnetic superexchange interactions, leading to a variety of complex magnetic structures. First, the one dimensional model is studied as a function of the electronic density and a ferromagnetic “polaron” phase, each polaron containing three spins only, is shown to exist in a large part of the diagram even at low density. The tendency towards phase separation is studied in detail. In a second part we consider spin ordering in 3-leg ladders present in the structure of the Fe Ludwigite: Fe(3)O(2)BO(3). We examine the interplay between magnetic ordering, charge ordering and structural instability observed in this system. Finally, we consider the controversy between charge ordering and “Zener Polaron” ordering in the manganite La(0.5)Ca(0.5)MnO(3). It is shown that this two types of ordering are exclusive, except within an extremely reduced range of parameters., L'objectif de cette thèse est de contribuer à l'étude de l'ordre de spins classiques localisés couplés à des électrons itinérants par un modèle de double échange de Zener. La tendance au ferromagnétisme due au mouvement électronique est contrecarrée par la compétition avec des interactions de super-échange antiferromagnétiques, ce qui conduit à une variété de structures magnétiques complexes. Nous étudions tout d'abord le modèle à une dimension en fonction de la densité électronique, et montrons qu'une phase de « polarons » ferromagnétiques, chaque polaron contenant seulement trois spins, existe dans une grande partie du diagramme de phase même à faible densité. La tendance vers une séparation de phases est étudiée en détail. Dans une deuxième partie nous considérons l'ordre de spins dans les échelles de spins à trois montants présentes dans la structure de la ludwigite de Fe : Fe(3)O(2)BO(3). Nous examinons la relation entre l'ordre magnétique, l'ordre de charge et l'instabilité structurale observée dans ce système. Finalement, nous abordons la controverse entre l'ordre de charge et l'ordre de « polarons de Zener » dans la manganite La(0.5)Ca(0.5)MnO(3). Nous montrons que ces deux types d'ordre sont exclusifs, sauf pour une gamme extrêmement réduite des paramètres.

We show that the semi-classical model of conduction breaks down if the mean free path of charge carriers is smaller than a typical extension of their wavefunction. This situation is realized for sufficiently slow charge carriers and leads to a transition from a metallic like to an insulating like regime when scattering by defects increases. This explains the unconventional conduction properties of quasicrystals and related alloys. The conduction properties of some heavy fermions or polaronic systems, where charge carriers are also slow, present a deep analogy., 4 pages

Resonant X-ray scattering (RXS) is a spectroscopy where both the power of site selective diffraction and the power of local absorption spectroscopy regarding atomic species are combined. By virtue of the dependence on the core level state energy and the three dimensional electronic structure of the intermediate state, this technique is specially suited to study charge, orbital or spin orderings and associated crystal distortions. In the case of charge ordering, we exploit the fact that atoms with closely related site symmetries but with small charge differences exhibit resonances at slightly different energies. The sensitivity of this effect allows for quantitative estimations of the charge disproportion. Opposite to fluorescence or absorption measurements, the power of diffraction relies on the capability of detecting differences that are smaller than the inverse lifetime of the core hole level. To account for the uncertainty of the crystallographic structure and the fact that the charge ordering must be disentangled from the associated atomic displacements, a complete methodology is proposed and applied to the low temperature phase of magnetite. Relative sensitivity on spin, toroidal and orbital ordering is also shown and compared in different transition metal oxide compounds, like V2O3 and GaFeO3.

International audience; Although the local resistivity of semiconducting silicon in its standard crystalline form can be changed by many orders of magnitude by doping with elements, superconductivity has so far never been achieved. Hybrid devices combining silicon's semiconducting properties and superconductivity have therefore remained largely underdeveloped. Here we report that superconductivity can be induced when boron is locally introduced into silicon at concentrations above its equilibrium solubility. For sufficiently high boron doping (typically 100 p.p.m.) silicon becomes metallic(1). We find that at a higher boron concentration of several per cent, achieved by gas immersion laser doping, silicon becomes superconducting. Electrical resistivity and magnetic susceptibility measurements show that boron-doped silicon (Si:B) made in this way is a superconductor below a transition temperature T-c approximate to 0.35 K, with a critical field of about 0.4 T. Ab initio calculations, corroborated by Raman measurements, strongly suggest that doping is substitutional. The calculated electron-phonon coupling strength is found to be consistent with a conventional phonon-mediated coupling mechanism(2). Our findings will facilitate the fabrication of new silicon-based superconducting nano-structures and mesoscopic devices with high-quality interfaces.

