Characterization of magnetic properties of Sr2CuWO6 and Sr2CuMoO6

In this work we examine the low-temperature magnetic properties of the two double-perovskite compounds Sr2CuWO6 and Sr2CuMoO6 using magnetic susceptibility, muon spin rotation and relaxation, and neutron powder diffraction measurements. Additionally, the most relevant spin exchange interaction constants are derived from ab initio electronic structure calculations, aided by x-ray absorption spectroscopy. The compounds exhibit quasi-two-dimensional magnetic properties, with broad maxima at Tmax = 83 and 95 K for Sr2CuWO6 and Sr2CuMoO6, respectively. However, three-dimensional long-range order takes place below TN = 24(1) and 28(2) K for Sr2CuWO6 and Sr2CuMoO6, respectively. Our results show that the low-dimensional magnetic correlations are mainly due to the significant next-nearest-neighbor interactions in the ab plane of the double-perovskite structure, whereas three-dimensional long-range magnetic order is caused by weaker next-nearest-neighbor interactions along the c axis. Next-nearest-neighbor interactions are also slightly frustrated by weaker nearest-neighbor interactions within the ab plane. Based on these results we predict the low-temperature magnetic structure in these compounds to be type-II antiferromagnetic order of the double-perovskite lattice.Peer reviewe

Magnetic properties near the surface of cuprate superconductors studied using Beta-Detected NMR

Beta-detected Nuclear Magnetic Resonance (β-NMR) uses highly spin polarized β-emitting nuclei as a probe. Besides its use in nuclear physics, it has also become a powerful and sensitive tool in condensed matter physics and materials science. At TRIUMF, β-NMR of ⁸Li+ has been developed to study materials in a depth-resolved manner, where the implantation depth of ⁸Li+ is controlled via electrostatic deceleration. In this thesis, β-NMR of ⁸Li+ has been used to study high-Tc cuprate superconductors (HTSC). The objective of this work is to search for spontaneous magnetic fields generated by a possible time-reversal symmetry breaking (TRSB) superconducting state near the surface of hole-doped YBa₂Cu₃O₇−d (YBCO), and study the nature of the vortex lattice (VL) in YBCO and electron-doped Pr₂−xCexCuO₄−d (PCCO). For several advantages, our measurements were carried out by implanting ⁸Li+ in thin silver films evaporated on the superconductors. In our TRSB studies, the magnetic field distribution p(B) is measured 8 nm away from the Ag/YBCO interface in magnetic fields B₀ = 5 to 100 G, applied parallel to the interface. p(B) showed significant broadening below the Tc of ab- and c-axis oriented YBCO films. The broadening signals the existence of weak disordered magnetic fields near the surface of YBCO. From the broadening’s temperature and field dependence we draw an upper limit of 0.2 G on the magnitude of spontaneous magnetic fields associated with TRSB. To study the VL, p(B) is measured at average implantation depths ranging from 20 to 90 nm away from the Ag/YBCO or Ag/PCCO interface in B₀ = 0.1 to 33 kG, applied perpendicular to the surface. p(B) showed a dramatic broadening below Tc as expected from the emerging field lines of the VL in the superconductor. In YBCO, p(B) is symmetric and the dependence on B0 is much weaker than expected from an ideal VL, indicating that the vortex density varies across the face of the sample on a long length scale, likely due to vortex pinning at twin boundaries. In PCCO, a 2D VL is established due to the high anisotropy of the superconductor leading to a nearly symmetric p(B).Science, Faculty ofPhysics and Astronomy, Department ofGraduat

Contribution à la synthèse d'observateurs non linéaires pour des classes de systèmes dynamiques hybrides