International audience

The lower $({H}_{c1})$ and upper $({H}_{c2})$ critical fields of ${\mathrm{Mg}}_{1\ensuremath{-}x}{\mathrm{Al}}_{x}{\mathrm{B}}_{2}$ single crystals (for $x=0$, $0.1$, and $\ensuremath{\gtrsim}0.2$) have been deduced from local magnetization and specific heat measurements, respectively. We show that ${H}_{c1}$ and ${H}_{c2}$ are both decreasing with increasing doping content. The corresponding anisotropy parameter ${\ensuremath{\Gamma}}_{{H}_{c2}}(0)={H}_{c2}^{ab}(0)∕{H}_{c2}^{c}(0)$ value also decreases from $\ensuremath{\sim}5$ in pure ${\mathrm{MgB}}_{2}$ samples down to $\ensuremath{\sim}1.5$ for $x\ensuremath{\gtrsim}0.2$ whereas ${\ensuremath{\Gamma}}_{{H}_{c1}}(0)={H}_{c1}^{c}(0)∕{H}_{c1}^{ab}(0)$ remains on the order of 1 in all samples. The small and large gap values have been obtained by fitting the temperature dependence of the zero-field electronic contribution to the specific heat to the two-gap model for the three Al concentrations. Very similar values have also been obtained by point contact spectroscopy measurements. The evolution of those gaps with Al concentration suggests that both band filling and interband scattering effects are present.

International audience; We report on specific heat and Hall probe magnetization measurements in magnesium diboride single crystals. A magnetic field dependence of the coherence length has been deduced from the former assuming that the electronic excitations are localized in field dependent vortex cores in which case is related to the Sommerfeld coefficient = Cp /T T→0 throughout, H /a0 2 a0 being the vortex spacing . The reversible part of the magnetization has been analyzed with a phenomenological Ginzburg-Landau model introducing field dependent parameters i.e., penetration depth and which account for the decreasing contribution of the -band with increasing field. This approach perfectly reproduces the experimental data by combining the field dependence of deduced from Cp 1/ 2 B with an almost linear increase of from 450 Å at low field to 700 Å close to Hc2. These field dependences can then be used to consistently describe the field dependence of the critical current density, small angle neutron scattering form factor, and muon spin relaxation rate.

International audience; SnO2-based ceramics doped with Sb and/or Zn, have been prepared by solid state reaction at 1300 °C. The effect of dopants on electronic properties and sintering behavior has been studied. While undoped SnO2 pellets displayed very low electrical conductivities and lower densities, the Sb-doped ceramics showed higher electrical conductivity, with almost no densification and a significant antimony loss. On the contrary, a high densification and low conductivity are obtained for Zn-doped ceramics. Therefore, it is worthwhile to investigate SnO2 ceramics co-doped with Sb and Zn (SnO2:Sb:Zn) to combine the advantages of both dopants. X-ray photoelectron spectroscopy (XPS) analysis confirmed that Sb5+ is mainly substituted at the Sn4+ site for Sb-doped ceramics and is in agreement with Hall-measurements. In the case of SnO2 samples co-doped with Sb and Zn, XPS and Hall data confirmed the presence of both Sb5+ and Sb3+. The SnO2:Sb:Zn system exhibited enhanced electrical conductivity and high densities. In addition, the presence of Zn prevented the Sb evaporation during sintering.

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 version

International audience; The radial breathing modes and tangential modes have been systematically measured on a large number of individual semiconducting single-wall carbon nanotubes (thin bundles) suspended between plots (free-standing single-wall carbon nanotubes). The strong intensity of the Raman spectra ensures the precision of the experimentally determined line shapes and frequencies of these modes. The diameter dependence of the frequencies of the tangential modes was measured. This dependence is discussed in relation with recent calculations. The present data confirm/contradict some previous interpretations.

The recent introduction of virtual realities into scientific data processing tools inaugurates a major turning in the history of the man machine interfaces, by allowing a real-time interaction with virtual spaces. ln parallel, the emergence of nanosciences requires the installation of innovative tools which, beyond the measurement and classical data display, will allow action and understanding at a scale where physical phenomena are singular and typically intangible. This work aims to develop of a nanomanipulator, an instrumental interface of augmented reality based on the coupling of an Atomic Force Microscope, a simulator and a multisensory platform. This nanomanipulator conforms like a real tool allowing to interact naturally and directly with the environment of the nanoworld. The multisensoriality is allowed by the real-lime rebuilding of a mixed space (real and virtual), energetically and dynamically coherent, where the user can interact with force feedback, animated images and sounds. Reality is increased thanks to the insertion of virtual mass-point physical models, interacting conjointly with the user and nanoscene. The modularity and the opening of this instrument are presented, besides the issues induced by the scaling and by the technological and modeling systems. The teaching impact of such a device, for the formal training of intangible physical phenomena, within a practical teaching framework in nanophysic, constitutes a first evaluation of the tool. Its potential for research was highlighted through some applications of determination of real nanometric situations through recursive modeling and sensory comparisons with their virtual counterparts.; L'introduction récente des réalités virtuelles dans les outils informatiques scientifiques inaugure un tournant majeur dans l'histoire des interfaces homme-machine, en permettant l'interaction temps réel avec des espaces virtuels. Or l'émergence des nanosciences requiert la mise en place d'outils novateurs qui, au-delà de la mesure et de la représentation classique de données, permettront l'action et la compréhension à une échelle où les phénomènes physiques sont singuliers et typiquement intangibles. Ce travail vise au développement d'une interface instrumentale de réalité augmentée basée sur un Microscope à Force Atomique. Ce nanomanipulateur se conforme comme un outil réel permettant d'interagir naturellement avec le nanomonde. La multisensorialité est permise par la reconstruction temps-réel d'un espace mixte (réel et virtuel), énergétiquement et dynamiquement cohérent, où l'utilisateur peut interagir avec un retour de force, d'images animées et de sons. La réalité est augmentée par insertion de modèles physiques particulaires virtuels, interagissant à la fois avec l'utilisateur et la nanoscène. La modularité et l'ouverture de cet instrument sont exposées, ainsi que les problèmes induits par le changement d'échelle et par les systèmes technologiques et de modélisation. L'impact pédagogique, dans l'apprentissage formel de phénomènes physiques non tangibles en enseignement pratique en nanophysique, constitue une première évaluation de l'outil. Son potentiel pour la recherche s'est affirmé à travers quelques applications de détermination de situations nanométriques réelles par modélisations récursives et comparaison sensorielle avec leurs homologues virtuels.

We have altered the superconductivity of a suspended rope of single walled carbon nanotubes, by coating it with organic polymers. Upon coating, the normal state resistance of the rope changes by less than 20%. But superconductivity, which on the bare rope shows up as a substantial resistance decrease below $300\phantom{\rule{0.3em}{0ex}}\mathrm{mK}$, is gradually suppressed. We correlate this to the suppression of radial breathing modes, measured with raman spectroscopy on suspended single and double-walled carbon nanotubes. This points to the breathing phonon modes as being responsible for superconductivity in ropes of single walled carbon nanotubes.