Ces dernières années, beaucoup de chercheurs se sont intéressés à l'étude des Systèmes Dynamiques Hybrides (SDH). Ces recherches sont motivées par le fait que le comportement hybride apparaît dans de nombreux systèmes réels. En effet, ces phénomènes hybrides existent aussi bien dans des processus industriels que dans des processus naturels. La synthèse d'observateurs est un thème fondamental en théorie des systèmes. En effet, les états d'un système dynamique ne sont pas souvent tous accessibles à la mesure ce qui peut compromettre la mise en oeuvre de certaines lois de commande. La connaissance de l'état peut aussi être nécessaire dans des problèmes de surveillance ou de diagnostic. Il devient alors nécessaire de les reconstruire. Dans cette thèse, nos travaux autour des SDH concernent différentes classes de SDH: les systèmes à commutations avec ou sans saut, avec ou sans mémoire discrète. Dans ce contexte nos contributions portent sur le problème de la synthèse d'observateurs non linéaires pour ces classes de systèmes. L'observateur hybride que nous proposons a pour objectif d'estimer à tout instant, aussi bien le mode discret appelé é-galement l'état discret que les états continus. Nous insisterons particulièrement sur deux aspects originaux de ce travail, l'un portant sur l'intérêt de l'utilisation des observateurs à modes glissants et particulièrement ceux d'ordres deux (Super Twisting Aigorithm), l'autre intérêt porte sur la convergence en temps fini de l'erreur d'observation, une propriété majeure dans l'étude des systèmes hybrides. Ainsi, l'originalité de l'observateur proposé est de synthétiser un observateur pour la partie continue; l'état continu observé constituant le capteur (la sortie) nécessaire à la synthèse de l'observateur de la partie discrète.These last decades, the works on Hybrid Dynamical Systems (HDS) have attracted great attention. Researchers are motivated by the fact that the hybrid behaviors appears in numerous real systems. The hybrid phenomenon can occur in industrial processes as weil as in natural processes. The observer synthesis is a fundamental topic in the system's theory. Indeed, in most cases, the states of a dynamic system are not available for measurement which can compromise the state feedback control design. The knowledge of the system's states can also be necessary in diagnosis problems. Consequently, the observation problem is always a chal1enging one and especial1y for new classes of systems as the HDS. The work presented in this thesis considers several classes of HDS: switched systems with or without jumps, with or without discrete memory. In that context, our contributions are focused on the problem of the synthesis of non-linear observers for those HDS classes. The proposed hybrid observer allows estimating at each moment of time both discrete mode (also cal1ed the discrete state) and continuous states. We focused on two particular and original aspects. The first one regarding the interest of using sliding mode observers and especially of second order (Super Twisting Aigorithm) and the second one regarding the convergence of the estimation error in finite time which is a major property in proposed study for the considered classes of hybrid systems. Moreover, the main idea of the proposed observer is to synthesize an observer for the continuous dynamics, while the continuous estimated state constitutes the sensor (output) for the synthesis of the observer for the discrete part.CERGY PONTOISE-BU Neuville (951272102) / SudocSudocFranceF

Arrays of copper rings with tunable dimensions via electro-colloidal lithography

Electro-colloidal lithography (ECL) is an easy, fast, straightforward and cheap patterning process that has been used here to design patterned copper film. ECL consists in assembling polystyrene (PS) micro-beads in 2D hexagonal crystals onto an electrode using AC (Alternative Current) electric field, in sticking them onto the electrode surface where copper is then deposited using Cu2+-containing multi-lamellar vesicles (MLVs) submitted to a DC (Direct Current) electric field. Copper rings are then formed; their separation, ranging from 2 to 4.2 mu m, could be varied playing on either PS beads size or on AC electric field parameters (frequency and amplitude). The ring internal diameter (100-880 nm range), the ring height (28-400 nm range) as well as the copper film thickness (6-102 nm range) were controlled through the time of DC field application. AFM, SEM, microscopic imaging as well as mathematical modeling show that PS beads behave as a template for copper ring growth, and that this patterning is due to an accumulation of MLVs around the dielectric PS spheres on the electrode. (C) 2015 Elsevier B.V. All rights reserved

STM of CrCl3-graphite intercalation compounds: Islands, graphite in-plane defects

Samples of stage-2 chromium chloride graphite intercalation compounds have been studied by Scanning Tunneling Microscopy (STM) with atomic resolution. Due to the existence of Daumas-Herold domains with one and two graphite layers on the guest layer closest to the surface, which exhibit different STM images, the first direct evidence is given for the existence of small intercalated islands. The island shape is shown to be very complicated. In addition, a new kind of defects has been observed in some images, where extra carbon atoms give rise to the presence of heptagons and triangles inside the hexagonal lattice of the carbon layers. These defects could be responsible for the well known particular chemical properties of Kish graphite and for the shift of its Fermi level relative to pristine graphite, as deduced from magnetic susceptibility experiments.La microscopie à effet tunnel (STM) a été utilisée pour étudier les composés d'insertion graphite-CrCl3 de stade-2. Deux types de domaines, montrant des images différentes en STM, peuvent être reliés aux domaines de Daumas-Herold avec un et deux plans de graphite, respectivement, recouvrant le plan intercalé le plus proche de la surface. Pour la première fois, l'existence d'îlots de petite taille est démontrée. Cette étude montre que la forme des îlot est très compliquée. Enfin, des défauts ont été observés dans les plans de carbone, et sont interprétés comme des atomes interstitiels dans les plans. Ceux-ci créent des cycles à sept et trois atomes dans les plans de symétrie hexagonale. Ces défauts pourraient être responsables des propriétés chimiques particulières bien connues du graphite Kish, et du déplacement de son niveau de Fermi par rapport au graphite pur, que mettent en évidence des mesures de susceptibilité magnétique