We present an x-ray diffuse scattering study of the charge-density wave (CDW) structure in the doped blue bronzes K{sub 0.3}(Mo{sub 1-x}V{sub x})O{sub 3} (x=2.8%), Rb{sub 0.3}(Mo{sub 1-x}V{sub x})O{sub 3} (x=1.44% and 0.28%), and K{sub 0.3}(Mo{sub 1-x}W{sub x})O{sub 3} (x=2%). At low temperature, the {+-}2k{sub F} satellite reflections are broadened in all four compounds, and shifted in V-doped compounds. Moreover, we have observed an intensity asymmetry of the {+-}2k{sub F} satellite reflections relative to the pure compound, and a profile asymmetry of each satellite reflections. We show that both effects, intensity and profile asymmetry, give access to the local properties of CDW in disordered systems, including the pinning and the phase shifts around impurities. This leads us to propose a complete scenario of the pinning in doped blue bronzes. In the V-doped compounds, we show that the profile asymmetry is due to Friedel oscillations around the charged V substituent and that the intensity asymmetry is related to the strong pinning of the CDW. In the W-doped crystal, the CDW is weakly pinned in regions containing a few tens of neutral W substituents and the CDW phase is slightly distorted around the W impurities. Exact calculations are presented to sustain these ideas.

International audience; The crystallization of electrons in quasi low-dimensional solids is studied in a model which retains the full three-dimensional nature of the Coulomb interactions. We show that restricting the electron motion to layers (or chains) gives rise to a rich sequence of structural transitions upon varying the particle density. In addition, the concurrence of low-dimensional electron motion and isotropic Coulomb interactions leads to a sizeable stabilization of the Wigner crystal, which could be one of the mechanisms at the origin of the charge ordered phases frequently observed in such compounds.

International audience; We report on the magneto-optical spectroscopy and cathodoluminescence of a set of wurtzite (Ga,Mn)N epilayers with a low Mn content, grown by molecular beam epitaxy. The sharpness of the absorption lines associated to the Mn$^{3+}$ internal transitions allows a precise study of its Zeeman effect in both Faraday and Voigt configurations. We obtain a good agreement if we assume a dynamical Jahn-Teller effect in the 3d$^{4}$ configuration of Mn, and we determine the parameters of the effective Hamiltonians describing the $^{5}T_{2}$ and $^{5}E$ levels, and those of the spin Hamiltonian in the ground spin multiplet, from which the magnetization of the isolated ion can be calculated. On layers grown on transparent substrates, transmission close to the band gap, and the associated magnetic circular dichroism, reveal the presence of the giant Zeeman effect resulting from exchange interactions between the Mn$^{3+}$ ions and the carriers. The spin-hole interaction is found to be ferromagnetic.

11 pages; International audience; We compute the velocity correlation function of electronic states close to the Fermi energy, in approximants of quasicrystals. As we show the long time value of this correlation function is small. This means a small Fermi velocity, in agreement with previous band structure studies. Furthermore the correlation function is negative on a large time interval which means a phenomenon of backscattering. As shown in previous studies the backscattering can explain unusual conduction properties, observed in these alloys, such as for example the increase of conductivity with disorder.

In organic field-effect transistors (FETs), charges move near the surface of an organic semiconductor, at the interface with a dielectric. In the past, the nature of the microscopic motion of charge carriers--which determines the device performance--has been related to the quality of the organic semiconductor. Recently, it was discovered that the nearby dielectric also has an unexpectedly strong influence. The mechanisms responsible for this influence are not understood. To investigate these mechanisms, we have studied transport through organic single-crystal FETs with different gate insulators. We find that the temperature dependence of the mobility evolves from metallic-like to insulating-like with increasing dielectric constant of the insulator. The phenomenon is accounted for by a two-dimensional Fröhlich polaron model that quantitatively describes our observations and shows that increasing the dielectric polarizability results in a crossover from the weak to the strong polaronic coupling regime. This represents a considerable step forward in our understanding of transport through organic transistors, and identifies a microscopic physical process with a large influence on device performance.

International audience; p-type {111} homoepitaxial diamond layers were grown by Microwave Plasma-Enhanced Chemical Vapor Deposition. The variation of the gas phase boron concentration led to solid-state incorporation of boron in the 6 · 1016–3 · 1021 cm– 3 range. Confocal Raman spectroscopy and Raman imaging have been used to investigate this series of homoepitaxial films. As already observed for undoped or phosphorous-doped {111} epilayers, a first noticeable feature was the presence of many sharp and weak lines peaking at random in the 500–2000 cm– 1 range. These peaks were all the most observed that the doping level was low. A number of boron-related Raman lines centered at about 610, 925, 1045 cm– 1 were observed for solid state boron concentrations in the 1.5 · 1018–9 · 1019 cm– 3 range. Above a boron concentration of 3 · 1020 cm– 3, the usual Raman signal of heavily boron-doped diamond was recorded. The thickness of the epitaxial layers, in the 0.2–2 μm range, was too low to allow a more detailed analysis of the zone-center diamond optical phonon.

International audience; We compute the velocity correlation function of electronic states close to the Fermi energy, in approximants of quasicrystals. As we show the long time value of this correlation function is small. This means a small Fermi velocity, in agreement with previous band structure studies. Furthermore the correlation function is negative on a large time interval which means a phenomenon of backscattering. As shown in previous studies the backscattering can explain unusual conduction properties, observed in these alloys, such as for example the increase of conductivity with disorder.

International audience

“MgB2 : an exotic conventionnal superconductor”..... Behind this apparently paradoxical qualification is hiding the great originality of the system which was the subject of my work. Conventionnal in the electron-phonon coupling mechanism, MgB2 is exotic in the presence of two weakly coupled superconducting gaps. The existence of those two gaps leads to a “pathological” behaviour of the fundamental parameters : the coherence length, xsi and penetration depth, lambda.Whereas a consensus on the multigap nature of MgB2 was quickly established, the published values of these fundamental parameters and of the associated critical fields expressed a disconcerting dispersion.My work falls under this period of browsing of the MgB2 properties. It has the originality to use complementary techniques: Hall-probe magnetometry and alternative specific heat. First, we present a thermodynamic study of the H-T phase diagram carried out on single crystals. Specific heat measurements enabled the determination of the upper critical field Hc2. The lower critical field Hc1 has been deduced from magnetic measurements. Its determination required a thorough study of the penetration mechanisms of the vortices. The field distribution deduced from Hall probe array measurements on a serie of samples with various size ratios highlighted the presence of the geometrical barriers that delay the entry of the vortices. It is essential to take into account these barriers in the deduction of Hc1.The anisotropy of the critical fields is temperature dependent due to the existence of two pairs of slightly coupled superconductive bands whose relative influence varies according to the position in the phase diagram H-T.This leads to a field dependence of these fundamental parameters.Finally, a study of doped and irradiated samples were performed. The substitution of Mg by Al modifies the electronic doping and the critical fields. On the other hand, the effect of disorder introduced by electron irradiation led to a brutal increase of the peak effect which still remains to be explained., “MgB2, un supraconducteur conventionnel exotique”... Derrière cette qualification apparemment antinomique se cache la grande originalité du composé qui fut le sujet de mon travail. Conventionnel dans son mécanisme de couplage électron-phonon, MgB2 est exotique de part la présence de deux gaps supraconducteur faiblement couplés. L'existence de ces deux gaps va alors conduire à un comportement “pathologique” des paramètres fondamentaux de la supraconductivité : la longueur de cohérence, xsi et la profondeur de pénétration, lambda.Alors qu'un consensus sur la nature multigap de MgB2 s'est rapidement établi, les résultats publiés concernant ces paramètres fondamentaux et ceux des champs critiques associés présentaient une dispersion déconcertante. Mon travail s'inscrit dans cette période flou expérimental. Il allie deux techniques complémentaires : la magnétométrie à microsondes de Hall et la chaleur spécifique alternative.Nous présentons tout d'abord une étude thermodynamique du diagramme de phases H-T effectuée sur des monocristaux. Des mesures de chaleur spécifique ont permis de déterminer le champ critique supérieur Hc2. Le champ critique inférieur Hc1 a lui été évalué par des mesures magnétiques.Sa détermination a nécessité une étude approfondie des mécanismes de pénétration des vortex. La répartition spatiale de l'induction magnétique obtenue avec un réseau de microsondes sur une collection d'échantillons de rapports de dimensions différents a mis en évidence la présence de barrières géométriques retardant l'entrée des vortex. La prise en compte de ces barrières est essentielle à la déduction de Hc1.L'anisotropie des champs critiques présente une dépendance en température due à la présence de deux paires de bandes supraconductrices faiblement couplés et dont l'influence relative varie en fonction de la position dans le diagramme H-T. Cette présence conduit également à une évolution de xsi et lambda en fonction du champ magnétique que nous présentons ici.Enfin, une étude d'échantillons dopés et irradiés a été abordée. L'effet de la substitution de Mg par Al modifie le dopage électronique et les champs critiques. En revanche, l'effet du désordre ponctuel introduit par l'irradiation aux électrons conduit à une brutale augmentation de l'effet de pic dont l'origine reste encore à établir.