Static and cyclic crack propagation in Ce-TZP ceramics with different amounts of transformation toughening

International audienceCeria partially stabilized zirconia ceramics (Ce-TZP) with identical grain size and different amounts of transformation toughening were processed to investigate the influence of phase transformation on static and cyclic fatigue crack growth. Static crack growth is governed by environmentally stress induced corrosion at the crack tip and it is highly influenced by the crack shielding due to the phase transformation. Three fatigue mechanisms are expected to be operative at different proportions depending on the amount of transformation: wedge effect due to debris, degradation of bridging and modification of the shielding effect of the transformation zone. However, it is difficult to separate the contribution of the different mechanisms as grain bridging is induced by crack arrest due to phase transformation

Spatial Heterogeneity of Glassy Polymer Films

By studying the morphology of polystyrene films subjected to a fast structuration method, we demonstrate the spatial heterogeneity of their surface viscoelasticity at temperatures well below the glass transition temperature, Tg. Our results point to a nonrandom arrangement of zones of different elastic modulus deep in the glassy state. The spatial distribution of viscoelastic properties of the film was temperature dependent; we observed the presence of soft zones (surrounded by areas of larger elastic modulus) spanning increasinglylarger areas of the surface as T gets closer to Tg. The heterogeneity on viscoelastic properties of the film is supported by the temporal evolution and relaxation of the structured surfaces and by direct measurements of the spatial distribution of the local elastic modulus of polystyrene films

Fine-tuning the assembly of highly stable oppositely charged ceriumoxide nanoparticles in solution and at interfacesSribharani

tIn this work we have shown how to fine-tune the electrostatic assembly of oppositely charged ceriumoxide nanoparticles (NPs) in a solution and at a solid/liquid interface. In a first stage, we have developedanionic and cationic CeO2NPs stable in a highly saline environment and a wide pH range via efficientelectro-steric interactions. Anionic NPs were obtained by coating bare NPs with short polyacrylic acid(PAA) chains through a Precipitation-Redispersion method. Whereas cationic NPs were formulated bytethering branched polyethyleneimine (bPEI) chains onto PAA-coated ceria NPs through the formation ofamide bonds with the help of imide cross-linkers. The covalent attachment of bPEI chains was evidenced through NMR characterizations while thermogravimetric (TGA), dynamic light scattering (DLS) and zetapotential measurements confirmed the successful encapsulation by both types of weak polyelectrolytes.In a second stage, the high stability of both NP dispersions toward high ionic strength (up to 4 M NH4Cl)enabled to generate hybrid clusters in a controlled way by fine tuning their co-assembly following adesalting transition route monitored by DLS. In a last part, we have extended our approach to surfaces andgenerated all-nanoparticle layer grown directly from a liquid/solid interface monitored by Quartz CrystalMicrobalance (QCM)...

STM of CrCl3-graphite intercalation compounds

Scanning tunneling microscopy has been used to image stage-2 CrCl 3-graphite intercalation compounds with atomic resolution. Images obtained after cleaving the samples are characteristic of two types of domains. One (the most frequent) exhibits an hexagonal lattice where all carbon atoms are equivalent, whereas the other shows a centered hexagonal structure similar to that of pristine graphite. These two domains can be related to Daumas-Herold domains with one and two graphite layers, respectively, covering the guest layer closest to the surface.La microscopie à effet tunnel a été utilisée pour étudier les composés d'insertion graphite-CrCl3 de stade-2. Les images obtenues de la surface après clivage sont caractéristiques de deux types de domaines. Le plus fréquent apparaît sous forme d'un plan de structure hexagonale dont tous les atomes de carbone sont équivalents, tandis que l'autre montre une structure hexagonale centrée proche de celle du graphite pur. Ces deux types de domaines peuvent être reliés aux domaines de Daumas-Herold avec un et deux plans de graphite, respectivement, recouvrant le plan intercalé le plus proche de la surface

Graphitization of Pyrocarbons - An STM Study

Scanning tunneling microscopy (STM) has been used to image at atomic resolution the surface of pyrocarbons deposited at 1680-degrees-C and heat-treated at temperatures up to 2800-degrees-C. Information is obtained on the increase of surface order with HTT. Of special interest are the presence, in HTT 2000 samples, of point defects arranged in a (square-root 7 x square-root 7) R 19. 1-degrees superstructure and attributed to an array of interstitial carbon atoms, and the observation of a moire pattern in the HTT 2200 sample, which could be due to the turbostratic nature of the layer stacking