“MgB2 : an exotic conventionnal superconductor”..... Behind this apparently paradoxical qualification is hiding the great originality of the system which was the subject of my work. Conventionnal in the electron-phonon coupling mechanism, MgB2 is exotic in the presence of two weakly coupled superconducting gaps. The existence of those two gaps leads to a “pathological” behaviour of the fundamental parameters : the coherence length, xsi and penetration depth, lambda.Whereas a consensus on the multigap nature of MgB2 was quickly established, the published values of these fundamental parameters and of the associated critical fields expressed a disconcerting dispersion.My work falls under this period of browsing of the MgB2 properties. It has the originality to use complementary techniques: Hall-probe magnetometry and alternative specific heat. First, we present a thermodynamic study of the H-T phase diagram carried out on single crystals. Specific heat measurements enabled the determination of the upper critical field Hc2. The lower critical field Hc1 has been deduced from magnetic measurements. Its determination required a thorough study of the penetration mechanisms of the vortices. The field distribution deduced from Hall probe array measurements on a serie of samples with various size ratios highlighted the presence of the geometrical barriers that delay the entry of the vortices. It is essential to take into account these barriers in the deduction of Hc1.The anisotropy of the critical fields is temperature dependent due to the existence of two pairs of slightly coupled superconductive bands whose relative influence varies according to the position in the phase diagram H-T.This leads to a field dependence of these fundamental parameters.Finally, a study of doped and irradiated samples were performed. The substitution of Mg by Al modifies the electronic doping and the critical fields. On the other hand, the effect of disorder introduced by electron irradiation led to a brutal increase of the peak effect which still remains to be explained.; “MgB2, un supraconducteur conventionnel exotique”... Derrière cette qualification apparemment antinomique se cache la grande originalité du composé qui fut le sujet de mon travail. Conventionnel dans son mécanisme de couplage électron-phonon, MgB2 est exotique de part la présence de deux gaps supraconducteur faiblement couplés. L'existence de ces deux gaps va alors conduire à un comportement “pathologique” des paramètres fondamentaux de la supraconductivité : la longueur de cohérence, xsi et la profondeur de pénétration, lambda.Alors qu'un consensus sur la nature multigap de MgB2 s'est rapidement établi, les résultats publiés concernant ces paramètres fondamentaux et ceux des champs critiques associés présentaient une dispersion déconcertante. Mon travail s'inscrit dans cette période flou expérimental. Il allie deux techniques complémentaires : la magnétométrie à microsondes de Hall et la chaleur spécifique alternative.Nous présentons tout d'abord une étude thermodynamique du diagramme de phases H-T effectuée sur des monocristaux. Des mesures de chaleur spécifique ont permis de déterminer le champ critique supérieur Hc2. Le champ critique inférieur Hc1 a lui été évalué par des mesures magnétiques.Sa détermination a nécessité une étude approfondie des mécanismes de pénétration des vortex. La répartition spatiale de l'induction magnétique obtenue avec un réseau de microsondes sur une collection d'échantillons de rapports de dimensions différents a mis en évidence la présence de barrières géométriques retardant l'entrée des vortex. La prise en compte de ces barrières est essentielle à la déduction de Hc1.L'anisotropie des champs critiques présente une dépendance en température due à la présence de deux paires de bandes supraconductrices faiblement couplés et dont l'influence relative varie en fonction de la position dans le diagramme H-T. Cette présence conduit également à une évolution de xsi et lambda en fonction du champ magnétique que nous présentons ici.Enfin, une étude d'échantillons dopés et irradiés a été abordée. L'effet de la substitution de Mg par Al modifie le dopage électronique et les champs critiques. En revanche, l'effet du désordre ponctuel introduit par l'irradiation aux électrons conduit à une brutale augmentation de l'effet de pic dont l'origine reste encore à établir.

Transport through a single-molecular conductor is considered, showing negative differential conductance behavior associated with phonon-mediated electron tunneling processes. This theoretical work is motivated by a recent experiment by Leroy et al. using a carbon nanotube contacted by a scanning tunneling microscope tip [Nature 432, 371 (2004)], where negative differential conductance of the breathing-mode phonon side peaks could be observed. A peculiarity of this system is that the tunneling couplings which inject electrons and those which collect them on the substrate are highly asymmetrical. A quantum dot model is used, coupling a single electronic level to a local phonon, forming polaron levels. A ``half-shuttle'' mechanism is also introduced. A quantum kinetic formulation allows us to derive rate equations. Assuming asymmetric tunneling rates and in the absence of the half-shuttle coupling, negative differential conductance (NDC) is obtained for a wide range of parameters. A detailed explanation of this phenomenon is provided, showing that NDC is maximal for intermediate electron-phonon coupling. In addition, in the absence of a gate, the ``floating'' level results in two distinct lengths for the current plateaus, related to the capacitive couplings at the two junctions. It is shown that the half-shuttle mechanism tends to reinforce the negative differential regions, but it cannot trigger this behavior on its own.

Because of its wide band gap of 5.45 eV, diodes made with diamond as semiconductor have huge potentialities such as working at temperatures higher then those supported by other semiconductors, or a breakdown voltage of several kV. The purpose of this work was to grow and study homoepitaxial CVD diamond layers slightly boron-doped prepared with addition of oxygen, that are suited to these diodes, and to study properties of contacts (ohmic and Schottky) made on these layers. Different analytical techniques such as micro-Raman spectroscopy and cathodoluminescence have been used which provide informations about crystalline quality and the nature of defects present in these epilayers. A study of the optimization of dry etching using O2 plasma allowed to realise ohmic contacts on p+ layer and mesa structure. The characteristics of ohmic contacts (Au/Ti and Si/Al/Si) and Schottky contacts (Ar/Ti and Ni/Er) are studied. The generation and passivation of defects and their interaction with the compensation of acceptors introduce by boron, are determined by electrical and transient measurements.; Les diodes réalisées avec le diamant comme semi-conducteur comportent de nombreuses potentialités dues à sa bande interdite de 5.45 eV. Parmi les principales, il y a le fonctionnement à des températures très supérieures à celles supportées par les autres semi-conducteurs, ainsi qu'une tension de claquage de plusieurs kV. Le but de ce travail est de réaliser des couches de diamant homoépitaxié faiblement dopées bore et préparées avec ajout d'oxygène, adaptées à ces diodes. Des méthodes de caractérisation telles que la spectroscopie Raman et la cathodoluminescence fournissent des informations sur la qualité cristalline des couches et la nature des défauts présents. Une étude de l'optimisation de la gravure par plasma O2 a permis de réaliser des contacts ohmiques sur la couche p+ et des structures mesa. Les caractéristiques de contacts ohmiques Au/Ti et Si/Al/Si et de contacts Schottky Ag/Ti et Ni/Er sont étudiées. La génération et la passivation des défauts révélés ainsi que leur interaction avec la compensation des accepteurs introduits par le bore sont déterminées par des mesures électriques et transitoires.

Except for small molecules, it is impossible to solve many electrons systems without imposing severe approximations. If the configuration interaction approaches (CI) or Coupled Clusters techniques \cite{FuldeBook} are applicable for molecules, their generalization for solids is difficult. For materials with a kinetic energy greater than the Coulomb interaction, calculations based on the density functional theory (DFT), associated with the local density approximation (LDA) \cite{Hohenberg64, Kohn65} give satisfying qualitative and quantitative results to describe ground state properties. These solids have weakly correlated electrons presenting extended states, like $sp$ materials or covalent solids. The application of this approximation to systems where the wave functions are more localized ($d$ or $f$-states) as transition metals oxides, heavy fermions, rare earths or actinides is more questionable and can even lead to unphysical results : for example, insulating FeO and CoO are predicted to be metalic by the DFT-LDA...

MEMS and NEMS (Micro and Nano Electro-Mechanical Systems) presently undergo a wide development within the framework of the Nanomechanics. The efficient operation of these MEMS / NEMS requires the control and knowledge of fundamental interactions between surfaces separated by distances of the order of 100 nm to 1 micron. At this scale, dominant forces can have different origins (van der Waals, Casimir effect, electrostatic, magnetic...). This thesis presents an experimental study by AFM of the electrostatic and Van der Waals/Casimir interactions between a microsphere and a metallic surface. First, characterisation of the sensors that is based on a microsphere glued onto a cantilever beam is presented. Sensitivity of our measurements determined by noise analysis is shown to provide us with a resolution in the subpiconewton range. In static mode (i.e. cantilever deflection measurement), we have precisely measured capacitive forces of the order of few dozens of piconewton in the 100 to 500 nanometer range. We have also measured and quantified the cantilever deflection effects on the determination of the sphere-surface distance. By dynamic measurements (cantilever as an oscillator at resonance), we studied the coupling between the oscillator mechanically or thermally excited and a surface under the influence of Casimir effect. Finally, dynamic mode measurements allowed us to study dissipation mechanisms associated to long range interactions. For example, we have observed the damping induced by electromechanical coupling., Les MEMS et NEMS (Micro et Nano ElectroMechanical Systems) connaissent un développement important notamment dans le cadre de la Nanomécanique. Le bon fonctionnement de ces MEMS/NEMS est soumis au contrôle des interactions de surfaces séparées par des distances de l'ordre de 100 nm à 1 Μm. A cette échelle, les forces dominantes peuvent être de nature variée (Van der Waals, Casimir, électrostatique, magnétique,...). Ce travail de thèse est consacré à l'étude expérimentale par AFM des interactions électrostatiques et de Van der Waals/Casimir entre une microsphère et une surface d'or. Une attention particulière a été apportée à la préparation et à la caractérisation des sondes (microsphère collée sur un microlevier). Ensuite, à partir de l'analyse du bruit et de la sensibilité du système de mesure, nous avons pu montrer notre capacité à mesurer des forces faibles avec une résolution inférieure au piconewton. Par une détection statique, nous avons mesuré précisément des forces capacitives de l'ordre de quelques dizaines de piconewton et pour des distances comprises entre 100 et 500 nm. Nous avons mesuré et quantifié les effets de la déflexion du microlevier sur la détermination de la distance sphère-surface. Par des mesures dynamiques, nous avons étudié le couplage entre un oscillateur (sphère-microlevier) excité mécaniquement ou thermiquement et une surface sous l'effet de la force de Casimir. Enfin, les mesures en mode dynamique, nous ont permis d'aborder l'étude des mécanismes de dissipation associés aux interactions à longue distance. Nous avons ainsi observé la dissipation par couplage électromécanique.